FINAL REPORT
UTILIZATION OF RADAR IN
SHORT RANGE FORECASTING
Contract No. N189s-88l64
5 August 1953
ILLINOIS STATE WATER SURVEY
AT
UNIVERSITY OF ILLINOIS
URBANA, ILLINOIS
FINAL REPORT
UTILIZATION OF RADAR IN SHORT-RANGE
WEATHER FORECASTING
Contract No. N 1 8 9 s - 8 8 l 6 4
The r e s e a r c h reported in this document h a s been
made possible through support and s p o n s o r s h i p extended
by the B u r e a u of Aeronautics P r o j e c t AROWA, Naval Air
Station, Norfolk, Virginia, under Contract No. N 1 8 9 s 88164. It is published for technical information only,
and does not n e c e s s a r i l y r e p r e s e n t r e c o m m e n d a t i o n s or
conclusions of the supporting agency.
P r e p a r e d by
H. W. H i s e r and S. G. Bigler
G. E. Stout, Group L e a d e r
A. M. Buswell, Chief
5 August 1953
UTILIZATION OF RADAR IN SHORT-RANGE WEATHER
FORECASTING
TABLE OF CONTENTS
PREFACE
ABSTRACT
INTRODUCTION
PART I
RADAR F U N D A M E N T A L S P E R T I N E N T
i . .
Page
i
ii
1
TO AEROLOGY
RADAR C H A R A C T E R I S T I C S .
P r i n c i p l e s of R a d a r
Scope P r e s e n t a t i o n
Radar Equation
R a d a r Wavelengths for W e a t h e r Use
RADAR S C O P E I N T E R P R E T A T I O N S
B l o c k i n g By O b s t r u c t i o n s
S e a R e t u r n and G r o u n d C l u t t e r .
Attenuation.
Precipitation
Range . . . . . . . . . . .
Anomalous Propagation
Receiver Noise
RADAR P E R F O R M A N C E F A C T O R S AND L I M I T A T I O N S . . . .
Stability and C a l i b r a t i o n s
Refraction
B e a m Width
DESIRED RADARS AND O P E R A T I O N A L A S P E C T S
Usage o f P P I , RHI, and " A " Scope P r e s e n t a t i o n s . . . .
R e c e i v e r Gain C o n t r o l s
Calibrated Receiver-Gain Control
Video Inversion
Automatic Receiver-Gain Control
Plotting Devices
C a m e r a s for T r a i n i n g P u r p o s e s , R e s e a r c h , and
"Hindcasting"
2
2
2
3
4
4
4
4
5
5
5
6
6
7
7
7
7
7
7
8
9
9
9
9
10
P A R T II APPLICATIONS TO FORECASTING
J P P E R WIND DATA F R O M R A D A R E C H O E S
Introduction
Scope a n d T e c h n i q u e s U s e d
C o r r e l a t i o n of L a y e r M e a n W i n d s w i t h E c h o M o v e m e n t .
A c c u r a c y of D e t e r m i n a t i o n s
11
11
11
12
13
- 2 C o r r e l a t i o n o f Winds a t S p e c i f i c L e v e l s w i t h E c h o
Movements
„
A c c u r a c y of D e t e r m i n a t i o n s
C o m p a r i s o n of R e s u l t s .
N o m o g r a m s f o r D e t e r m i n i n g Winds
. .
R u l e s for I n c r e a s e d A c c u r a c y of Wind D e t e r m i n a t i o n s . .
PRECIPITATION INTENSITY MEASUREMENTS
A r e a l Method
Radial Method (Point) . .
N o m o g r a m s F o r Rainfall E s t i m a t e s
V I S I B I L I T Y DURING RAINSHOWERS . . . . . . . . . . . . . .
Introduction .
Approach to P r o b l e m
Data Used
T r e a t m e n t of Data
R e s u l t s of Data A n a l y s i s
Mean Reduction in Visibility
M a x i m u m Reduction in Visibility
Determination o f Visibility Reduction b y Radar
. . . . .
Atlas' Equation for Determining Visibility f r o m
Rainfall Rate .
S Q U A L L Y WIND D E T E R M I N A T I O N S . . .
M e c h a n i s m s P r o d u c i n g Squally Winds.
R a d a r E c h o e s a n d S q u a l l y Winds
R u l e s for E s t i m a t i n g S q u a l l y Winds f r o m R a d a r E c h o e s .
P A R T III
13
14
15
16
16
17
17
18
18
20
20
20
20
21
21
21
22
22
23
23
24
25
25
CASE S T U D I E S
HIGH L E V E L S H O W E R S A N D T H U N D E R S T O R M S DURING
NIGHT O F 2 3 - 2 4 J U L Y 1951
Synoptic Situation
D i s c u s s i o n o f R a d a r and Synoptic Data . . . . . . . . . .
Upper Winds .
Conclusions
P R E - F R O N T A L S Q U A L L L I N E O F 1 2 S E P T E M B E R 1951 . . .
Synoptic Situation
D i s c u s s i o n o f R a d a r and S y n o p t i c D a t a . . . . . . . . . . .
Conclusions
P R E - F R O N T A L S Q U A L L L I N E O F 8 - 9 A U G U S T 1952
S y n o p t i c S i t u a t i o n and W e a t h e r F o r e c a s t
Radar Data
Conclusions
SHOWER F O R M A T I O N S ON A S T A T I O N A R Y F R O N T 14
O C T O B E R 1952
S y n o p t i c S i t u a t i o n and W e a t h e r F o r e c a s t
27
27
28
29
29
30
30
30
31
32
32
32
33
34
34
- 3 -
D i s c u s s i o n of R a d a r and Synoptic Data
34
Conclusions
„
.
36
??QUALL LINE OF 17 NOVEMBER 1952 SHOWING
E F F E C T S OF ATTENUATION
37
Synoptic Situation and Weather F o r e c a s t . . . . . . . . .
37
D i s c u s s i o n of Radar and Synoptic Data
37
Conclusions
39
??ADAR OBSERVATION OF A TORNADO ON 9 A P R I L 1953 . . 40
Synoptic Situation
40
Radar D a t a .
40
Conclusions
41
PARALLEL PRECIPITATION BANDS O F 1 7 APRIL 1953 . . .
42
Synoptic Situation and Weather F o r e c a s t
42
D i s c u s s i o n of Radar and Synoptic Data
42
Conclusions .
44
LIST OF R E F E R E N C E S
45
- i PREFACE
This m a n u s c r i p t is to be considered the final r e p o r t covering one
? ? e a r ' s r e s e a r c h by the Illinois State Water Survey on the operational use
??f r a d a r as a weather forecasting aid under BuAer c o n t r a c t n u m b e r
??189s-88l64. The m a t e r i a l is presented in a m a n n e r so that the sponsoring agency m a y publish it as an Aerology bulletin.
Additional r e s e a r c h is needed for further development and refinement of techniques in the field of r a d a r weather f o r e c a s t i n g . More
accurate d e t e r m i n a t i o n s of echo intensity as related to other w e a t h e r
phenomena should be p u r s u e d .
It was decided to include the f i r s t section of the r e p o r t even
though there a r e s e v e r a l other a r t i c l e s available. Only those f a c t o r s
affecting the use of r a d a r in meteorological work a r e d i s c u s s e d . The ,
f o r e c a s t e r must.become thoroughly familiar with the c h a r a c t e r i s t i c s
of his r a d a r set before p r o p e r i n t e r p r e t a t i o n s of the scope can be m a d e .
The authors a r e indebted to Floyd A. Huff for editing, to Glenn
E. Stout for valuable a s s i s t a n c e and to Duane Yetter and Donald Worden
for much of the plotting and calculations. The a u t h o r s a r e a l s o indebted
to Dr. H. R. B y e r s of the University of Chicago, D e p a r t m e n t of Meteorology, for reviewing the r e p o r t .
All scope photographs appearing in this r e p o r t w e r e made with
the Water S u r v e y ' s APS-15 r a d a r .
- ii ABSTRACT
The r a d a r data used in this r e p o r t were collected with a modified
A P S - 1 5 , 3-cm r a d a r during the period 1950-52. This r a d a r s e t was
equipped with an automatic gain reduction device which afforded a method
of m e a s u r i n g echo intensity. Surface and upper a i r o b s e r v a t i o n s by the
U. S. Weather B u r e a u stations in Illinois and rainfall r e c o r d s from a 95
s q u a r e m i l e raingage network w e r e used for c o r r e l a t i o n p u r p o s e s .
R a d a r c h a r a c t e r i s t i c s , r a d a r scope i n t e r p r e t a t i o n s , r a d a r performance factors and l i m i t a t i o n s , and d e s i r e d r a d a r s and operational
a s p e c t s a r e discussed in P a r t I.
P a r t II of the r e p o r t deals with the study of the utility of r a d a r in:
the d e t e r m i n a t i o n of u p p e r - l e v e l winds; the prediction of rainfall intensity;
the calculation of visibility during rainfall; and the d e t e r m i n a t i o n of squally
winds.
Sixty-nine c a s e s of r a d a r echo m o v e m e n t s w e r e c o r r e l a t e d with the
winds aloft. The echoes w e r e classified into two g r o u p s : one of e l e m e n t s
of squall lines and one of isolated e c h o e s . The m o v e m e n t s of the e c h o e s
were c o r r e l a t e d with the m e a n wind for three l a y e r s : 2000-20, 000 ft; 200026, 000 ft; and 6000-30, 000 ft; and with winds at t h r e e specific l e v e l s : 6000
ft, 10, 000ft, and 20,000 ft. Standard e r r o r s of e s t i m a t e w e r e computed for
each of the l a y e r s and levels to obtain an estimation of the a c c u r a c y of the
wind d e t e r m i n a t i o n s . A list of r u l e s for i n c r e a s e d a c c u r a c y of wind determinations is p r e s e n t e d . N o m o g r a m s for operational use a r e also included.
A graph based on t h e o r e t i c a l calculations by Wexler was constructed
which gives the theoretical rainfall intensity produced by an echo at a given
r a n g e . Quantitative c o m p a r i s o n s were made between the t h e o r e t i c a l graph
values and the actual m e a s u r e d rainfall produced by echoes over the r a i n gage network. R e s u l t s indicate that m o r e r e s e a r c h is needed on effects of
drop size d i s t r i b u t i o n and attenuation on 3-cm wavelength r a d a r in o r d e r to
develop a c c u r a t e equations for rainfall d e t e r m i n a t i o n from such r a d a r .
P e r c e n t a g e reductions in visibility produced by v a r i o u s intensities
of showers and thunder s h o w e r s w e r e computed from surface o b s e r v a t i o n s
taken at nine Weather B u r e a u and Air F o r c e stations in Illinois. P e r c e n t a g e
reductions in visibility can be applied to showers of different i n t e n s i t i e s as
observed on the r a d a r . In this m a n n e r , the visibility reduction to be expected
with the p a s s a g e of a shower of a given intensity can be determined from r a d a r .
The d e t e r m i n a t i o n of squally winds from r a d a r echoes r e m a i n s l a r g e l y
qualitative due to the lack of good wind data a s s o c i a t e d with echoes of measured intensity. C o m p a r i s o n s of wind gusts and shower intensities w e r e
made from Weather B u r e a u surface o b s e r v a t i o n s taken at stations in the
range of the r a d a r . The r e s u l t s as expected indicate that the a v e r a g e and
iii
??eak gusts from heavy showers and thunder showers a r e c o n s i d e r a b l y
??igher than those from light or moderate intensities of s h o w e r s and
??hunder s h o w e r s . The findings of the T h u n d e r s t o r m P r o j e c t (8) w e r e
??f g r e a t value as s u p p l e m e n t a r y information in this study.
P a r t III of this r e p o r t gives special case studies of specific
s t o r m s which w e r e made to aid in training a e r o l o g i s t s in the use of
radar. These studies show the good relationships which exist b e ??ween synoptic conditions and r a d a r data, and i l l u s t r a t e how each
??an aid in p r o p e r i n t e r p r e t a t i o n of the other. These c a s e s also show
??hat considerable weather can e x i s t undetected by the p r e s e n t networks
??f surface observation s t a t i o n s , such as those in the Midwest. By the
??se of r a d a r the f o r e c a s t e r is able to keep a constant vigilance over,
??he weather in a r e a s lacking r e p o r t i n g stations and to watch for new
levelopments.
- 1 -
INTRODUCTION
The advent of r a d a r h a s provided the field of Aerology with one of its
??st useful tools for keeping continuous vigilance over the w e a t h e r in t i m e
?? space. As an a n a l y s i s and forecasting aid to a e r o l o g i s t s , who a r e
quently exposed to operational forecasting with v e r y limited synoptic
a, r a d a r has proved to be e x t r e m e l y valuable.
The p o s s i b i l i t i e s for application of r a d a r to Aerology w e r e recognized
World War II soon after the equipment b e c a m e g e n e r a l l y a v a i l a b l e . In the
??t few y e a r s it has b e c o m e a r e g u l a r p a r t of weather o b s e r v i n g and fore? ? t i n g s y s t e m s ; however, development of specific forecasting techniques
?? not yet been pursued sufficiently to p e r m i t effective utilization of r a d a r
the operational f o r e c a s t e r .
The purpose of this r e p o r t is to d e m o n s t r a t e quantitative techniques
the operational use of r a d a r in weather forecasting. The e x a m p l e s a r e
??ited to the use of P P I ( P l a n - P o s i t i o n Indicator) r a d a r .
- 2 -
PART I
RADAR CHARACTERISTICS
principles of Radar
A r a d a r set e m i t s a short, intense pulse of e l e c t r o - m a g n e t i c e n e r g y
ch is confined within a n a r r o w b e a m as it t r a v e l s outward. If a t a r g e t
? ? r c e p t s the b e a m , some of the energy contained in the pulse is reflected.
m a l l portion of the reflected energy r e t u r n s as an " e c h o " to the point of
t r a n s m i s s i o n , where it is r e c e i v e d , amplified, and p r e s e n t e d on differtypes of s c o p e s . The range of the t a r g e t is d e t e r m i n e d by the elapsed
e between t r a n s m i s s i o n of the pulse and its r e t u r n as an echo. The
muth of the t a r g e t , with r e s p e c t to the r a d a r set, is given by the d i r e c t i o n
vhich the beam is pointed. Raindrops a c t as s c a t t e r e r s of the r a d a r b e a m
her than d i r e c t r e f l e c t o r s ; each individual d r o p r e t u r n s a p a r t of the
iated pulse to the r a d a r . (1) (2) The p r o c e s s is s o m e t i m e s called Rayleigh
ttering after Lord Rayleigh who studied the effect in g r e a t detail 40 y e a r s
scope P r e s e n t a t i o n s
There a r e three common types of scope p r e s e n t a t i o n s of r a d a r e c h o e s
ch a r e valuable for a e r o l o g i c a l u s e . These a r e the P l a n - P o s i t i o n Indicator
(I), Range-Height Indicator (RHI), and " A " scope. The P P I scope gives a
izontal plane projection of the echoes so that a m a p can be drawn with the
ar station at the c e n t e r . As the r a d a r antenna t u r n s in azimuth, a s y n onous sweep line r o t a t e s on the P P I scope. When the b e a m i n t e r c e p t s a
get, a b r i g h t spot a p p e a r s on the sweep line at a r a d i a l distance c o r r e nding to the range of the t a r g e t . The face of the scope is coated with a
s p h o r e s c e n t m a t e r i a l , which continues to glow for a few seconds after the
??ep has p a s s e d , thus p r e s e r v i n g between sweeps the i m p r e s s i o n of the
get on the scope.
The RHI scope p r e s e n t s a v e r t i c a l c r o s s section of the echo along the
muth on which the antenna is pointed. This type of presentation r e q u i r e s
antenna which s c a n s in the v e r t i c a l as well as sweeping in azimuth.
The third type is an "A" scope in which the e l e c t r o n s t r e a m sweeps
? ? r a l l y , tracing out a fixed ho rizontal time b a s e on a s c r e e n . If a signal
letected by the r a d a r r e c e i v e r at any instant during the sweep, the e l e c n s t r e a m is deflected v e r t i c a l l y by an amount c o r r e s p o n d i n g to the signal
ength. T a r g e t s which a r e intercepted by the b e a m s appear as v e r t i c a l
t u b e r a n c e s (pips) on a horizontal base line. The b a s e line is calibrated
ectly in r a n g e . "A" scopes a r e g e n e r a l l y used to accompany P P I and RHI
p e s , and a r e useful in detecting v e r y weak e c h o e s , and in tuning the r a d a r
eiver.
- 3 ??ar Equation
The following equation gives the a v e r a g e power r e c e i v e d from a
? ? t t e r i n g - t y p e echo which completely fills the r a d a r b e a m with s p h e r e s
??niform d i a m e t e r (3) (4):
Quantity
Range of the t a r g e t s
Reflectivity per unit volume of s t o r m
Attenuation factor depending on media
Constants of proportionality for units
used
Power t r a n s m i t t e d
Minimum detectable signal
P u l s e length
Number of d r o p s / u n i t volume
Sixth power of drop d i a m e t e r
Wave length
B e a m width (horizontal)
B e a m width (vertical)
Antenna gain
Values for*
Modified APS-15
.......Km
......Cm -1
D i m e n s i o n l e s s Const.
6. 1 x 1 0 " 1 6
9 to 14 x 10 watts
5 x 10-13 watts
300 m e t e r s
3. 2 cm
2. 5 d e g r e e s
2. 9 d e g r e e s
Dimensionless
This equation is applied l a t e r under " P r e c i p i t a t i o n Intensity M e a s u r e ? ? t s " , w h e r e values for R, P r , and Pt a r e used to solve for the reflectivity
??n rain drops.
T a r g e t s in the form of s c a t t e r i n g p a r t i c l e s such as r a i n clouds have
? ? a r a c t e r i s t i c not common to other types of t a r g e t s , namely, they can be
??etrated by the r a d a r b e a m . Thus it is possible to view the r a i n - c l o u d
??oes in depth and to see one behind the o t h e r . The precipitating clouds do
? ? e v e r , attenuate, that i s , r e d u c e the intensity of the b e a m so that t h e r e
??limit to the penetration. This attenuation will be d i s c u s s e d in detail in
??ter section.
It is i n t e r e s t i n g to note that the s m a l l e s t cloud p a r t i c l e s detectable
??adar methods at o r d i n a r y r a n g e s and at 3-to 10-cm wavelengths a r e
??r those that have an a p p r e c i a b l e fall velocity and a r e therefore classified
??recipitation.
values a r e for the r a d a r set used by the Illinois State Water Survey in this
??search.
- 4 ??ar Wavelengths for Weather Use
The m i l i t a r y s e r v i c e s and other organizations have used both 3-cm
10-cm wavelength r a d a r s for s t o r m detection. The 3 - c m is m o r e
sitive than the 10-cm for this p u r p o s e . The 3 - c m wavelength p r e s e n t s
??-defined echoes but attenuates too much when solid l i n e s of heavy echoes
p r e s e n t and when widespread light r a i n e x i s t s . This attenuation s o m e ??es r e s u l t s in a failure to detect showers behind a solid line of echoes
??ring toward the r a d a r station, or failure to detect the extent of widespread
? ? t r a i n . The 10-cm r a d a r has the advantage of penetrating through heavy
??es and extensive a r e a s of rain. It is favored for h u r r i c a n e and t h u n d e r ??m tracking. However, it fails to give the d e s i r e d definition in many
??es and fails completely to detect light s h o w e r s .
Probably the b e s t wavelength for weather r a d a r o p e r a t i o n s l i e s in
5-6 cm band b e c a u s e attenuation is negligible and good definition can be
ined. R e s e a r c h and development of these m o r e d e s i r a b l e wavelengths
?? p r o g r e s s in the m i l i t a r y s e r v i c e s .
RADAR SCOPE INTERPRETATIONS
??king by Obstructions
Radar s e t s operating from both shipboard and land s t a t i o n s might be
??ect to blocking. Nearby s u p e r s t r u c t u r e s , buildings, or mountains p r o ??ing into the b e a m r e s u l t in blank segments or shadows in which weather
??ot be detected. In F i g u r e s la and l b , blocking effects can be noted due
? ? e e s to the south and to another r a d a r a n t e n n a to the southwest.
Return and Ground Clutter
Echoes from the sea near the r a d a r set show a g r e a t e r r e s e m b l a n c e
recipitation echoes than those from solid fixed land t a r g e t s . Visual ob--ation at close range from the ship will usually afford a m e a n s of checking
ther the echoes a r e sea echoes or precipitation. e c h o e s . Sea echoes
??lly show r a n d o m v a r i a t i o n to a m o r e m a r k e d d e g r e e than precipitation
??es (5).
Buildings, t r e e s , h i l l s , and even slight undulations in topography in
??vicinity of the r a d a r station a p p e a r as "ground c l u t t e r " when the antenna
??inted horizontally or at low elevations. Ground r e t u r n is usually m o r e
??e and has m o r e well-defined edges than precipitation echoes on a P P I
??e. On the "A" scope ground clutter gives s t r o n g , steady pips r a t h e r
the fluctuating v a r i e t y produced by precipitation. After precipitation
o c c u r r e d , the ground clutter is often m o r e extensive. Some of the m o r e
??ant isolated ground t a r g e t s a r e ideal for orienting r a d a r s on land if these
FIG. I RADAR SCOPE PHOTOS
- 5 -
t a r g e t s can be identified on a m a p of the a r e a . F i g u r e la shows the ground
clutter around the Water S u r v e y ' s r a d a r station. Some s m a l l p r e c i p i t a t i o n
echoes appear 20 to 30 m i l e s w e s t and northwest of the station.
Attenuation
Attenuation is the loss of energy by the r a d a r b e a m due to a b s o r p t i o n
or s c a t t e r i n g while traveling to and from the t a r g e t . T h e r e a r e two t y p e s :
one due to precipitation and the other due to r a n g e .
P r e c i p i t a t i o n . The g r e a t e s t attenuation is produced by the r a i n d r o p s
t h e m s e l v e s . When the r a d a r b e a m p a s s e s through a d i s t r i b u t i o n of w a t e r
d r o p s , some of the e n e r g y is s c a t t e r e d in all d i r e c t i o n s and s o m e is a b s o r b e d .
The amount of scattering and absorption depends upon the s i z e , concentration
of the d r o p s and upon the frequency of the r a d a r signal. Scattering is n e c e s s a r y if the disturbance is to be detected, but absorption d e c r e a s e s the depth
of penetration.
When the attenuation is s m a l l , it is possible to see through n e a r b y
p r e c i p i t a t i o n a r e a s and to observe h e a v i e r precipitation echoes beyond these.
When pronounced attenuation o c c u r s on a 3 - c m wavelength r a d a r , a p r e c i p i tation a r e a m a y be obscured by another which l i e s between it and the r a d a r
s e t , or a given disturbance m a y not be detected to its full r a d i a l depth.
F i g u r e lb is an example of precipitation attenuation when a rainfall r a t e of
0. 24 i n / h r was occurring at the r a d a r station. Attenuation can usually be
distinguished by the feathered edges and faded-out a p p e a r a n c e of the edge of
the echo with increasing r a n g e .
Range. Range attenuation r e s u l t s from divergence of the t r a n s m i t t e d
b e a m so that the b e a m is not filled in c r o s s section by an echo at c o n s i d e r a b l e
r a n g e . If the target has uniform reflectivity throughout its c r o s s section, the
total reflected power r e m a i n s constant over the range in which the b e a m is
completely filled. Beyond this r a n g e , the fraction of the b e a m occupied by a
t a r g e t of given size d e c r e a s e s as the s q u a r e of the d i s t a n c e . The e n e r g y
s c a t t e r e d by the t a r g e t a l s o d i v e r g e s so that the power density along the r e t u r n path from the t a r g e t d e c r e a s e s as the square of the d i s t a n c e .
Range attenuation m u s t be taken into account when e s t i m a t i n g s t o r m
i n t e n s i t i e s . This factor will be d i s c u s s e d under " P r e c i p i t a t i o n Intensity
Measurements".
The portion of a s t o r m which lies below the horizon i n c r e a s e s with
r a n g e and b e c o m e s significant at long r a n g e . The fraction of the b e a m filled
by a s t o r m at any given range is thereby further diminished by this e a r t h
c u r v a t u r e factor as the range i n c r e a s e s .
- 6 -
Anomalous P r o p a g a t i o n
Under v e r y stable a t m o s p h e r i c conditions with low-level t e m p e r a t u r e
i n v e r s i o n s or high concentrations of m o i s t u r e n e a r the e a r t h ' s s u r f a c e , a
bending of the r a d a r b e a m may o c c u r . This r e s u l t s in anomalous p r o p a g a t i o n
when the c u r v a t u r e of the b e a m b e c o m e s equal to or g r e a t e r than that of the
e a r t h . The attenuation of power with d i s t a n c e is noticeably diminished s i n c e
the e n e r g y is not allowed to s p r e a d out (diverge) in the v e r t i c a l plane. Consequently, t a r g e t s which lie within the trapping layer or "duct" can be
detected at unusually g r e a t ranges as in F i g u r e l c . In this figure s m a l l ,
s h a r p l y defined, elongate echoes, typical of anomalous propagation, a r e
detected to a r a n g e of 60 m i l e s to the west and 40 m i l e s to the south and e a s t .
"Sea r e t u r n " m a y a l s o be observed at extended r a n g e s under such
c o n d i t i o n s . Sea r e t u r n unfortunately a p p e a r s v e r y much like p r e c i p i t a t i o n
on either the P P I or "A" scope.
Elevation of the antenna 2 or 3 d e g r e e s will usually eliminate anomalous propagation involving land or sea e c h o e s , or cause enough change in
their position to distinguish them from precipitation e c h o e s . A l s o , ground
t a r g e t s (solid objects) detected under such conditions can usually be distinguished f r o m precipitation echoes by use of the "A" scope. The "V scope
" p i p s " produced by precipitation echoes show c o n s i d e r a b l y m o r e s h o r t - p e r i o d
fluctuations and have a fuzzier a p p e a r a n c e than those produced by ground
t a r g e t s . (Figure Id)
Any p r o c e s s which tends to d e s t r o y or oppose h o r i z o n t a l s t r a t i f i c a t i o n
of the a t m o s p h e r e is unfavorable for anomalous propagation. T h e r e f o r e ,
anomalous propagation is not likely to occur on days when conditions a r e
favorable for well-developed s h o w e r s or t h u n d e r s t o r m s , except after thunders t o r m r a i n has stratified the low level air (6).
R e c e i v e r Noise
A c e r t a i n amount of electronic noise is produced by the t h e r m a l activity
in the e l e c t r o n i c components of a r a d a r set. As the r e c e i v e r gain is i n c r e a s e d ,
the noise level i n c r e a s e s and a p p e a r s as s m a l l r a n d o m l y fluctuating pips or
" g r a s s " on the "A" scope. Any r e c e i v e d signal is s u p e r i m p o s e d on the noise
and the s u m of the signal and noise m u s t be somewhat g r e a t e r than the noise
alone in o r d e r to be detected. Weather echoes on the "A" scope show a random fluctuation much like the n o i s e , except that the p i p s have g r e a t e r amplitude. The fuzzy base line a c r o s s the "A" scope in F i g u r e Id is produced by
receiver noise.
This noise a l s o shows on the P P I and RHI s c o p e s as background light.
This background light intensity can be controlled by the r e c e i v e r gain and the
scope b r i l l i a n c e . The r e c e i v e r gain should be turned down and the scope
b r i l l i a n c e adjusted so that the sweep is faintly v i s i b l e . Then the r e c e i v e r
- 7 -
gain should be i n c r e a s e d until the " g r a s s " shows on the "A" scope and a
slight amount of background light a p p e a r s on the P P I or RHI scope.
RADAR PERFORMANCE FACTORS AND LIMITATIONS
Stability and Calibrations
When r a d a r is used for quantitative m e a s u r e of s t o r m i n t e n s i t i e s ,
occasional m e a s u r e m e n t s should be made of the t r a n s m i t t e d power and
the r e c e i v e r sensitivity. If the t r a n s m i t t e d power and the r e c e i v e r
sensitivity of the r a d a r v a r y as much as 3 d e c i b e l s , then quantitative
m e a s u r e m e n t s of s t o r m intensities r e q u i r e frequent calibrations of the
r a d a r . The r a t i o of r e c e i v e r power to t r a n s m i t t e d power should be
calculated for different r e c e i v e d gain settings (this will be d i s c u s s e d
l a t e r under " P r e c i p i t a t i o n Intensity M e a s u r e m e n t s " ) . It is r e c o m m e n d e d
that m e a s u r e m e n t s of r e c e i v e r sensitivity and t r a n s m i t t e d power be made
before or after each r a i n in o r d e r to obtain r e l i a b l e data.
Refraction
As a r e s u l t of refraction in the lower a t m o s p h e r e , the r a d a r b e a m
is gradually and continuously bent from a s t r a i g h t line path. The bending
is usually downward so that energy r e a c h e s below the g e o m e t r i c a l horizon.
The extent to which the r a d a r beam is bent depends upon the g r a d i e n t s of
t e m p e r a t u r e and m o i s t u r e in the lowest layer of the a t m o s p h e r e .
Under a v e r a g e weather conditions the c u r v a t u r e of the r a y s is l e s s
than that of the e a r t h and the horizon distance is i n c r e a s e d by about 15 per
cent. Line OB in F i g u r e 2 i l l u s t r a t e s this bending of the b e a m for a r a d a r
antenna located on the e a r t h ' s surface with the b e a m directed horizontally.
OA is a horizontal line and the upper curve shows the bending of a r a d a r
b e a m tilted upward 1 d e g r e e .
B e a m Width
F r o m the e a r l i e r d i s c u s s i o n of range attenuation resulting f r o m
divergence of the b e a m , it b e c a m e obvious that a r a d a r for quantitative
weather use should have a beam as n a r r o w as possible (3 d e g r e e s or l e s s )
so that r e l a t i v e l y s m a l l s t o r m s will fill the b e a m at long r a n g e s . The
r a d a r equation a s s u m e s the b e a m to be filled. F o r qualitative weather
s u r v e i l l a n c e wider beam widths a r e n e v e r t h e l e s s u s a b l e .
DESIRED RADARS AND OPERATIONAL ASPECTS
Usage of P P I , RHI, and "A" Scope P r e s e n t a t i o n
The P P I type of p r e s e n t a t i o n h a s the widest range of uses in Aerology.
Because it gives a horizontal plane or m a p - t y p e projection, the a r e a l extent,
FIG. 2
R A N G E — HEIGHT DIAGRAM FOR
CENTER OF RADAR B E A M
- 8 -
except as s o m e t i m e s affected by attenuation, and the d i r e c t i o n of m o v e m e n t
of precipitation can be a s c e r t a i n e d with r e a s o n a b l e a c c u r a c y . The formation and dissipation of s t o r m s can be r e a d i l y observed. Also, by the use of
a v a r i a b l e - p o s i t i o n , r e c e i v e r - g a i n control or an automatic g a i n - r e d u c t i o n
device, (to be d e s c r i b e d l a t e r ) the intensity of the precipitation, the expected
associated visibility reduction, and changes in precipitation intensity with
time can be a s c e r t a i n e d .
The RHI type of p r e s e n t a t i o n h a s g r e a t value, especially when used
in conjunction with a P P I scope. RHI r a d a r gives the tops of e c h o e s at extended r a n g e s , provided the echoes a r e tall enough to compensate for the
e a r t h ' s c u r v a t u r e and to protrude into the r a d a r b e a m . The b a s e can a l s o
be d e t e r m i n e d at close r a n g e s ; but often when precipitation r e a c h e s the
ground, the base a p p e a r s at or below ground level. The use of the b a s e and
height data will be d i s c u s s e d under "Upper Wind Data f r o m Radar E c h o e s " .
The v e r t i c a l growth r a t e s as indicated on RHI give some indication of the
violence of the s t o r m . RHI r a d a r provides a useful m e a n s of identifying
cumulo-nimbus clouds hidden from view by a r e a s of light precipitation of
the m o r e stable types, as commonly a s s o c i a t e d with w a r m frontal overrunning. It is probable that the l a r g e s t gusts experienced by a i r c r a f t in
flight a r e associated with the edges of s t r o n g and sharply-defined columns
of weather echo as shown on RHI. Also, the evidence strongly s u p p o r t s the
belief that w e l l - m a r k e d columns of echoes as shown on RHI a r e a s s o c i a t e d
with s t r o n g updrafts, while w e l l - m a r k e d troughs in the echo a r e indicative
of downdrafts (7). Updrafts at the top can be m e a s u r e d roughly by noting
the r a t e of upward growth. . It is believed that rapidly-growing tops a r e
associated with hail(8).
The "A" scope is ideal for use in conjunction with either an RHI or
P P I scope. As pointed out previously, it is a valuable aid in tuning and in
identifying precipitation echoes and ground t a r g e t s .
When tuning the r a d a r r e c e i v e r manually, the antenna should be stopped while pointing at some fixed ground t a r g e t . The r e c e i v e r tuner should
then be adjusted so that the "pip" on the "A" scope produced by the ground
target is at its m a x i m u m height.
R e c e i v e r - G a i n Controls
The r e c e i v e r - g a i n controls the intensity of the echo image on the P P I
scope. By reducing the r e c e i v e r - g a i n , the weaker echoes can be obliterated
from the scope, thus affording a method of m e a s u r i n g the s t o r m intensity.
P r i o r to the development of the automatic r e c e i v e r - g a i n control, only
two methods of obtaining echo intensities w e r e known to be in g e n e r a l use.
One method employed a manually controlled calibrated gain dial; the other
used v i d e o - i n v e r s i o n c i r c u i t s . (3) (9).
- 9 Calibrated R e c e i v e r - G a i n Dial. This method m e r e l y r e q u i r e s replacing the r e g u l a r r e c e i v e r - g a i n control knob with a c a l i b r a t e d dial so that
the r e c e i v e r - g a i n could be a c c u r a t e l y controlled. The dial was c a l i b r a t e d
in t e r m s of echo power n e c e s s a r y for the threshold of visibility on the P P I
s c o p e . The r a d a r o p e r a t o r changes the gain to v a r i o u s settings in o r d e r to
m e a s u r e the r e l a t i v e intensity of the s t o r m (9).
Video Inversion. The video inversion method of m e a s u r i n g s t o r m
intensity p r e s e n t s the s t o r m s t r u c t u r e a s a l t e r n a t e b r i g h t and d a r k b a n d s .
The light r a i n around the outside of the s t o r m a p p e a r s b r i g h t , while the
h e a v i e r r a i n a p p e a r s as a d a r k a r e a in the center of the s t o r m (9).
Automatic R e c e i v e r - G a i n Control. The b a s i s of the a u t o m a t i c gain
control c i r c u i t is a multiple-positioned "stepping s w i t c h " that r e d u c e s the
r e c e i v e r - g a i n by fixed s t e p s (10). An antenna switch is used to c o n t r o l the
stepping switch and can be used to t r i g g e r a scope c a m e r a if d e s i r e d .
A s e l e c t o r c i r c u i t enables the r a d a r o p e r a t o r to s e l e c t the n u m b e r of
s t e p s the c i r c u i t will automatically scan. The stepping c i r c u i t p r o v i d e s a
m a x i m u m of 10 s t e p s . With i n c r e a s e d r a n g e , fewer s t e p s will be r e q u i r e d
to reduce a given rainfall r a t e to the threshold of detection on the r a d a r
scope due to the range effect on s c a t t e r i n g of the e n e r g y .
This automatic s y s t e m is p a r t i c u l a r l y valuable for use with a scope
c a m e r a , whereby a continuous r e c o r d of the s t o r m i n t e n s i t y can be made
with s h o r t i n t e r v a l s of time between p h o t o g r a p h s . F i g u r e 3 shows a s e r i e s
of photographs of a squall line using j 7 gain s t e p s .
R e g a r d l e s s of the g a i n - r e d u c t i o n method used, t h e r e should be a
fixed initial setting for the r e c e i v e r - g a i n to aid in p r o p e r i n t e r p r e t a t i o n of
the s cope.
Plotting Devices
R e s e a r c h in r a d a r Aerology usually m a k e s use of photographs of the
s c o p e s which a r e e n l a r g e d for p r o c e s s i n g at some l a t e r d a t e .
Operational usage of r a d a r m a k e s it m a n d a t o r y to plot the scope data
in some fashion in o r d e r to keep a c u r r e n t r e c o r d and to check on the movem e n t and development of e c h o e s . B e c a u s e m o s t r a d a r s use s m a l l P P I and
RHI s c o p e s , some device for enlarging these p r e s e n t a t i o n s is d e s i r a b l e .
This is e s p e c i a l l y true for P P I p r e s e n t a t i o n s when it is d e s i r e d to t r a c k
echoes for d e t e r m i n i n g upper level winds. F o u r methods that might be
employed a r e :
F i r s t , freehand plottings of the radar echoes can be made in pencil
on some type of plotting c h a r t using r a n g e and azimuth as a means of orientation. A p l a s t i c o v e r l a y can be used which shows s u r r o u n d i n g towns and
FIG. 3 RAIN INTENSITY PATTERNS AS PRESENTED ON THE
SCOPE WITH THE AUTOMATIC SENSITIVITY CONTROL
PPI
- 10 other land m a r k s which aid in i n t e r p r e t a t i o n if the r a d a r station is on land.
The m a i n objections to this s y s t e m a r e the p o s s i b i l i t i e s for e r r o r in orientation, and the difficulty of showing the details which often indicate growth
or decay.
Secondly, an optical c o m p a r a t o r can be used which p e r m i t s the
o p e r a t o r to view the scope and plotting board independently by m e a n s of
l e n s e s , p r i s m s , and eye pieces which make the scope face and plotting
b o a r d appear synonymous (11). A plotting board adjusted for at l e a s t a
24-inch projection of the scope p r e s e n t a t i o n is d e s i r a b l e . This will allow
for tracing of details and make it easy to t r a c k echoes for d e t e r m i n i n g
upper wind data. With this s y s t e m the t r a c i n g s can be made in pencil and
filed for future r e f e r e n c e . This s y s t e m is somewhat simple in c o n s t r u c t i o n
and avoids the maintenance p r o b l e m s of an e l e c t r o n i c device.
A third method employs a polaroid oscilloscope c a m e r a , such as the
F a i r c h i l d Model F - 2 8 4 , which will produce a finished black and white scope
photograph in about one minute. These can be blown up to m o r e d e s i r a b l e
s i z e s by use of an opaque p r o j e c t o r . This method p e r m i t s quick c o m p a r i s o n s
of detailed shapes and positions of echoes from photo to photo.
The fourth and m o s t d e s i r a b l e type of operational p r o j e c t o r m a k e s use
of a d a r k t r a c e oscilloscope tube and has a complete control panel for its
operation. The Projection Plan Position Indicator, type CG-55AFN (VG-3
R e p e a t e r ) , used with Navy Radar Model S P - 1 M , is an example of a d e s i r a b l e
type of d a r k t r a c e p r o j e c t o r . This s y s t e m provides a l a r g e plotting surface
and improved a c c u r a c y with sufficient b r i l l i a n c e to p e r m i t detailed t r a c i n g s
of the weakest e c h o e s .
C a m e r a s for Training P u r p o s e s , R e s e a r c h , and "Hindcasting"
A l 6 - m m or 35-mm scope c a m e r a should be used at l e a s t p a r t of the
time on some shipboard r a d a r s in o r d e r to a c q u i r e good training films of
s t o r m s at sea. Also, these would provide excellent m a t e r i a l for " h i n d c a s t i n g "
to improve techniques and for r e s e a r c h .
The Navy type "A" scope c a m e r a is ideal for this purpose since it has
a built-in clock and data card which a r e r e c o r d e d on the film. (Figure 3).
The data card also h a s six s m a l l lights which m a y be used in v a r i o u s combinations for r e c o r d i n g gain step n u m b e r s . This c a m e r a can be t r i g g e r e d
automatically by a switch on the rotating antenna. A c a m e r a control box is
provided so that p i c t u r e s can be taken on e v e r y scan, every second, e v e r y
fourth, or e v e r y twelfth scan. F o r m o s t antenna s p e e d s , every second or
e v e r y fourth scan gives sufficient d e t a i l s .
- 11 PART II
UPPER WIND DATA FROM RADAR ECHOES
Introduction
F o r e c a s t e r s a r e s o m e t i m e s faced with the p r o b l e m of d e t e r m i n i n g
upper wind s t r u c t u r e s when soundings a r e not available. Also, land stations
and ships at s e a , equipped for. taking upper air soundings, often cannot make
balloon r e l e a s e s due to high winds or other a d v e r s e w e a t h e r conditions at
the station,, The use of r a d a r m a k e s limited upper wind data available under
such c i r c u m s t a n c e s .
Since the clouds producing showers and thunder s h o w e r s a r e embedded
in a t m o s p h e r i c l a y e r s of considerable thickness, both specific level winds
and layer m e a n winds a r e significant. L a y e r mean winds a r e p a r t i c u l a r l y
valuable for operation of jet a i r c r a f t when a flat parabolic path is flown,
and for firing of projectiles which t r a v e l a parabolic path, because the
parabolic paths tend to integrate wind conditions through a l a y e r .
Scope and Techniques Used
A study was made to d e t e r m i n e the r e l a t i o n s h i p s between localized
echo movements and the large scale wind s t r u c t u r e as p o r t r a y e d by streamline c h a r t s covering s e v e r a l m i d w e s t e r n s t a t e s . The c a s e s of echo movement were selected without r e g a r d to synoptic conditions favorable for
c o r r e l a t i o n p u r p o s e s in o r d e r to introduce the p r o b l e m s that might be
encountered when only P P I r a d a r information is available.
The study was limited to an investigation of squall lines and a i r m a s s
showers during s u m m e r and fall. About 25 per cent of the c a s e s studied
o c c u r r e d in the fall. Radar echo m o v e m e n t s , as portrayed on the P P I scope,
were c o r r e l a t e d with the upper a i r circulation as indicated by s t r e a m l i n e
c h a r t s constructed from 6-hourly Pibal and Rawin observations made by
Weather B u r e a u and Air F o r c e stations over the Middle West.
Sixty-nine c a s e s of r a d a r echoes w e r e investigated. The r a d a r echoes
were traced from 3 5 - m m photographs of the P P I scope onto a b a s e m a p using
a r e c o r d a k m i c r o f i l m viewer. The echoes w e r e classified into two groups
consisting of isolated echoes and e l e m e n t s of squall l i n e s . Isolated echoes a r e
those which a r e not associated with a definite squall line. These frequently
r e p r e s e n t a i r m a s s showers and t h u n d e r s t o r m s . E l e m e n t s of squall lines a r e
the individual echoes which a r e a p a r t of a line or a n a r r o w squall zone.
Figure 4 shows the v e c t o r s used to d e s c r i b e the motion of e l e m e n t s of
a squall line. "A" is the component of motion p a r a l l e l to the line. " B " is
the n o r m a l component which gives the forward motion of the squall line. " C "
FIG. 4 SQUALL LINE
OF AUGUST 9, 1952.
- 12 is the r e s u l t a n t vector which d e s c r i b e s the actual path of an e l e m e n t as
o b s e r v e d on r a d a r . Vector " C " was c o r r e l a t e d with the upper level winds.
The e c h o e s were identified at s u c c e s s i v e t i m e s by following the h e a v i e s t
c o r e s , as shown by the automatic gain-reduction device, or by p r o m i n e n t
f e a t u r e s of the echo, if gain reductions w e r e not m a d e .
Streamline c h a r t s , as illustrated in F i g u r e 5 w e r e p r e p a r e d for each
2000-ft level from 2000 ft to 20, 000 ft and for the 25, 000-ft and 30, 000-ft
l e v e l s . F o r convenience of s t a t i s t i c a l a n a l y s i s data for the 2000-ft levels
between 20, 000 ft and 30, 000 ft w e r e obtained by interpolating from the
c h a r t s for the 20, 000-ft, 25, 000-ft and 30, 000-ft l e v e l s .
The movements of both isolated echoes and e l e m e n t s of squall lines
w e r e c o r r e l a t e d with the mean wind d i r e c t i o n and speed for t h r e e l a y e r s :
2000-20, 000 ft, 2000-26, 000 ft, and 6000-30, 000 ft; and with the wind dir e c t i o n and speed for three individual l e v e l s ; 6000 ft, 10, 000 ft, and
2 0 , 0 0 0 ft.
C o r r e l a t i o n of Layer Mean Winds With Echo Movement
Thirty-four c a s e s of the movement of isolated echoes were analyzed.
R e s u l t s showed about equal c o r r e l a t i o n for the combination of speed and dir e c t i o n for the l a y e r s , 2000-20, 000 ft and 2000-26, 000 ft. The 6000-30, 000ft layer showed c o r r e l a t i o n with direction as good as the other l a y e r s but
somewhat p o o r e r c o r r e l a t i o n with speed (Table 1).
F i g u r e 6 shows scatter d i a g r a m s for the relation between the move m e n t s of isolated echoes and the mean wind for the three investigated l a y e r s .
The slope of the r e g r e s s i o n lines indicates that for the 2000-20, 000 ft l a y e r ,
the echoes moving faster than 2 0 knots a r e moving faster than the mean wind,,
F o r the 2000-26, 000-ft layer, the echoes moving faster than 30 knots a r e
moving faster than the mean wind. It m u s t be r e m e m b e r e d that this study
was concerned with the overall wind field obtained from s t r e a m l i n e c h a r t s ,
and that the wind field in the vicinity of the t h u n d e r s t o r m may be somewhat
different from the over all wind field. It is possible that local maxima of
wind speeds exist in the vicinity of fast moving e c h o e s .
Thirty-five c a s e s of the movement of e l e m e n t s of squall lines were
c o m p a r e d with the mean winds. Results show the c o r r e l a t i o n coefficients
for all three l a y e r s to be about the same (Table 1).
A set of s c a t t e r d i a g r a m s s i m i l a r to those in F i g u r e 6 was constructed
to show the relation between the layer mean winds and the movements of elem e n t s of squall l i n e s . The complete set of d i a g r a m s was p r e s e n t e d in the
technical r e p o r t covering the wind study (12). As in the case of isolated e c h o e s ,
the e l e m e n t s of squall lines move faster than the winds at the higher wind s p e e d s .
In this c a s e , propagation of the squall line m a y cause observed speeds to be too
fast; however, in this study attempts were made to follow an individual e l e m e n t
and to avoid following other echoes that m a y have developed nearby.
FIG. 5
2,000 FT. STREAMLINES
AUGUST 15, 1952
1500 C S T
FIG. 6 RADAR ECHOES VS. MEAN WIND FOR THREE LAYERS
ISOLATED ECHOES
- 13 Accuracy of determinations.
Standard e r r o r s of estimate were
c o m p u t e d for e a c h of the l a y e r s to o b t a i n an e s t i m a t i o n of t h e a c c u r a c y of
the wind d e t e r m i n a t i o n s .
V a l u e s o f two s t a n d a r d e r r o r s , o r the a c c u r a c y
to be e x p e c t e d 95 p e r c e n t of the t i m e , a r e g i v e n in T a b l e 1 a l o n g w i t h the
c o r r e l a t i o n coefficients.
T h e s e c o r r e l a t i o n c o e f f i c i e n t s and s t a n d a r d e r r o r s w e r e c o m p u t e d
f r o m a l l the d a t a and i n c l u d e s o m e c a s e s t h a t could h a v e b e e n e l i m i n a t e d
s i n c e the e c h o e s had e i t h e r h i g h - b a s e s o r low t o p s .
For example, a
t h u n d e r s t o r m b a s e d at 6 0 0 0 ft and e x t e n d i n g to 3 5 , 000 ft could not be exp e c t e d t o c o r r e l a t e w e l l with the 2 0 0 0 - 2 0 , 000-ft l a y e r .
TABLE 1
RELATION OF THE MOVEMENT CF ISOLATED ECHOES
AND E L E M E N T S O F S Q U A L L L I N E S WITH L A Y E R M E A N WINDS
Correlation
Coefficients
Dir.
Speed
L a y e r (ft)
Isolated Echoes
E l e m e n t s of
Squall lines
Two
Standard E r r o r s
Dir.
Speed
(
Knots)
(deg)
2 0 0 0 - 2 0 , 000
2 0 0 0 - 2 6 , 000
6 0 0 0 - 3 0 , 000
.87
.90
.88
.72
.67
.60
±40
±34
±34
±13
±16
±19
2 0 0 0 - 2 0 , 000
2 0 0 0 - 2 6 , 000
6 0 0 0 - 3 0 , 000
.79
.84
.82
. 72
.73
. 72
±32
±26
±26
±13
±13
±15
P r e s e n t i n g the d a t a i n t h i s m a n n e r s h o w s the m a x i m u m e r r o r t o b e
e x p e c t e d i f only P P I r a d a r d a t a a r e u s e d f o r u p p e r wind d e t e r m i n a t i o n s .
It
i s t o b e e x p e c t e d t h a t g r e a t e r a c c u r a c y w i l l b e a c h i e v e d when s u p p l e m e n t a r y
d a t a f r o m RHI r a d a r and s u r f a c e weather o b s e r v a t i o n s a r e a v a i l a b l e .
C o r r e l a t i o n o f Winds a t S p e c i f i c L e v e l s w i t h E c h o M o v e m e n t s
T h e c o r r e l a t i o n o f the m o v e m e n t o f i s o l a t e d e c h o e s w i t h the wind a t
s p e c i f i c l e v e l s w a s not a s good a s w i t h the l a y e r m e a n w i n d s .
This is not
s u r p r i s i n g s i n c e the c u m u l u s - t y p e cloud i s e m b e d d e d i n a s o m e w h a t t h i c k
a t m o s p h e r i c l a y e r and s h o u l d b e s t e e r e d b y the w i n d s i n t h i s e n t i r e l a y e r
r a t h e r t h a n b y the wind a t one s p e c i f i c l e v e l .
T h e w i n d s a t 10, 000 f t g a v e
the b e s t c o r r e l a t i o n c o n s i d e r i n g both s p e e d and d i r e c t i o n ( T a b l e 2 ) .
The
6 0 0 0 - f t a n d 2 0 , 000-ft w i n d s s h o w g o o d c o r r e l a t i o n s w i t h d i r e c t i o n but
p o o r e r c o r r e l a t i o n s with s p e e d .
- 14 F i g u r e 7 s h o w s the s c a t t e r d i a g r a m s for the r e l a t i o n b e t w e e n the
i s o l a t e d e c h o m o v e m e n t a n d wind v e l o c i t y a t the t h r e e i n v e s t i g a t e d l e v e l s .
T h e s e d i a g r a m s e x h i b i t the s a m e g e n e r a l d i s t r i b u t i o n a s the d i a g r a m s i n
F i g u r e 5 . A t s p e e d s e x c e e d i n g 2 0 k n o t s , the e c h o e s g e n e r a l l y m o v e
f a s t e r t h a n the wind at the 6 0 0 0 - f t and 10, 000 ft l e v e l s b u t s l o w e r t h a n the
v/ind at t h e 2 0, 000-ft l e v e l .
A s with i s o l a t e d e c h o e s , the c o r r e l a t i o n o f the m o v e m e n t o f e l e m e n t s o f s q u a l l l i n e s with the wind a t s p e c i f i c l e v e l s w a s not a s good a s
with the l a y e r m e a n w i n d s .
The c o r r e l a t i o n coefficients indicate that
t h e s e e c h o e s a r e b e t t e r s u i t e d t o d e t e r m i n i n g the 10, 000-ft w i n d s t h a n
the 6 0 0 0 - f t or 2 0 , 000-ft w i n d s . ( T a b l e 2)
T h e s c a t t e r d i a g r a m s for t h e s e r e l a t i o n s h i p s w i l l not b e r e p r o d u c e d h e r e s i n c e they a r e s i m i l a r t o t h o s e i n F i g u r e 7 , and a r e a v a i l a b l e
i n the t e c h n i c a l r e p o r t c o v e r i n g the wind s t u d y (10).
The d i a g r a m s i n d i c a t e the s a m e r e l a t i o n s h i p b e t w e e n e c h o s p e e d and wind s p e e d , a s i n
Figure 7.
The e c h o e s t r a v e l i n g a t m o r e t h a n a b o u t 2 0 k n o t s m o v e f a s t e r
than the wind s p e e d s in the o v e r a l l wind field. The p r o p a g a t i o n of the s q u a l l
l i n e , a l r e a d y m e n t i o n e d , m a y b e the e a u s e o f t h e s e f a s t e r s p e e d s .
Accuracy of determinations. Standard e r r o r s of estimate were also
c o m p u t e d for e a c h of the l e v e l s to o b t a i n an e s t i m a t i o n of the a c c u r a c y of
the wind d e t e r m i n a t i o n s .
T a b l e 2 g i v e s the v a l u e s o f two s t a n d a r d e r r o r s
of e s t i m a t e and the c o r r e l a t i o n c o e f f i c i e n t s .
T ABLE 2
R E L A T I O N O F THE M O V E M E N T O F I S O L A T E D E C H O E S AND
E L E M E N T S O F S Q U A L L L I N E S WITH WINDS A T S P E C I F I C L E V E L S
L e v e l s (ft)
Isolated Echoes
E l e m e n t s of
Squall Lines
Correlation
Coefficients
D i r . Speed
Two
Standard E r r o r s
Dir.
Speed
(Knots)
(deg)
6 000
10, 000
2 0 , 000
.84
. 75
. 82
.44
.62
. 54
±53
±54
±44
±15
±16
±24
6000
10,000
2 0 , 000
.56
. 80
.63
.44
.68
.64
±75
±25
±45
±2 0
±14
±2 0
An e x a m i n a t i o n of the s t a n d a r d e r r o r s shows s o m e quite l a r g e v a l u e s .
T h e s e w e r e a l s o c o m p u t e d w i t h o u t a q u a l i t a t i v e i n s p e c t i o n o f the c a s e s t o
FIG. 7 RADAR ECHOES VS. WIND FOR THREE LEVELS
ISOLATED ECHOES
- 15 d e t e r m i n e the ones that could be eliminated. There w e r e o n l y a few c a s e s
w h e r e the echo movement and winds did not c o r r e l a t e well with any of the
t h r e e l a y e r s or l e v e l s , indicating further that if attention had been given to
the height of the b a s e s and tops, the c o r r e l a t i o n could have been improved.
The high standard e r r o r and low c o r r e l a t i o n coefficient for the
6000-ft level for e l e m e n t s of squall lines indicates that frequently the squall
l i n e s used in the study were above the frontal s u r f a c e , while the 6000-ft
wind was beneath the frontal surface and had no effect on the m o v e m e n t s of
the e l e m e n t s of the squall l i n e s .
Comparison of Results
Some c o m p a r i s o n s of r e s u l t s were made with the findings of H. B.
B r o o k s in 1945, the T h u n d e r s t o r m P r o j e c t in 1946-47, and r e c e n t work by
Ligda, (13, 8, 14).
Brooks used 46 s t o r m s and found that "large s t o r m s " move m o s t
frequently with the 11, 000-ft winds, while " s m a l l s t o r m s " move m o s t frequently with the 5000-ft winds. Ligda used data for one 12-month period,
which probably included a considerable amount of stable a i r m a s s precipitation types. His r e s u l t s indicate that convective cells in the New England
a r e a moved e s s e n t i a l l y with the velocity of the 700-mb geostrophic wind.
In this study, the winds at the 6000-ft and 10, 000-ft levels w e r e c o r r e l a t e d
with echo movements to give some f o r m of c o m p a r i s o n with the findings of
B r o o k s and Ligda. The r e s u l t s obtained for the 10, 000-ft level a r e in fair
a g r e e m e n t with L i g d a ' s work. Brooks defined s m a l l s t o r m s as echoes having
d i a m e t e r s l e s s than 7. 5 m i l e s . If isolated a i r m a s s s h o w e r s can be conside r e d " s m a l l " s t o r m s , since their d i a m e t e r s w e r e usually l e s s than 7. 5
m i l e s , then these r e s u l t s a r e not in a g r e e m e n t with the findings of B r o o k s ,
b e c a u s e the echoes were found to move considerably faster than the 6000-ft
winds.
The T h u n d e r s t o r m Project, using wind observations from their own
network in the vicinity of the e c h o e s , found that clouds move slower than the
m e a n wind for the layer from the gradient level to 20, 000 feet. Also, this
difference was found to i n c r e a s e with i n c r e a s i n g wind s p e e d s . The two studies
a r e in a g r e e m e n t for slow moving echoes but in this study the fast moving
e c h o e s usually had speeds g r e a t e r than the m e a n wind speed for the l a y e r ,
2000-20, 000 ft. It m u s t be r e m e m b e r e d that this study was concerned with
the l a r g e scale wind s t r u c t u r e , as it is related to the movement of r a d a r
e c h o e s . It is possible that local maxima of wind speeds e x i s t in the vicinity
of fast moving e c h o e s . These local conditions would have been m e a s u r e d by
the T h u n d e r s t o r m P r o j e c t s ' m i c r o - n e t w o r k of upper a i r soundings, with 10
upper wind stations in about 350 s q u a r e m i l e s . Also, some of the disagreem e n t s a r e probably due to differences in the heights of b a s e s and the v e r t i c a l
extent of the echoes studied by each investigator, which would have a b e a r i n g
on their movement.
- 16 N o m o g r a m s F o r Determining Winds
All of the n o m o g r a m s shown a r e based on s u m m e r - t i m e observations. F i g u r e 8 shows n o m o g r a m s for determining winds from isolated
e c h o e s . The bracketed v a l u e s a r e two standard e r r o r s of e s t i m a t e , or
95 per cent confidence l i m i t s . The r e g r e s s i o n lines w e r e taken from the
original graphs of echo m o v e m e n t s v e r s u s winds, as shown in F i g u r e s 6
and 7. It is i n t e r e s t i n g to note in the upper right hand n o m o g r a m how
much the speed of isolated echoes usually exceeded the 6000-ft wind speed.
This was mentioned e a r l i e r in comparing r e s u l t s of this study with those of
Brooks in which he found that s m a l l s t o r m s moved m o s t frequently with the
5, 000-ft winds. F i g u r e 9 shows a set of n o m o g r a m s for d e t e r m i n i n g winds
from e l e m e n t s of squall l i n e s .
Some of the n o m o g r a m s can be expected to give b e t t e r r e s u l t s than
o t h e r s , as indicated by the confidence l i m i t s . F o r p u r p o s e s of c o m p a r i s o n ,
n o m o g r a m s w e r e p r e p a r e d for all l a y e r s and levels studied. However, the
n o m o g r a m s with the b r o a d e s t 95 per cent confidence bands should be used
with caution.
The n o m o g r a m s w e r e constructed using all of the c a s e s studied in
o r d e r to a r r i v e at c o n s e r v a t i v e e s t i m a t e s of the a c c u r a c y of wind d e t e r m i nations to be made from them. Likewise, there was no initial selection of
c a s e s for study. The following r u l e s for i n c r e a s i n g the a c c u r a c y of wind
d e t e r m i n a t i o n s will r e s u l t in m o r e a c c u r a t e wind d e t e r m i n a t i o n s than indicated by the confidence l i m i t s on the g r a p h s . The g r a p h s p r e s e n t the overall a c c u r a c y to be expected when all echoes a r e used r e g a r d l e s s of their
adaptability to wind d e t e r m i n a t i o n s .
Rules for Increased A c c u r a c y of Wind Determinations
An inspection of the c a s e s where poor c o r r e l a t i o n s existed r e s u l t e d
in the e s t a b l i s h m e n t of the following set of r u l e s for i n c r e a s i n g the a c c u r a c y
of the wind d e t e r m i n a t i o n s :
1.
U s e RHI data to d e t e r m i n e the base and top of the echo
w h e r e p o s s i b l e , and use such observations to choose
the p r o p e r l a y e r for c o r r e l a t i o n .
2.
If RHI data a r e not available, plot the h e a v i e s t c o r e s
of echoes on the P P I scope. These a r e m o r e likely
to have sufficient v e r t i c a l extent to reflect upper
level wind conditions.
3.
A r e a s of m o r e intense precipitation within a r e a s of light
r a i n should be avoided. These m o r e intense a r e a s may
be a r e s u l t of n a t u r a l seeding (14) (15) from higher clouds,
SPECIFIC LEVELS
FIG. 9 NOMOGRAMS FOR DETERMINING WINDS, FROM ELEMENTS OF
SQUALL LINES
- 17 and the precipitation a r e a s will move with a velocitys i m i l a r to the clouds at the seeding level.
4.
If t h u n d e r s t o r m b a s e s a r e likely to be high, e. g. in
the case of w a r m a i r o v e r - r u n n i n g a cold d o m e , a
h i g h e r - b a s e d l a y e r , such as from 6000-30, 000 ft,
should c o r r e l a t e b e s t .
5.
Use echoes within about 50 m i l e s r a n g e when choosing
an e l e m e n t of a squall line. At longer r a n g e s , it is
often quite difficult to s e p a r a t e the e c h o e s and keep
t r a c k of a p a r t i c u l a r one.
It m u s t be emphasized that g r e a t care m u s t be taken in making s u c c e s sive plots of the s a m e echo to avoid identification e r r o r s . The o p e r a t o r should
pick an echo that is isolated or is e a s i l y distinguishable f r o m its neighbors
e i t h e r by its shape or s i z e . The tracking of a p a r t i c u l a r echo r e q u i r e s a l m o s t
constant vigilance of the scope b e c a u s e of rapid fluctuations in the a p p e a r a n c e s
of e c h o e s .
PRECIPITATION INTENSITY MEASUREMENTS
To make full use of r a d a r for m e a s u r e m e n t s of s t o r m i n t e n s i t i e s for
predicting the a s s o c i a ted surface wind gusts and v i s i b i l i t i e s , it is n e c e s s a r y
to know what rainfall r a t e s or water d r o p concentrations produce a given
intensity of echo. An a r e a l method and a r a d i a l method have been employed
for comparing rainfall r a t e s with r a d a r echo i n t e n s i t i e s .
A r e a l Method
A quantitative study of the a r e a l r e l a t i o n s h i p s between one minute r a d a r
isoecho m a p s and one minute isohyetal maps d r a w n from raingage r e c o r d s from
a m i c r o n e t w o r k was made in o r d e r to compare t h e o r e t i c a l with observed values
and to e s t a b l i s h operational techniques.
The one-minute rainfall a m o u n t s used in drawing isohyetal m a p s were
obtained from fifty B e n d i x - F r i e z dual t r a v e r s e , r a i n gages located on a 96s q u a r e - m i l e network c e n t e r e d 18 m i l e s w e s t - n o r t h w e s t of the r a d a r station.
The isoecho m a p s have contours enclosing a r e a s of p r e c i p i t a t i o n echoes
equal to or g r e a t e r than a c e r t a i n intensity as d e t e r m i n e d by an automatic
r e c e i v e r g a i n - r e d u c t i o n device (Refer to the d i s c u s s i o n of the Automatic R e c e i v e r Gain Control). The isohyetal m a p s have contours enclosing a r e a s with amounts
of p r e c i p i t a t i o n equal to or g r e a t e r than the chosen contour v a l u e . Graphs were
plotted of a r e a s v e r s u s intensities f r o m both the isohyetal and i s o e c h o m a p s .
Rainfall equivalents of the r a d a r echo intensities w e r e then obtained by c o m p a r i n g
like a r e a s on these g r a p h s .
- 18 Radial Method (Point)
E a r l i e r work (16) made use of a r a d i a l method of analysis which gave
r e s u l t s comparable to the a r e a l method and afforded additional data for comp a r i s o n with the theoretical v a l u e s . In the r a d i a l method each gain s t e p on
the r a d a r isoecho map was matched with a c e r t a i n isohyet on the rainfall m a p .
To compensate for the fall time and drift of r a i n d r o p s from the
average height of 2000-3000 feet observed by the r a d a r , s u c c e s s i v e o n e minute m a p s of r a d a r contours and surface rainfall were used. The o n e minute rainfall r a t e s were determined from the slopes of the curves on the
raingage c h a r t s .
The direction and amount of drift w e r e approximated from the movem e n t of the r a d a r cores for s e v e r a l consecutive m i n u t e s . The r a d a r and
rainfall profiles were matched by comparing a chosen one-minute r a d a r
profile along an a p p r o p r i a t e r a d i a l from the r a d a r station with consecutive
one minute surface rainfall profiles along the s a m e r a d i a l , after applying a
drift c o r r e c t i o n to the surface rainfall pattern,,.
Only those c a s e s where the indicated time lag was l e s s than five
minutes with drift not g r e a t e r than 1. 5 m i l e s w e r e used in this a n a l y s i s .
Cases involving intervening r a i n and core c e n t e r s moving off the network
w e r e excluded. Where the o p e r a t o r ' s log showed the possibility of instrum e n t a l e r r o r or other observational i n c o n s i s t e n c i e s , the data were d i s c a r d e d .
Examples of v e r y light shower activity w e r e not c o n s i d e r e d .
P r e v i o u s studies by other i n v e s t i g a t o r s have indicated that the received power from r a i n s t o r m s should closely follow the Rayliegh s c a t t e r i n g
effect. According to Spilhaus' information on drop size (17), the r e c e i v e d
power should be exponentially related to rainfall intensity. A proportionally
g r e a t e r received power has been noted with the APS-15 from higher rainfall
r a t e s than from the lower r a t e s . These proportionally g r e a t e r r e c e i v e d power values at higher rainfall r a t e s may be a r e s u l t of changes of the m e a n
d r o p size and shape. The Illinois State Water Survey is c u r r e n t l y collecting
data on r a i n d r o p sizes and d r o p shapes at different rainfall r a t e s . * The results of this work will be published at a l a t e r date.
N o m o g r a m s for Rainfall E s t i m a t e s
Figure 10 p r e s e n t s a method of d e t e r m i n i n g the intensity of precipitation and the corresponding visibility reduction from r a d a r echoes. This
graph is based on t h e o r e t i c a l calculations r a t h e r than quantitative data from
independent m e a s u r e m e n t s by r a d a r and r a i n g a g e s . The r a d a r equation
(Eq. 2) used in constructing the graph is that of Wexler (4). The equation
was applied with the r a d a r p a r a m e t e r s for the Illinois State Water S u r v e y ' s
modified A P S - 1 5 r a d a r . (Refer to the b a s i c r a d a r equation and APS-15 param e t e r s under "Radar C h a r a c t e r i s t i c s " ) This gives:
*Under s p o n s o r s h i p of Evans Signal L a b o r a t o r y , Contract No. DA-36-039
SC 42446
FIG. 10
THEORETICAL
RAIN
INTENSITY AND ASSOCIATED VISIBILITY REDUCTION — APS-15 RADAR
- 19 -
where
Pr
Pt
I
R
is
is
is
is
power received in watts
power t r a n s m i t t e d in watts
intensity of rainfall in i n . / h r .
range in nautical m i l e s .
The constant, 10. 94 r e p r e s e n t s the r a d a r set used and c o m p e n s a t e s
for horizontal and v e r t i c a l b e a m widths, antenna a p e r t u r e , pulse length,
wave length, and the refractive index of w a t e r . The constant 1. 53 r e l a t e s
rainfall intensity to drop size distribution, and according to W'exler (4)
c o r r e c t s to some extent the deviation from the Rayleigh s c a t t e r i n g for 3 - c m
wavelength r a d a r when l a r g e r a i n d r o p s a r e p r e s e n t .
The abscissa on the graph in F i g u r e 10 can be plotted in two f o r m s .
The P r / P t r a t i o works to b e s t advantage for r a d a r s e t s with varying transmitted power (Pt). F o r these s e t s , P r and P t m e a s u r e m e n t s should be made
as often as possible and at c e r t a i n selected values of r e c e i v e r gain. E c h o e s
on the P P I scope can be reduced to their threshold of visibility by reducing
the r e c e i v e r gain manually or by use of an automatic gain reduction device.
These reduced gain settings can be converted to P r / P t r a t i o s , and used in
conjunction with the echo range to d e t e r m i n e the rainfall r a t e in inches per
hour from F i g u r e 10. The per cent reduction in visibility is then d e t e r m i n e d
from the rainfall r a t e . This will be d i s c u s s e d in the following section on
visibility. The rainfall r a t e s in inches per hour in F i g u r e 10 have been
classified as light, m o d e r a t e , and heavy s h o w e r s according to Weather
B u r e a u Circular N s t a n d a r d s .
F o r a r a d a r set with a constant t r a n s m i t t e d power, the a b c i s s a of
F i g u r e 10 can be plotted d i r e c t l y in t e r m s of r e c e i v e r gain, and l e s s frequent
checks of received and t r a n s m i t t e d power will be n e c e s s a r y . With e i t h e r set
of a b s c i s s a values, the shower intensity may be d e t e r m i n e d d i r e c t l y from the
graph.
This p a r t i c u l a r graph is applicable only to the Illinois State Water
S u r v e y ' s modified APS-15 r a d a r s e t and is presented as an e x a m p l e . Each
r a d a r would r e q u i r e its own graph because received and t r a n s m i t t e d power
values a r e different for each set and the r a d a r p a r a m e t e r s (beam width,
pulse length, e t c . ) a r e different for each m o d e l .
Also, d r o p size and shape distributions probably affect the r e c e i v e d
power ( P r ) of 3-cm wavelength r a d a r to a g r e a t e r extent than the chosen
constant 1. 53 in Equation 2 takes into account.
The r e s u l t is that on
n u m e r o u s occasions the graph in F i g u r e 10 did not check by the " A r e a l
Method" or the "Radial Method" of c o r r e l a t i o n between r a d a r and raingage
measurements.
- 20 A graph such as F i g u r e 10, set up with Equations 1 and 2 and using
the p a r a m e t e r s for a 10-cm r a d a r set, should give b e t t e r rainfall intensitye s t i m a t e s than the 3-cm r a d a r - r a i n f a l l graph because the r a t i o of maxim u m r a i n d r o p d i a m e t e r to r a d a r wavelength is much s m a l l e r for 10-cm
r a d a r than for 3-cm r a d a r . Consequently, the Rayleigh s c a t t e r i n g law
holds much better for the 10-cm wavelength. Also attenuation of the signal
by the r a i n d r o p s , which is appreciable at 3 - c m , is negligible for 10-cm
radar.
Additional r e s e a r c h on attenuation and on r a i n d r o p size and shape
d i s t r i b u t i o n s as they affect s c a t t e r i n g at 3 - c m wavelengths is needed in
o r d e r to develop m o r e a c c u r a t e equations for d e t e r m i n i n g rainfall r a t e s
from 3 - c m r a d a r echo i n t e n s i t i e s .
VISIBILITY DURING RAINSHOWERS
Introduction
Knowledge of visibility reduction during rainfall is p e r t i n e n t to
the planning and a c c o m p l i s h m e n t of c e r t a i n fleet o p e r a t i o n s . A m e a s u r e
of the intensity of rainfall can be obtained f r o m the r a d a r echo intensity
as observed on the P P I s c o p e . Since visibility reduction during rainfall
is related to the intensity of the rainfall, it should be p o s s i b l e to e s t a b l i s h
quantitative e m p i r i c a l r e l a t i o n s between r a i n intensity and visibility reductions. With the e s t a b l i s h m e n t of such r e l a t i o n s , r a d a r could be utilized
for determining visibility reductions during rainfall when weather station
observations a r e not a v a i l a b l e . Also the visibility reduction values can be
used for forecasting v i s i b i l i t i e s once the expected shower intensities have
been f o r e c a s t by any method other than r a d a r .
This section s u m m a r i z e s the r e s u l t s of a limited investigation to
establish quantitative r e l a t i o n s between the intensity of shower-type precipitation and visibility reduction. The r e s u l t s a r e based upon data from
n i n e Illinois weather stations for the s u m m e r s of 1951 and 1952. Consequently, the r e s u l t s a r e applicable only to r e g i o n s with s i m i l a r a t m o s p h e r i c
conditions. It is obviously d e s i r a b l e that the investigation be extended to
other regions and other types of precipitation.
Approach to P r o b l e m
Data Used. Microfilm copies of h o u r l y weather observation r e c o r d s
for nine Illinois stations for the s u m m e r s of 1951 and 1952 w e r e the s o u r c e
of analytical data. These stations included Bradford, Joliet, Moline, P e o r i a ,
Quincy, Rockford, Springfield, Vandalia, and Chanute Field. Because night
visibility o b s e r v a t i o n s a r e m o r e subject t o e r r o r s than daylight o b s e r v a t i o n s ,
the study was limited to the h o u r s between 0500 and 1900. Because the
object of the investigation was to d e t e r m i n e the reduction in visibility due to
- 21 rainfall, c a s e s were selected in which the visibility preceding rainfall was
not r e s t r i c t e d by fog, s m o k e , or h a z e . The study was further limited to
the unstable types of r a i n s h o w e r s and t h u n d e r s h o w e r s , which p r e d o m i n a t e
during s u m m e r . The t w o - s u m m e r period provided a total of about 531
s t a t i o n - d a y s of precipitation which met the preceding r e q u i r e m e n t s for
inclusion in the a n a l y s i s .
T r e a t m e n t of data. The rainfall data w e r e divided into the four
c l a s s e s of i n t e n s i t i e s as defined by the U . S . Weather B u r e a u ; v e r y
light, light, m o d e r a t e , and heavy i n t e n s i t i e s . By definition, no accumulation o c c u r s with v e r y light r a i n , while light, m o d e r a t e and heavy
i n t e n s i t i e s c o r r e s p o n d to r a t e s of a t r a c e to 0. 10 i n . / h r , 0. 11 to 0. 30
i n . / h r . and over 0. 30 i n . / h r , r e s p e c t i v e l y .
In p r o c e s s i n g the data, visibility o b s e r v a t i o n s preceding and during
each shower w e r e tabulated. The p e r c e n t a g e reduction in visibility due to
rainfall was then calculated by dividing the difference between the visibility
p r i o r to and during rainfall by the visibility preceding precipitation, and
multiplying by 100.
The data were f i r s t analysed using observations when no fog, h a z e ,
or smoke (visibility g r e a t e r than six m i l e s ) was r e p o r t e d before the onset
of precipitation, and none was r e p o r t e d during the o c c u r r e n c e of rainfall.
These r e s u l t s , t h e r e f o r e , should r e p r e s e n t the reduction in visibility due
to rainfall. L a t e r , a study was made of those c a s e s where fog, h a z e , or
smoke o c c u r r e d during, but not preceding, r a i n f a l l . Such o c c u r r e n c e s a r e
r e p r e s e n t a t i v e of conditions under which e x t r e m e reductions in visibility
m a y occur during precipitation. Unfortunately, insufficient data w e r e available to complete the second study. It is complicated by the fact that the r a i n
actually removed p a r t i c l e s from the a t m o s p h e r e previously r e s t r i c t i n g the
visibility.
Results of Data Analysis
Mean Reduction in Visibility. Using 1951-1952 data for the nine
Illinois s t a t i o n s , an a v e r a g e p e r c e n t a g e reduction in visibility for each
type and intensity of r a i n s h o w e r was computed for those c a s e s in which no
fog, h a z e , or smoke o c c u r r e d p r i o r to or during rainfall. The r e s u l t s have
been e x p r e s s e d graphically in F i g u r e 11.
Two r e s u l t s of p a r t i c u l a r i n t e r e s t a r e : f i r s t , the visibility reduction
with t h u n d e r s t o r m s is always g r e a t e r than the corresponding r a i n s h o w e r intensity. Two possible explanations for this a r e suggested. The o b s e r v e r m a y
consistently r e c o r d the shower intensity a c c o r d i n g to the frequency of thunder
r a t h e r than the rainfall r a t e ; for e x a m p l e , t h u n d e r s t o r m s having m o d e r a t e
rainfall r a t e s may be r e c o r d e d as light b e c a u s e of the infrequency of thunder.
The other possibility is a variation in d r o p s i z e , with s m a l l e r drops o c c u r r i n g
with thunder s t o r m s than with r a i n s h o w e r s of the same intensity. S m a l l d r o p s
would reduce visibility m o r e than l a r g e d r o p s with equal amounts of liquid
w a t e r content p r e s e n t in both c a s e s .
FIG 11
VARIOUS
PERCENTAGE REDUCTION IN VISIBILITY FOR
INTENSITIES OF SHOWERS.
- 22 -
Second r e s u l t is the s m a l l i n c r e a s e in visibility reduction between
m o d e r a t e r a i n s h o w e r s and thundershowers and heavy ones. Drop size
variation m a y also be an explanation for this; the visibility reduction due
to the i n c r e a s e in liquid water content per unit volume m a y be p a r t i a l l y
offset by an i n c r e a s e in the r a t i o of large to s m a l l d r o p s .
The above r e s u l t s a r e applicable only to a r e a s of s i m i l a r atmos p h e r i c conditions where the visibility r a n g e s a r e about the s a m e . The
southwestern portion of the United States, where the turbidity is much
l e s s , would probably have g r e a t e r visibility than Illinois during a shower
of a given rainfall intensity.
Maximum Reduction in Visibility. As mentioned previously, the
study of the m a x i m u m reductions in visibility to be e x p e c t e d when fog,
h a z e , or smoke f o r m during the precipitation period did not prove as
fruitful as expected. The number of observations was very s m a l l , and
the v a r i a t i o n s between observations were so large that it was decided
not to include this phase of the study in this r e p o r t . As expected, the
r e s u l t s indicated an increase in visibility reduction for the lighter rainfall
r a t e s . During the two-year period studied, no fog, h a z e , or smoke formed
during p e r i o d s of heavy r a i n s h o w e r s or t h u n d e r s h o w e r s .
Determination of Visibility Reduction by Radar
It is unlikely that v e r y light showers will be detected by m o s t r a d a r
s e t s except at s h o r t r a n g e s . However, as shown by F i g u r e 11, the a v e r a g e
reduction in visibility during such s t o r m s is v e r y s m a l l and, t h e r e f o r e , of
little significance in the accomplishment of m o s t fleet o p e r a t i o n s . F o r the
preceding r e a s o n s , v e r y light showers w e r e not considered in developing
techniques for a s c e r t a i n i n g visibility reduction by r a d a r .
The intensity of a r a d a r echo (rainfall intensity) can be determined
by use of automatic or manual gain reduction, as d e s c r i b e d under "Receiver
Gain C o n t r o l s " . To make the r e s u l t s of F i g u r e 11 applicable to r a d a r
a n a l y s i s , the a v e r a g e percentage reductions in visibility for light, m o d e r a t e ,
and heavy intensities were e x p r e s s e d in t e r m s of rainfall intensity, using
the U . S . Weather Bureau C i r c u l a r N c r i t e r i a for intensity. The r e s u l t s
a r e shown in Table 3
Since the rainfall intensity can be d e t e r m i n e d from the r a d a r echo
intensity, the average percentage reduction in visibility during a shower
due to r a i n d r o p s can be estimated by entering Table 1 with the approximate
intensity value. Since Weather Bureau r e c o r d s w e r e used to develop the
r e l a t i o n between rainshower intensity and visibility reduction (Figure 11),
Table 1 is based upon the assumption that the classification of light, m o d e r a t e ,
and heavy s h o w e r s by Weather Bureau o b s e r v e r s conforms with Circular N
c r i t e r i a for rainfall intensity. While the v e r a c i t y of this assumption may be
questionable, it r e p r e s e n t s the b e s t e s t i m a t e available at the p r e s e n t time.
- 23 T ABLE
3
RELATION BETWEEN PERCENTAGE REDUCTION
I N VISIBILITY AND R A I N F A L L I N T E N S I T Y F O R
GIVEN T Y P E S A N D I N T E N S I T I E S O F S H O W E R S
Shower Type
a n d Inte n s i t y
RWTRWRW
TRW
RW+
TRW+
Average Percenta g e R e d u c tion in
Vi s i b i l ity
25
38
66
75
78
87
Circular N
C r i t e r i a for
Intensity
in. / h r .
T r a c e to 0. 10
"""
0. 11 to 0. 30
"""
O v e r 0. 30
""
These percentage reductions in visibility have been entered in
F i g u r e 1 1 , a n d the r a i n f a l l r a t e s c o r r e s p o n d i n g t o l i g h t , m o d e r a t e , a n d
h e a v y s h o w e r s h a v e b e e n s h a d e d in„ T h i s p e r m i t s a d i r e c t e s t i m a t i o n o f
the v i s i b i l i t y r e d u c t i o n f r o m the e c h o i n t e n s i t y ,
A t l a s ' E q u a t i o n for D e t e r m i n i n g V i s i b i l i t y F r o m R a i n f a l l R a t e
T h e following u n p u b l i s h e d e q u a t i o n b y A t l a s (18) for d e t e r m i n i n g
visibility during rainfall has received attention:
(Eq. 3 )
V=12. 5 R - 6 7 , w h e r e V i s v i s i b i l i t y i n K m
and R i s r a i n f a l l r a t e i n m m / h r .
R e c e n t l y A t l a s h a s i n d i c a t e d c h a n g e s i n the v a l u e s o f the c o n s t a n t s
and exponent.
B e c a u s e only a few d i r e c t o b s e r v a t i o n s b e t w e e n r a d a r e c h o
i n t e n s i t y and v i s i b i l i t y w e r e a v a i l a b l e i n t h i s s t u d y , v e r i f i c a t i o n o f the
A t l a s e q u a t i o n w a s not p o s s i b l e .
S Q U A L L Y WIND D E T E R M I N A T I O N S
O n l y a s m a l l a m o u n t o f q u a n t i t a t i v e d a t a h a s b e e n a v a i l a b l e for the
s t u d y of s q u a l l y s u r f a c e w i n d s . A s t u d y w a s m a d e of s q u a l l y w i n d s in
connection with t h u n d e r s t o r m s o c c u r r i n g a t Weather B u r e a u f i r s t o r d e r
stations in Illinois.
The r e s u l t s i n d i c a t e , a s m i g h t b e e x p e c t e d , t h a t the
a v e r a g e and p e a k g u s t s t o b e e x p e c t e d f r o m h e a v y s h o w e r s and t h u n d e r s t o r m s a r e c o n s i d e r a b l y h i g h e r than those f r o m light o r m o d e r a t e i n t e n s i t i e s
o f s h o w e r s and t h u n d e r s h o w e r s .
- 24 -
Three aerovane r e c o r d i n g wind s y s t e m s have been operated for a
s h o r t time on the Water S u r v e y ' s Goose Creek raingage network. More
r e c o r d s from these would be r e q u i r e d in order to produce conclusive
quantitative r e s u l t s . However, available information from the Thunders t o r m P r o j e c t (8) and data collected in this study does make it possible
to draw helpful qualitative conclusions about squally winds a s s o c i a t e d
with r a d a r echoes.
M e c h a n i s m s Producing Squally Winds (8)
Squally surface winds associated with the p a s s a g e of isolated a i r
m a s s t h u n d e r s t o r m s , frontal t h u n d e r s t o r m s , o r s q u a l l - l i n e t h u n d e r s t o r m s
a r e produced p r i m a r i l y by the cold downdrafts of the t h u n d e r s t o r m cells
that have reached m a t u r i t y .
There a r e c h a r a c t e r i s t i c p a t t e r n s in the wind v e l o c i t y - d i v e r g e n c e
field associated with the v a r i o u s stages of the t h u n d e r s t o r m life c y c l e .
During the e a r l y cumulus stage, even 20 to 30 minutes before the r a d a r
echo a p p e a r s , the surface winds turn toward the a r e a s of convection,
where relatively weak convergence develops. The inflow, which is usually
light, frequently extends over a r a d i u s of six to eight m i l e s . When there
a r e n u m e r o u s cells in the vicinity, the outflow of one m a y so completely
dominate the surface wind flow that it is impossible to detect the inflow
field of another.
The cold downdraft is by far the m o s t significant feature pertaining
to squally surface winds. When the cold downdraft of a t h u n d e r s t o r m cell
r e a c h e s the surface t h e r e is an immediate r e v e r s a l from the inflow which
was p r e s e n t during the cumulus stage of the cloud development. The outward flowing cold air u n d e r r u n s the w a r m e r air and a discontinuity in the
wind field is e s t a b l i s h e d .
In relatively slow-moving or s t a t i o n a r y s t o r m s , the outflow is
a l m o s t r a d i a l , and as it continues, an a r e a of light winds develops immediately beneath the center of the downdraft a r e a . However, in m o s t c a s e s ,
the outflow field is a s y m m e t r i c a l , with higher wind s p e e d s on the downs t r e a m side. This is due to the reinforcement, or cancellation as the case
m a y be, of the r a d i a l outflow by the prevailing a i r m o v e m e n t .
Often in the c a s e of moving s t o r m s a l m o s t all of the cold downdraft
a i r s p r e a d s out in the direction of cell movement, giving a strong wind
discontinuity at the leading edge. A downward t r a n s p o r t of r e l a t i v e l y high
horizontal momentum from upper levels r e i n f o r c e s the surface outflow
wind speeds in the d i r e c t i o n of the cloud movement and r e t a r d s those in
the opposite d i r e c t i o n .
After the outflow h a s been spreading for 15 to 2 0 m i n u t e s , the discontinuity zone will have t r a v e l e d about five or six m i l e s from the cell c e n t e r .
- 25 -
The surface winds n e a r the discontinuity surface r e m a i n s t r o n g and gusty,
but f a r t h e r behind, inside the c o l d - a i r d o m e , the s u r f a c e wind speeds dec r e a s e so that the s t r o n g e s t winds a r e no longer underneath the cell itself.
This i n c r e a s e in wind speed as one a p p r o a c h e s the discontinuity zone f r o m
the c o l d - a i r side r e s u l t s from a continued settling of the outflow air„. The
s h a r p i n c r e a s e in wind speed at the leading edge of the discontinuity h a s
been t e r m e d "the f i r s t gust, " since it often a p p e a r s as a f i r s t m a j o r gust
of a period of high, gusty winds. After the cold a i r s p r e a d s outward, the
wind speed near the boundary of this a i r d e c r e a s e s . The r a t e of d e c r e a s e
depends mainly upon the rate of spreading of the c o l d - a i r dome and thus
the r a t e of d i s p e r s i o n of the downdraft e n e r g y .
Radar E c h o e s and Squally Winds
The T h u n d e r s t o r m P r o j e c t (8) found that the c e n t e r s of downdrafts
coincided with the c o r e s of heaviest precipitation which a r e shown by P P I
r a d a r . This p e r m i t s the d e t e r m i n a t i o n of s o u r c e s of strong surface wind
gusts by the use of r a d a r .
RHI r a d a r can be used to d e t e r m i n e the height of the base and tops
of echoes and their v e r t i c a l growth r a t e s . The fastest growing and t a l l e s t
echoes a r e likely to develop the s t r o n g e s t downdrafts and a s s o c i a t e d s u r f a c e
gusts.
Studies made by Jones (7) and A m e r i c a n A i r l i n e s (9), revealed that
the s e v e r e s t turbulence experienced by a i r c r a f t in flight was a s s o c i a t e d with
steep g r a d i e n t s of r a d a r echo c o n t o u r s . These s t e e p g r a d i e n t s can be
d e t e r m i n e d by the use of manual or a u t o m a t i c r e c e i v e r - g a i n reduction d e v i c e s .
Rules for E s t i m a t i n g Squally Winds from R a d a r E c h o e s
1. If a moving squall line or line of frontal t h u n d e r s t o r m s is maintaining or i n c r e a s i n g its intensity, then at any given time there will be some
m a t u r e t h u n d e r s t o r m cells with well developed d o w n d r a f t s . These downdrafts will afford a supply of energy for the surface wind g u s t s . Under such
c i r c u m s t a n c e s the e c h o e s can usually be prognosticated to maintain their
associated gust intensities as they r e a c h stations in their future path.
2. L a r g e a n d / o r v e r y intense, fast moving echoes will frequently
produce gusts of 40 to 50 m i l e s per hour. In general fast moving echoes
produce the m o s t intense g u s t s . Often the large intense echoes will a p p e a r
as one cell r a t h e r than m u l t i c e l l u l a r in s t r u c t u r e .
3. The s t r o n g e s t squalls occur in the e a r l y m a t u r e stage before the
energy of the downdraft has been widely d i s p e r s e d through s p r e a d i n g of the
c o l d - a i r d o m e . Strong echoes recently developed a r e the m o s t d a n g e r o u s .
- 26 -
4. Observations on n u m e r o u s occasions have indicated that the
m o s t s e v e r e surface winds a r e a s s o c i a t e d with s h a r p discontinuites or
wave a p p e a r a n c e s in the echo p a t t e r n . Figure 12 i l l u s t r a t e s this type
of echo. Gusts of 50 to 60 m i l e s per hour have been observed with the
passage of echoes of this shape.
5. High surface wind speeds and s e v e r e weather usually o c c u r
at the zone of intersection of two squall lines. This is in a g r e e m e n t with
T e p p e r ' s theory (19) on the i n t e r s e c t i o n of two p r e s s u r e jump l i n e s .
(Figure 12d)
6. L a r g e , fast moving t h u n d e r s t o r m s in unstable air at sea should
produce higher a s s o c i a t e d surface wind speeds than those of like, intensity
on land because l e s s surface friction occurs at sea to dispel the downdraft
energy. The energy not dispelled by friction is available for c r e a t i n g higher
surface gust speeds.
7. P r e - c o l d frontal and cold frontal squall lines and w a r m a i r m a s s
t h u n d e r s t o r m s usually have s t r o n g e r associated surface wind gusts than high
level t h u n d e r s t o r m s above a w a r m frontal surface. The stability of the
w a r m front tends to absorb some of the energy of downdrafts before they
r e a c h the e a r t h ' s surface, leaving l e s s downdraft energy for c o n v e r s i o n into
surface wind gusts. Synoptic information and RHI r a d a r data can be used as
. aids in determining the likelihood that the t h u n d e r s t o r m s a r e situated above
a frontal surface. The case study of 23 July 1951, presented l a t e r , illust r a t e s the use of synoptic information in determining whether the echoes a r e
high based above a frontal s u r f a c e .
FIG. 12
ECHOES WITH SHARP
ASSOCIATED
WITH
DISCONTINUITIES OR WAVE
SQUALLY
SURFACE WINDS
PATTERNS
- 27 P ART III
CASE STUDIES
The following case studies a r e presented to i l l u s t r a t e to the a e r o logist both the usefulness and limitations of r a d a r in s h o r t range forecasting.
Several different me teorological phenomena a r e i l l u s t r a t e d . The determination of visibility during the passage of echoes over the station has been
ommitted due to the limited observational p r o g r a m conducted at the r a d a r
station. See F i g u r e 5 for optimum operating range of the r a d a r used in these
studies.
The shipboard a e r o l o g i s t should make m a x i m u m use of the r a d a r
whenever surface hourly r e p o r t s a r e available to confirm r a d a r observations
of echo intensity, visibility reduction, and beginning and ending of precipitation. In this way, he can thoroughly f a m i l i a r i z e himself with the charact e r i s t i c s of the set he will use at sea where surface h o u r l y r e p o r t s a r e not
available.
Time used is Central Standard Time (CST) and distance is e x p r e s s e d
in nautical m i l e s .
HIGH L E V E L SHOWERS AND THUNDERSTORMS
DURING NIGHT OF 2 3-24 July 1951
This case i l l u s t r a t e s the usefulness of r a d a r in detecting unexpected
weather in a r e a s lacking surface reporting stations. The squall lines show
quite rapid developments and changes in orientation that a r e of i n t e r e s t . A l s o ,
the usefulness of surface synoptic data as an aid in determining upper winds
from P P I echoes is shown.
Synoptic Situation
At 1530* on 23 July 1951, a slow moving cold front was oriented e a s t west through n o r t h e r n Kentucky and southern Illinois (Figure 13). A postfrontal shower was reported at Louisville, and thunder showers were r e p o r t e d
in the w a r m air south of the cold front at Jackson and Knoxville, T e n n e s s e e ,
Bowling Green, Kentucky, Maiden, M i s s o u r i and Little Rock, A r k a n s a s . By
2130, the front had become stationary slightly south of its 1530 position. At
2130 t h u n d e r s t o r m s were reported at T e r r e Haute, Indiana, Maiden, M i s s o u r i ,
Walnut Ridge, A r k a n s a s , and at stations f a r t h e r southwest in Oklahoma and
Texas. F i g u r e 14 shows the wind profile in the echo a r e a as determined from
streamline charts.
*Time used is Central Standard Time.
FIG.
13
FIG. 14
23 JULY
23 JULY
1951
1951
I530CST
2I00CST
SYNOPTIC
WIND
CHART
PROFILE
- 28 D i s c u s s i o n of Radar and Synoptic Data
At 2 011 (Figure 15a), the 3-cm r a d a r showed developing s h o w e r s
west of Vandalia (VLA) Illinois, and over a large a r e a from T e r r e Haute
(HUF), Indiana, southward for 60 m i l e s * . Surface reporting s t a t i o n s w e r e
lacking in this a r e a . The echo p a t t e r n indicated the development of an
ENE-WSW squall line just south of T e r r e Haute. By 2030 (Figure 15b),
the r a d a r showed a definite ENE-WSW squall line just north of T e r r e Haute.
The Weather Bureau hourly total precipitation for T e r r e Haute between 2000
and 2100 was 0. 21 inches. A few other echoes of l e s s e r i m p o r t a n c e w e r e
still p r e s e n t west of Vandalia and 50 to 60 m i l e s south-southwest of T e r r e
Haute. At 2130 (Figure 15c), the isolated echoes west of Vandalia had inc r e a s e d in intensity. Two well-developed isolated echoes w e r e situated in
southeast Illinois about 50 m i l e s southwest of T e r r e Haute. Most of the
echoes appeared to develop along the surface position of the s t a t i o n a r y
front, 60 m i l e s south of T e r r e Haute, and move northeastward up the frontal
slope. Between 2100 and 2200, 0. 25 inches of r a i n fell at T e r r e Haute.
By 2230 (Figure 15d), s c a t t e r e d echoes were situated to the north,
west, and southwest of Vandalia, and the two intense isolated e c h o e s had
developed and moved toward a point 40 m i l e s southwest of T e r r e H a u t e .
The squall line north of T e r r e Haute was beginning to d i s s i p a t e . Shortly
after t h i s , other echoes began to a p p e a r 40 m i l e s southwest of T e r r e Haute.
By 2300 (Figure 15e), an ESE-WNW squall line had developed and moved
n o r t h e a s t to within 30 m i l e s of that station. By 2330 (Figure 1 5f) the squall
line was about 20 m i l e s south of T e r r e Haute and had become oriented in a
n e a r l y E-W d i r e c t i o n , possibly becoming linked with echoes to the north of
Vandalia and with other echoes in Indiana, southeast of T e r r e Haute. The
squall line had a total E-W extent of about 120 m i l e s at this time. Some
evidence of the older squall line north of T e r r e Haute was still p r e s e n t .
By 0030, 24 July, (Figure 15g), the h e a v i e s t portions of the E-W
squall line had moved northward a l m o s t to T e r r e Haute. Most of the e c h o e s
n o r t h e a s t of Vandalia, on the west end of the line, had dissipated; a l s o , m o s t
of the e l e m e n t s of the older squall line north of T e r r e Haute had d i s s i p a t e d .
The individual e l e m e n t s of the major line moved from about 248° at 17 knots.
The Weather B u r e a u map for 0030, July 24 showed a t h u n d e r s t o r m
at Evansville, 91 m i l e s south of T e r r e Haute, and a rainshower at Indianapolis,
Indiana, 6l m i l e s e a s t - n o r t h e a s t . o f T e r r e Haute, with no indication of an E-W
squall line along the frontal surface between these s t a t i o n s . However, the
r a d a r indicated a well-defined squall line at this hour.
At 0130 (Figure 15h), the major E-W squall line had moved northward
to a position north of T e r r e Haute, and isolated echoes extended from T e r r e
Haute southwest a l m o s t to Vandalia. Another line of echoes oriented ENE-WSW
*Distance is e x p r e s s e d in nautical m i l e s .
- 29 -
had formed 40 m i l e s to the n o r t h w e s t of the major squall line, at a position
about 10 m i l e s south of Danville (DAN), Illinois. Between 0100 and 0200,
0.28 inches of r a i n was r e c o r d e d at T e r r e Haute as the squall line moved
over that station.
By 0230 (Figure 15i), both squall lines had l a r g e l y d i s s i p a t e d and
s m a l l e r isolated echoes w e r e s c a t t e r e d over the region. These slowly
d i s s i p a t e d during the e a r l y m o r n i n g h o u r s .
Upper Winds
The upper wind p a t t e r n at 2100 (Figure 14) l e a v e s little doubt that
the echoes were h i g h - b a s e d in the w a r m a i r m a s s above the frontal surface
and s t e e r e d by the higher level winds. The winds at 2000 ft and 4000 ft in
the echo a r e a w e r e f r o m 70° to 90° at 18 to 20 knots. At 6000 ft, the winds
w e r e still from an e a s t e r l y d i r e c t i o n but w e r e v e r y light. Between 6000 and
8000 ft, the winds v e e r e d to the southwest and r e m a i n e d light. Above 8000
ft, the southwesterly flow began to i n c r e a s e in strength from 12 knots at
10, 000 ft to 30 knots at 30, 000 ft.
Since the echoes moved from about 248º at 17 k n o t s , they m u s t have
b e e n high based in the s o u t h w e s t e r l y c u r r e n t above the frontal s u r f a c e .
RHI r a d a r would have shown the b a s e and tops of these e c h o e s . However ,
in the a b s e n c e of such r a d a r s , s u r f a c e synoptic data, as i l l u s t r a t e d h e r e ,
can provide s u p p l e m e n t a r y information to the P P I r a d a r for d e t e r m i n i n g
the approximate base of e c h o e s . Due to the high b a s e of these e c h o e s , they
a r e b e s t suited for d e t e r m i n i n g the l a y e r m e a n winds for 6000-30, 000 ft.
The use of the n o m o g r a m s for d e t e r m i n i n g winds from the echo m o v e m e n t s
gave a mean wind of 255° at 23 knots for the l a y e r . The actual observed
m e a n wind for this l a y e r at 2100, as taken from s t r e a m l i n e c h a r t s , was
268° at 17 knots.
Conclusions
This is a typical example in which r a d a r d e t e c t s unexpected weather
developments in r a t h e r p a s s i v e synoptic situations and over a r e a s lacking
surface r e p o r t i n g s t a t i o n s . Also, it i l l u s t r a t e s the usefulness of r a d a r for
upper-wind d e t e r m i n a t i o n s and shows how available surface synoptic data
should be taken into c o n s i d e r a t i o n in selecting the layer of b e s t c o r r e l a t i o n .
FIG. 15
HIGH
LEVEL SHOWERS AND THUNDERSTORMS DURING NIGHT
OF 2 3 - 2 4 JULY 1951
(10-MILE RANGE MARKERS )
FIG. 15
HIGH
LEVEL SHOWERS AND THUNDERSHOWERS
OF 2 3 - 2 4 JULY 1951
( 1 0 - M I L E RANGE MARKERS)
DURING
NIGHT
- 30 P R E - F R O N T A L SQUALL LINE OF 12 SEPTEMBER 1951
This case i l l u s t r a t e s the utility of 3-cm r a d a r in detecting squall lines
and tracking such s t o r m s for a period of s e v e r a l h o u r s . Limitations in the
detection of light s h o w e r s and in d e t e r m i n i n g the depth and intensity of p r e cipitation zones due to range and precipitation attenuation a r e also i l l u s t r a t e d .
Synoptic Situation
The 0930 weather m a p drawn from h o u r l y r e p o r t s , (Figure 16), showed
a cold front extending south-southwestward from a major low in Canada through
e a s t e r n Minnesota, then through a weak low in n o r t h e a s t Iowa, and into s o u t h e a s t e r n K a n s a s and c e n t r a l Oklahoma.
An analysis of synoptic conditions indicated that the cold front would
p a s s the station by 2200 with t h u n d e r s t o r m s and r a i n s h o w e r s in advance of
the front.
Discussion of R a d a r and Synoptic Data
The r a d a r was turned on at 1435 (Figure 18a) and t h r e e groups of echoes
w e r e observed on the r a d a r scope. One group was 80 to 100 m i l e s northwest of
the station, while two groups w e r e located southeast of the station, one at 40
to 60 m i l e s and the other at 90 m i l e s .
Shortly thereafter, m o r e echoes appeared in the northwest. By 1522
(Figure 18b) they w e r e a r r a n g e d in a N-S band about 110 m i l e s long. The
echoes to the southeast had become a r r a n g e d into a NNW-SSE line about 70
m i l e s long. but w e r e not i n c r e a s i n g in intensity.
The 1530 hourly r e p o r t s indicated light showers in p r o g r e s s at P e o r i a
(PIA) and Springfield, (SPI); precipitation beginning at P e o r i a at 1500 and at
Springfield at 1528. The shower at P e o r i a was visible on the r a d a r but the
shower at Springfield was not. However, a l a t e r examination of the raingage
t r a c e at Springfield indicated a rainfall r a t e of only 0. 02 i n . / h r . , too light to
be detected by the r a d a r at that range due to attenuation. The raingage c h a r t
at P e o r i a was not available for c o m p a r i s o n with the rainfall r a t e s .
By 1602 (Figure 18c), the squall line to the southeast had a l m o s t c o m pletely disappeared from the scope, probably due to range attenuation. There
w e r e no r a d a r echoes over P e o r i a or Springfield at this t i m e , but the 1630
hourly r e p o r t s indicated light showers still in p r o g r e s s at both s t a t i o n s . Both
range and precipitation attenuation of the low-powered 3 - c m r a d a r could have
been the r e a s o n for not seeing this rainfall, since there w e r e echoes between
both stations and the r a d a r station.
The forward m o v e m e n t of the squall line at this t i m e was from 285,
at 25 knots. With the continuation of this speed and d i r e c t i o n of m o v e m e n t ,
FIG. 26
FIG. 27
17 NOV. 1952
17
NOV. 1952
I830CST
2I00CST
SYNOPTIC
WIND
CHART
PROFILE
- 31 m o d e r a t e l y heavy precipitation was expected to begin in Champaign in two
h o u r s , or about 1800.
The movement of the cells within the squall line was from 215° at
46 knots. This was compared with 0900 pibals (Figure 17). The wind
velocity was v e r y uniform between 2000 ft and 30, 000 ft, the d i r e c t i o n
varying between 200º and 215º, and the speed varying between 35 and 45
knots. Consequently, the m e a n winds for all three l a y e r s were approximately 207° at 41 knots.
Two adjacent squall lines b e c a m e distinctly visible by 1640 (Figure
18d). As the f i r s t line approached the station, m o r e echoes b e c a m e visible
to the r e a r indicating that the precipitation a r e a probably extended over a
relatively broad zone.
The range of the r a d a r was reduced to 30 m i l e s at 1645 to p e r m i t
a detailed study of the echoes as they passed over the dense raingage network centered 18 m i l e s w e s t - n o r t h w e s t of the station. By 1720 (Figure
18e), the echoes w e r e only 15 m i l e s west of Champaign. P r e c i p i t a t i o n
began in northwest Champaign at 1755 as a l a r g e echo developed to the
west southwest (Figure 18f). At 1750 a wind shift with gusts of 34 mph
was r e c o r d e d at the Water S u r v e y ' s weather station in Champaign.
Rainfall continued for a period of about two h o u r s at Champaign, and
was light except for the initial b u r s t at 1755. As mentioned p r e v i o u s l y , the
appearance of a second squall line at 1640 was indicative of the p r e s e n c e of
a broad rainfall zone. Because of the attenuation factors involved, the intensity of the precipitation echoes to the r e a r of the first squall line and the
exact depth of the rainfall zone could not be r e l i a b l y a s c e r t a i n e d with the
APS-15 (3-cm) r a d a r . With a m o r e powerful 3 - c m s e t , such as the CPS-9
m o r e detailed information on the depth and intensity of the rainfall zone could
have been obtained. F u r t h e r m o r e , the p r e s e n c e of a broad rainfall zone
might have become apparent at an e a r l i e r time with a m o r e powerful set.
Conclusions
Although attentuation factors masked the intensity and depth of the
squall zone in this c a s e , the ability of the r a d a r to detect and t r a c k the
forward edge of this zone for s e v e r a l h o u r s would have provided valuable
information for flight planning or other operations affected by such weather
phenomena, In analyzing such situations, knowledge of r a d a r limitations
due to attenuation factors is i m p o r t a n t to the f o r e c a s t e r in o r d e r to make
maximum utilization of the available data.
FIG. 18
PREFRONTAL SQUALL LINE OF 12 SEPTEMBER
( 1 0 - M I L E RANGE MARKERS)
1951
FIG. 19 9 AUGUST 1952
0030CST SYNOPTIC CHART
FIG. 20 9 AUGUST 1952 0300CST WIND PROFILE
- 32 P R E - F R O N T A L SQUALL LINE OF 8-9 AUGUST 1952
This c a s e i l l u s t r a t e s the growth and dissipation of a squall line as
observed by r a d a r and the effects of this formation on anomalous propagation that was o c c u r r i n g p r i o r to the formation. The r a d a r was a l s o useful
in locating an a r e a of lighter winds in southern Illinois and indicated l a r g e
openings in the squall line.
Synoptic Situation and Weather F o r e c a s t
T h e r e was a weak low c e n t e r located in c e n t r a l Wisconsin and a
cold front extended south-southwestward from t h i s low through e x t r e m e
w e s t e r n Illinois and c e n t r a l M i s s o u r i at 0030 on 9 August (Figure 19).
Some squall line activity had been a s s o c i a t e d with this cold front in ext r e m e n o r t h e r n Illinois, and, the p r e v i o u s evening, v e r y light shower
activity o c c u r r e d in c e n t r a l Illinois.
The cold front was expected to p a s s the station about 1000 on the
9th with s c a t t e r e d showers in advance of the cold front.
R a d a r Data
The f i r s t precipitation echoes b e c a m e visible about 2255 on the 8th
(Figure 21a) at a range of 90 to 100 m i l e s northwest of the station. T h e r e
was an e x c e s s i v e amount of ground clutter with some t a r g e t s visible as far
as 45 m i l e s from the station. The precipitation echoes w e r e quite s t r o n g ,
indicating a r a t h e r intense t h u n d e r s t o r m in p r o g r e s s .
By 0008 of the 9th (Figure 2 1 b ) the intensity of the echoes had dec r e a s e d , and w e r e located 70 to 100 m i l e s northwest of the station. The
ground p a t t e r n had begun to change shape, with fewer ground t a r g e t s visible
to the northwest. Convergence into the t h u n d e r s t o r m a r e a was d e s t r o y i n g
the inversion that produced the trapping. However, it was still difficult to
distinguish the ground t a r g e t s from the precipitation e c h o e s except by use
of the "A" scope.
More e c h o e s began to appear and at 0057 (Figure 21 c), t h r e e g r o u p s
of echoes w e r e p r e s e n t . By 0128 (Figure 21 d), a g e n e r a l intensification of
the echoes was e a s i l y noticeable. All signs of anomalous propagation w e r e
gone by this t i m e , indicating a r a t h e r wide a r e a of convergence in the thunderstorm area.
By 0210 (Figure 21e), the c e n t r a l portion of the squall line had developed
considerably, with the s o u t h e r n end only 35 m i l e s west of the station. T h e r e
w a s no a p p r e c i a b l e change in intensity of the e c h o e s to the north, but the e c h o e s
in the southwest had begun to d i s s i p a t e . A few light e c h o e s were visible to the
e a s t and south. The movement of the c e l l s in the center portion was from 255°
at 24 knots.
More development at the southern end of the line would be n e c e s s a r y
- 33 -
before a precipitation f o r e c a s t for the station would be r e q u i r e d , since the
e a s t - n o r t h e a s t e r l y movement of these cells would take them north of the
station.
The range of the r a d a r set was reduced to 30 m i l e s at this time to
study the detail of the s h o w e r s as they passed over the Water S u r v e y ' s
raingage network, centered 18 m i l e s w e s t - n o r t h w e s t of the station.
The echoes w e r e located only 20 m i l e s west of the station at 0237
(Figure 2 If) and no new development.had o c c u r r e d that would cause r a i n
- at the station.
The echoes p a s s e d to the north about 0345 without any r a i n o c c u r r i n g
at the r a d a r station. When the r a d a r range was i n c r e a s e d to 100 m i l e s at
0422 (Figure 21g), the squall line had lost m o s t of i t s c h a r a c t e r i s t i c s and
appeared as a c i r c u l a r a r e a of s h o w e r s . The intensity of the echoes continued to d e c r e a s e as they moved away from the station, probably a r e s u l t of
range attenuation. By 0534 (Figure 21h), they had b e c o m e a s m a l l a r e a of
s h o w e r s . A few isolated showers continued until about 0730.
The computed movement of the cells in the center portion, 255° at
24 knots at 0210, was compared with the 0300 P i b a l s (Figure 20). The wind
d i r e c t i o n was fairly constant between 2000 ft and 30, 000 ft, varying between
230° and 260°. The wind speeds ranged Between 20 and 30 knots up to 20, 000
ft, then i n c r e a s e d rapidly to 70 knots at 30, 000 ft. The 2, 000-26, 000-ft
layer or the 2000-20, 000-ft layer should yield equally satisfactory r e s u l t s for
c o r r e l a t i o n p u r p o s e s since there was no frontal surface aloft, and the echoes
probably did not extend above 20,000 or 25, 000 ft. Using n o m o g r a m s p r e sented in the wind study, the wind velocity from the movement of the echoes
should be 250° at 27 knots for the 2000-26, 000-ft l a y e r , and 245° at 22 knots
for the 2000-20, 000-ft l a y e r . The observed m e a n winds for the l a y e r s w e r e
243° at 29 knots and 244° at 25 knots r e s p e c t i v e l y . The 10, 000-ft level wind
d e t e r m i n e d from the n o m o g r a m was 253° at 21 knots, while the observed wind
was 230° at 23 k n o t s .
The echoes to the south of Springfield w e r e moving in the s a m e d i r e c tion as those in the center portion, but 6 knots s l o w e r , at a speed of only 18'
knots. The Pibal r e p o r t s indicated that lighter winds did exist in southern
Illinois.
Conclusions
This case i l l u s t r a t e s the utility of r a d a r in detecting and tracking e c h o e s
for d e t e r m i n a t i o n of the g e n e r a l wind field, echo intensity, s t o r m movement and
changes in a t m o s p h e r i c stability. It was also possible to detect an a r e a of
lighter winds by noting differences in the speed of m o v e m e n t of e c h o e s . The
a e r o l o g i s t should e x e r c i s e judgment in doing t h i s , h o w e v e r , since differences
in the speed of movement of echoes m a y be a r e s u l t of v e r t i c a l extent r a t h e r
than actual differences in the wind field.
FIG. 21
PREFRONTAL SQUALL LINE OF 8 - 9
( 2 0 - M I L E RANGE MARKERS)
AUGUST
1952
FIG. 21 PREFRONTAL SQUALL LINE OF 8-9 AUGUST 1952
(20-MILE RANGE MARKERS UNLESS OTHERWISE INDICATED)
- 34 SHOWER FORMATIONS ON A STATIONARY FRONT
14 OCTOBER 1952
This c a s e i l l u s t r a t e s another situation where RHI data or s u p p l e m e n t a r y surface synoptic data a r e needed in conjunction with P P I data to
s e l e c t the l a y e r of b e s t c o r r e l a t i o n for upper wind d e t e r m i n a t i o n s . A
l a r g e echo with severe surface weather conditions was detected as it
p a s s e d over D e c a t u r , Illinois. The utility of r a d a r in a s c e r t a i n i n g changing
s t o r m c h a r a c t e r i s t i c s is shown. The detection and tracking of r e l a t i v e l y
intense precipitation zones' within a r e a s of light rainfall is d e m o n s t r a t e d ,
and the n e c e s s i t y for avoiding use of echoes in such zones for d e t e r m i n i n g
upper winds is i l l u s t r a t e d .
Synoptic Situation and Weather F o r e c a s t
The weather m a p at 0630 (Figure 22) indicated a s t a t i o n a r y front
located along the Ohio River Valley. This front had p a s s e d the station the
night before as a slow-moving cold front. The only p r e c i p i t a t i o n with the
front on the 0630 m a p was a p a s t shower at Indianapolis, Indiana, but the
Weather B u r e a u f o r e c a s t called for occasional light r a i n s h o w e r s and
s c a t t e r e d light thunder s h o w e r s for soutitern and c e n t r a l Illinois.
Discussion of R a d a r and Synoptic Data
The r a d a r scope at 0815 showed no e c h o e s . The r a d a r was turned
on again at 0930 and s e v e r a l e c h o e s had developed in an E-W line, 40 to 95
m i l e s southeast of the station (Figure 25a). By 0947 (Figure 25b), the n u m b e r of s h o w e r s had i n c r e a s e d and spread westward, and a zone of s h o w e r s
extended from 40 m i l e s s o u t h - s o u t h e a s t of the r a d a r station to 20 m i l e s w e s t
of Indianapolis (Ind. ). There a l s o was an echo 20 m i l e s south of Springfield
(SPI), and an echo only 25 m i l e s southwest of the r a d a r station (Figure 25b).
F o r d e t e r m i n a t i o n of the d i r e c t i o n and speed of upper winds, the
6000-30, 000-ft l a y e r should c o r r e l a t e best in this c a s e , since the slow-moving
cold front had p a s s e d the station the night b e f o r e , and a frontal surface should
have been p r e s e n t aloft with the echoes based above this s u r f a c e . RHI r a d a r
would have been v e r y useful h e r e for determining the cloud b a s e , e s p e c i a l l y
if nothing had been known about the synoptic situation. The m o v e m e n t of the
echoes was from 240* at 37 knots. The use of the n o m o g r a m s p r e s e n t e d in the
wind study gave a 6000-30, 000-ft l a y e r m e a n wind of 250° at 38 knots. The
mean wind velocity (Figure 23) for the 6000-30, 000-ft layer as d e t e r m i n e d from
winds aloft c h a r t s was 249º at 43 knots, or a difference of 10° and 5 knots b e tween the mean wind d e t e r m i n e d from echo m o v e m e n t and the observed m e a n
wind. Neither of the two other l a y e r s or t h r e e levels gave good wind c o r r e l a tiona. The observed mean velocity for the 2000-20, 000-ft layer was 219° at
30 knots; the m e a n velocity for the 2000-26, 000-ft l a y e r was 225° at 35 knots.
FIG. 22
14 OCTOBER 1952
FIG. 23
0630CST
14 OCTOBER 1952
SYNOPTIC
0900CST WIND
CHART
PROFILE
- 35 -
The time of a r r i v a l for the shower located 25 m i l e s southwest of the
station was computed to be 102 0. Since the echo was v e r y weak, the rainfall was expected to be v e r y light. A brief shower did occur between 1015
and 1030, with only a t r a c e of p r e c i p i t a t i o n being r e c o r d e d at the r a d a r
station. At 1057", the zone of s h o w e r s , as indicated by the r a d a r , was about
140 m i l e s long and extended from 20 m i l e s w e s t of Springfield, to 20 m i l e s
n o r t h - n o r t h e a s t of T e r r e Haute (HUF), Indiana (Figure 25c). Another echo
had developed only 15 m i l e s southwest of the r a d a r station, and it was computed that the c e n t e r would p a s s a little n o r t h w e s t of the station. Light precipitation was expected to begin again at 1115.
The r a d a r antenna was tilted to 15° elevation at 1058 and was left
t h e r e until 1140 to collect echo data at close r a n g e . The m a x i m u m range
at this elevation angle is about 20 m i l e s , the c e n t e r of the b e a m being about
30, 000 ft high at this r a n g e .
A light thundershower began at the station at 1112 and lasted until
1135 with 0. 02 in. of r a i n falling at the r a d a r station. Quincy and Springfield r e p o r t e d a light r a i n s h o w e r and a light thundershower, r e s p e c t i v e l y ,
on the 1130 hourly w e a t h e r sequence, and Indianapolis r e p o r t e d a r a i n shower ending at 1101.
At 1140 ( F i g u r e 25d) the antenna was again lowered to z e r o elevation,
and a l a r g e intense echo was located 30 m i l e s w e s t of the station, extending
southward over Decatur (DEC), Illinois. The echo was about 40 m i l e s long
and eight m i l e s d e e p , and l a r g e r in a r e a l extent than m o s t echoes produced
by shower-type precipitation.
The automatic g a i n - r e d u c t i o n device indicated only one cell p r e s e n t
in the southern half of this echo. Usually t h e r e a r e two or t h r e e cells within t h u n d e r s t o r m s so that the existence of only one large cell m a y have b e e n
an indication of the s t o r m intensity. The stepping switch a l s o revealed a
r a t h e r steep gradient of only t h r e e m i l e s between the leading edge of the echo
and the center of the cell, as shown by the r a d a r . Attenuation of the r a d a r
b e a m prevented an a c c u r a t e d e t e r m i n a t i o n of the gradient on the trailing edge.
Studies made by Jones (7) and A m e r i c a n A i r l i n e s (9), r e v e a l e d that
the s e v e r e s t turbulence was a s s o c i a t e d with s t e e p gradients of r a d a r echo
c o n t o u r s , so that flight f o r e c a s t s could have b e e n issued to avoid this a r e a .
The Weather B u r e a u cooperative o b s e r v e r r e p o r t e d hail as the echo p a s s e d
o v e r Decatur. There was no r e c o r d i n g raingage at Decatur to d e t e r m i n e the
s t o r m rainfall r a t e but 0. 94 in. fell during the day.
As the s a m e echo center p a s s e d the r a d a r station, a p r e s s u r e r i s e of
0. 17 in. was r e c o r d e d , and a gust of 22 mph o c c u r r e d with a preceding m e a n
wind speed of 10 mph. The rainfall r a t e was 1. 5 i n . / h r with a total rainfall
of 0. 26 in.
- 36 The ability to isolate s t o r m a r e a s and to d e t e r m i n e changing chara c t e r i s t i c s was i l l u s t r a t e d l a t e r in the day. E c h o e s continued to form in
c e n t r a l Illinois and s p r e a d northward. During the late afternoon and evening, the c h a r a c t e r of the r a d a r echoes changed from well-defined, s h o w e r type echoes to poorly-defined echoes (Figure 25e), and finally m e r g e d into
an a r e a of g e n e r a l light r a i n with i n t e r d i s p e r s e d h e a v i e r p o r t i o n s . The
g e n e r a l r a i n was verified with 2030 r e p o r t s . The precipitation a r e a at
that time covered c e n t r a l and n o r t h e r n Illinois and n o r t h e r n Indiana.
Several m o r e intense a r e a s were visible within the precipitation
a r e a . Between 2130 and 2215, a fairly well-defined line of echoes passed
to the northwest of the station (Figure 25f). The computed echo movement
was from 244º at 72 knots.
The 2100 Pibal r e p o r t s (Figure 24) indicated v e e r i n g winds from
010° at 24 knots at 2000 ft to 090º at 13 knots at 6000 ft. Above 6000 ft the
wind direction was southwesterly. The wind speed did not r e a c h 70 knots
until the 16, 000-ft level, so that the echo speed m u s t have been r e l a t e d to
the winds n e a r this level. The cloud base was probably at about 6000 ft
where the frontal surface was located.
As mentioned in the wind study, natural seeding from high clouds
m a y produce a r e a s of m o r e intense precipitation within a r e a s of light rain.
Natural seeding from clouds at or about 16, 000 ft could have been the r e a s o n
for the fast movement of this line of echoes (11)(12).
The c o r r e l a t i o n of echo velocities and m e a n upper wind velocities
for this line of echoes was v e r y poor since the echo movement was related
to the winds at a high level. However, it was pointed out in the wind study
that c o r e s of m o r e intense precipitation within a r e a s of light r a i n should be
avoided for c o r r e l a t i o n p u r p o s e s .
Conclusions
This case i l l u s t r a t e s the use of r a d a r in routine s h o r t r a n g e forecasting. Two unusual phenomena o c c u r r e d : a s e v e r e t h u n d e r s t o r m and the
rapid movement of a line of echoes. The movement of this line of echoes
was related to the winds at 16, 000 ft.
FIG. 24
14
OCTOBER
1952
2I00CST
WIND
PROFILE
FIG. 25
SHOWER FORMATIONS ON A STATIONARY FRONT, 14 OCTOBER
( 2 0 - M I L E RANGE MARKERS UNLESS OTHERWISE INDICATED)
1952
- 37 -
SQUALL LINE OF 17 NOVEMBER 1952
SHOWING E F F E C T S OF ATTENUATION
This c a s e is presented as an example of the effects of attenuation on
low-powered, 3-cm r a d a r equipment. An examination of h o u r l y weather rep o r t s showed that what appeared to be an innocent-looking squall line was
actually an a r e a of w i d e s p r e a d shower activity with only the leading edge
v i s i b l e . The movement of the squall line was v e r y e r r a t i c and it dissipated
as it moved into a low-level jet s t r e a m .
Synoptic Situation and Weather F o r e c a s t
The 1830 w e a t h e r m a p of 17 November (Figure 26) showed a w e l l developed low center in e a s t e r n South Dakota with a w a r m front running
e a s t w a r d into c e n t r a l Wisconsin, and extending e a s t - s o u t h e a s t w a r d into
P e n n s y l v a n i a as a q u a s i - s t a t i o n a r y front. A slow moving cold front was
located southeastward from the low center through c e n t r a l Iowa, then south
southwestward into w e s t e r n M i s s o u r i , through a weak low in e a s t e r n Oklahoma and into c e n t r a l Texas. This cold front was expected to move into
w e s t e r n Illinois by 0630 of 18 N o v e m b e r . F i g u r e 27 gives the upper wind
profile at 2100.
The weather B u r e a u 1600 f o r e c a s t called for s c a t t e r e d light r a i n s h o w e r s and o c c a s i o n a l light thunder showers s p r e a d i n g e a s t w a r d through
Illinois. Some locally heavy t h u n d e r s t o r m s w e r e a l s o expected.
D i s c u s s i o n of Radar and Synoptic Data
A v e r y weak e c h o , located 118 m i l e s w e s t of the station, appeared
at 1650 (Figure 29a). The m o v e m e n t at 1726 (Figure 29b) was northeastward at about 45 knots. A definite velocity could not be d e t e r m i n e d due to
the long range and weak intensity of the echo. More e c h o e s formed and
continued to develop, r e s u l t i n g in a squall line with a N-S orientation. By
1818, this squall line had reached i t s g r e a t e s t length and intensity (Figure
29c). A new echo a p p e a r e d at this time and was located 105 m i l e s west of
the station.
On the 1830 w e a t h e r r e p o r t s , P e o r i a (PIA) r e p o r t e d lightning in the
west, Quincy, r e p o r t e d a light t h u n d e r s t o r m in p r o g r e s s , and Burlington,
Iowa, r e p o r t e d a light r a i n s h o w e r .
The original squall line to the northwest showed definite signs of
d i s s i p a t i o n by 1836, while the new echo to the west had developed into
a n o t h e r N-S squall line (Figure 29d). The new squall line continued to
grow, and at 1905 the line was 70 m i l e s long and v e r y n a r r o w , while the
o r i g i n a l squall line had a l m o s t completely dissipated (Figure 29e).
FIG. 26
FIG. 27
17 NOV. 1952
17
NOV. 1952
I830CST
2I00CST
SYNOPTIC
WIND
CHART
PROFILE
- 38 -
The computed m o v e m e n t of the individual cells
line was from 240° at 40 knots. The m o v e m e n t of the
from 280° at 25 knots. B a s e d upon these m o v e m e n t s ,
of p r e c i p i t a t i o n at the station was expected to be about
of the new squall
squall line was
the time of a r r i v a l
2210.
The a r e a l extent of the squall line continued to i n c r e a s e as it approached the station, and the s t o r m r e a c h e d an overall length of 140 m i l e s
about 2020 (Figure 29f).
Although the r a d a r indicated only a n a r r o w band of precipitation,
all the stations in Illinois w e s t of this line r e p o r t e d light to m o d e r a t e
s h o w e r s and thunder s h o w e r s in p r o g r e s s on the 2030 weather sequence
reports.
The s h o w e r s in the leading edge attenuated the r a d a r b e a m so that
the s h o w e r s to the r e a r w e r e not v i s i b l e . If low-powered, 3-cm equipment
is used, as in this c a s e , the absence of echoes behind a squall line does
not n e c e s s a r i l y indicate that s h o w e r s a r e not o c c u r r i n g .
At 2122, the squall line was s t i l l located 30 to 40 m i l e s west of the
station, but was beginning to show a slight d e c r e a s e i n a r e a l e x tent(Figure
29g). The t h u n d e r s t o r m s within the squall line w e r e v e r y distinct. It
b e c a m e a p p a r e n t at this time that the m o v e m e n t of the squall line was becoming v e r y e r r a t i c , having moved 40 m i l e s between 1905 and 2020, but
only 10 m i l e s between 2020 and 2122, so that a definite f o r e c a s t of the
time of a r r i v a l could not be e s t a b l i s h e d . However, the utility of the r a d a r
was s t i l l a p p r e c i a b l e since the exact position and intensity of the leading
squall line and any changes in its c h a r a c t e r i s t i c s were still e a s y to determine,,
The m o v e m e n t of the c e l l s was computed to be 214° at 37 knots, a
m o r e southerly d i r e c t i o n than the p r e v i o u s l y computed d i r e c t i o n of 240°.
The speed of m o v e m e n t of the individual cells was about the s a m e as b e f o r e .
The d e c r e a s e in speed of m o v e m e n t of the squall line and the r e s u l t a n t inc r e a s e in the effect of the wind field on the cell m o v e m e n t probably is the
explanation.
The range of the r a d a r was reduced to 30 m i l e s at this time to inc r e a s e the detail of the s h o w e r s within the squall line as they p a s s e d over
the Illinois State Water S u r v e y ' s network of 50 r a i n g a g e s . By 2150 (Figure 29h), the intensity of the echoes w e s t of the station showed some signs
of d e c r e a s i n g , and m o s t of the squall line s t r u c t u r e had dissipated by 2215
(Figure 29i). The n e a r e s t echo was only 12 m i l e s west of the station. At
this point, without any a p p a r e n t r e a s o n the squall line ceased to advance.
E c h o e s w e r e located around the station at 2353 ( F i g u r e 29j) and
p r e c i p i t a t i o n began s h o r t l y after in the form of light r a i n s h o w e r s and
continued through the night with a total accumulation of 0. 44 in.
- 39 -
The computed movement of the cells at 2130 (214°, 37 knots) was
compared with the 2100 Pibal r e p o r t s (Figure 27). The variation in wind
velocity through the f i r s t 25, 000 ft was such that the c o r r e l a t i o n of the
d i r e c t i o n and speed of movement of the cells and the m e a n winds was
equally s a t i s f a c t o r y for all t h r e e of the l a y e r s mentioned in the wind study.
The a v e r a g e wind velocity for all three l a y e r s was about 220", 41 knots.
Isotachs (lines of equal wind speed) d r a w n on the 2000-ft (Figure
28) and 4000-ft level s t r e a m l i n e c h a r t s indicated an a r e a of m a x i m u m
wind speed extending from s o u t h e a s t e r n Illinois into n o r t h w e s t e r n Indiana
and c e n t r a l lower Michigan. The wind d i r e c t i o n at these levels was p a r allel to the orientation of the squall line. It h a s been thought by some
m e t e o r o l o g i s t s that a r e a s to the west of a northward-flowing, low-level
jet a r e conducive to squall line formation because of the p r e s e n c e of cyclonic
vorticity, and that squall lines would dissipate after they c r o s s e d the major
axis of the jet because of the anticyclonic v o r t i c i t y to the e a s t . F r o m this
a n a l y s i s it a p p e a r s that this p a r t i c u l a r squall line dissipated as it moved
into the center of the low-level jet.
Conclusions
This case indicates a number of i n t e r e s t i n g m e t e o r o l o g i c a l conditions.
F i r s t , a weak squall dissipated while a second one developed and dissipated
as they moved into the center of the low-level jet. F u r t h e r studies a r e
r e c o m m e n d e d to d e t e r m i n e the exact influence of the low-level j e t upon
precipitation. Pronounced attenuation is p r e s e n t as surface r e p o r t i n g
stations indicate rainfall while the r a d a r did not d e t e c t such. A f o r e c a s t
of the time of a r r i v a l of rainfall was impossible due to the changing d i r e c t i o n
of m o v e m e n t of the cells and the d e c e l e r a t i o n and dissipation of the squall
line. However, a f o r e c a s t e r without r a d a r would not have had knowledge of
t h i s changing situation.
FIG. 28
17 NOV.
1952
2000-FT.
WINDS
2I00CST
FIG.
29
SQUALL LINE OF
( 2 0 - M I L E RANGE
17 NOVEMBER 1952, SHOWING
MARKERS UNLESS OTHERWISE
EFFECT OF ATTENUATION
INDICATED)
FIG. 29
SQUALL LINE OF 17 NOVEMBER 1952,
( 1 0 - M I L E RANGE MARKERS UNLESS
SHOWING EFFECT OF ATTENUATION
OTHERWISE INDICATED)
- 40 RADAR OBSERVATION OF A TORNADO OF 9 APRIL 1953
On 9 A p r i l 1953, the Illinois State Water Survey was fortunate to
observe by r a d a r the development, growth, and p a r t i a l decay of a t o r nado that caused three million d o l l a r s in p r o p e r t y damage and two fatalities.
Synoptic Situation
The surface weather map d r a w n from hourly sequence r e p o r t s i n d i cated a cold front lying about 70 m i l e s west of the a r e a of tornado f o r m a tion in e a s t c e n t r a l Illinois (Figure 30). The front was moving e a s t w a r d at
25-30 knots. A low of 995 mb was located in the vicinity of B r a d f o r d , Illinois,
about 100 m i l e s northwest of the a r e a where the tornado was f i r s t o b s e r v e d .
A w a r m front extended e a s t w a r d f r o m the low a c r o s s n o r t h e r n Illinois. The
low center was deepening, moving rapidly n o r t h e a s t w a r d , and occlusion was
beginning to take place. F i g u r e 31 gives the upper wind profile at 1500.
The tornado, which moved e a s t - n o r t h e a s t w a r d at about 45 knots,
appeared to be a s s o c i a t e d with a squall zone in a d v a n c e of the front. R e p o r t s from Weather B u r e a u stations and interrogation of r e s i d e n t s in the
tornado a r e a indicated that r a i n and hail w e r e o c c u r r i n g to the north of
the tornado path, but no p r e c i p i t a t i o n was falling in the immediate vicinity
of the tornado.
The Chanute Air F o r c e B a s e Weather Station, about eight m i l e s
north of the tornado, r e p o r t e d a heavy thundershower with hail.
Radar Data
The photographs in F i g u r e 32 s u m m a r i z e the h i s t o r y of the tornado
from its detection until it c r o s s e d the Indiana state line. All except the l a s t
photograph a r e on 30-mile range with 10-mile r a n g e m a r k e r s .
The first photograph (1644) shows the r a d a r precipitation echo of
the cloud with which the tornado was a s s o c i a t e d , 15 minutes p r i o r to the
f i r s t detection of the tornado formation. The f i r s t appearance of an a p pendage on the southwest edge of the mother cloud is faintly visible in the
second photo at 1700. The r a d a r b e a m extended from the surface to an
elevation of about 2000 ft at this t i m e .
The next two photographs taken at 1705 and 1710 show p r o g r e s s i v e
development of the tornado. In the photograph at 1713, taken about 13 m i n u t e s
after the initial detection, development of a cyclonic c u r l b e c o m e s c l e a r l y
evident at the south end of the appendage. F u r t h e r cyclonic development is
illustrated in the photographs at 1716 and 1719, and the appearance of an
" e y e " in the s t o r m is d i s c e r n i b l e in the l a t t e r photograph. O b s e r v e r s
FIG. 30
FIG.
31
9 APRIL
1953
9 APRIL 1953
I730CST
I500CST
SYNOPTIC
WIND
CHART
PROFILE
- 41 reported seeing the tornado extend from a cloud b a s e s t r a i g h t to the ground.
The position of this cloud was determined to have approximately coincided
with the south end of the appendage s e e n on the r a d a r .
The photograph at 1735 indicates that the mother cloud has d e c r e a s e d
in size and has developed a s p i r a l a p p e a r a n c e . Meanwhile, the tornado echo
has i n c r e a s e d in a r e a l extent. The photograph at 1738 is s i m i l a r to 1735.
At about this time (1738), the tornado was causing its g r e a t e s t p r o p e r t y
damage in Illinois. The width and length of the tornado path was g r e a t e r
than r e p o r t e d for m o s t t o r n a d o e s .
The l a s t photo at 1800 shows 100-mile r a n g e with 20-mile range
m a r k e r s . Other precipitation echoes in the surrounding a r e a a r e shown.
The tornado was about 35 m i l e s from the r a d a r station at the time of this
photograph.
The size of the cyclonic circulation, as shown by r a d a r , and the
width and nature of the path of destruction indicate that t h e r e may have
been a tornado cyclone, as proposed by Brooks (20), having one or m o r e
funnels within. This s m a l l intense cyclonic circulation m a y have developed
alongside the mother cloud and continued to grow until it began to envelop
the m o t h e r cloud in its l a t e r s t a g e s .
Conclusions
This case shows that r a d a r can detect tornadoes and show their
development and movement, provided they a r e of sufficient size or have a
peculiar circulation p a t t e r n of the above d e s c r i b e d type associated with
them. Some s m a l l e r , l e s s destructive tornadoes m a y a l s o have circulation
p a t t e r n s associated with t h e m that would be detectable with r a d a r .
FIG.32 RADAR PPI PHOTOS WITH 10-MILE RANGE MARKERS;
ILLUSTRATING TORNADO DEVELOPMENT ON 9 APRIL 1953.
FIG. 32 CONTINUED
MOVEMENT AT
DEVELOPMENT
45 MPH.
AND EAST-NORTHEAST
- 42 P A R A L L E L PRECIPITATION BANDS OF 17 A P R I L 1953
This case i l l u s t r a t e s the utility of r a d a r for m i c r o - s y n o p t i c s t u d i e s
by showing the s t r u c t u r a l f e a t u r e s of precipitation as produced by upslope
motion on a stationary front.
Synoptic Situation and Weather F o r e c a s t
The weather m a p at 0630, on 17 A p r i l (Figure 33) showed a m a j o r
low c e n t e r located between Lake S u p e r i o r and J a m e s Bay. The cold front
from this low extended a c r o s s c e n t r a l Ohio, southern Indiana, and s o u t h e r n
Illinois into a flat wave in southeast Kansas and then into another low cent e r in the Texas Panhandle. The cold front had passed the r a d a r station
about midnight. Intermittent light snow and r a i n o c c u r r e d at the r a d a r
station during m o s t of the day.
The 1000 Weather B u r e a u regional f o r e c a s t called for light snow
showers and occasional s l e e t in portions of c e n t r a l Illinois and s p r e a d i n g
e a s t w a r d into c e n t r a l and s o u t h e r n Indiana and Ohio.
By 1230 the front had b e c o m e q u a s i - s t a t i o n a r y a c r o s s the s o u t h e r n
tip of Illinois and southern Indiana. A weak wave was p r e s e n t in s o u t h e a s t
Kansas and southwest M i s s o u r i . Most of the stations in Illinois and Indiana
w e r e r e p o r t i n g light snow, light r a i n , or light freezing r a i n .
Discussion of R a d a r and Synoptic Data
The Chanute Field rawin and Springfield pibal for 0900 w e r e not
available. It was d e t e r m i n e d from the s t r e a m l i n e c h a r t s that a s h e a r zone
at lower levels existed acr.oss c e n t r a l Illinois. This s h e a r zone plus the
absence of the above two wind o b s e r v a t i o n s , made it difficult to d e t e r m i n e
a c c u r a t e wind computations, in the vicinity of the e c h o e s , from the s t r e a m line c h a r t s . The mean winds, e s t i m a t e d from the s t r e a m l i n e c h a r t s , for
the l a y e r s 2000-20, 000 ft and 2000-26, 000 ft w e r e 270º at 40 knots and 272º
at 47 knots, r e s p e c t i v e l y (Figure 34).
The r a d a r set was turned on at 0955. There w e r e some light, i l l defined echoes around the station to a d i s t a n c e of 30 m i l e s . The precipitation was m o s t l y in the f o r m of light snow s h o w e r s and located m a i n l y to the
south of the r a d a r station. By 1024 (Figure 35a) the precipitation had formed
WNW-ESE bands and was moving from 240º at about 28 knots, as computed
from plots made at short i n t e r v a l s of t i m e .
As mentioned in the wind study, such echoes in widespread a r e a s of
stable precipitation a r e not ideally suited for upper wind d e t e r m i n a t i o n s
unless the b a s e and top of the e c h o e s can be m e a s u r e d with RHI r a d a r or
e s t i m a t e d from synoptic conditions. In this c a s e , the Chanute Field 0900
FIG.
33
FIG. 34
17 APRIL
1953
17 APRIL 1953
0630CST
0900CST
SYNOPTIC
WIND
CHART
PROFILE
- 43 -
radiosonde showed the b a s e of the frontal surface to be at about 3500 ft
above sea level. The echoes w e r e apparently in the w a r m a i r m a s s
above this frontal s u r f a c e . Although the s t r e a m l i n e c h a r t s a r e questionable for the l a y e r 4000-10, 000 ft, there a r e indications that the winds in
this " s t e e r i n g " l a y e r did a g r e e with the echo m o v e m e n t . On the 1030
w e a t h e r s e q u e n c e , stations to the north and n o r t h e a s t , not r e p o r t i n g precipitation, w e r e c a r r y i n g ceilings of 6000-10, 000 ft. The WBAN facsimile
weather c h a r t s showed the front to be located about 50 m i l e s north of
Chanute Field on the 850 mb c h a r t and just south of Joliet, Illinois, on the
700 mb c h a r t . This is further evidence that the cloud deck producing the
precipitation was just above the frontal surface and sloped with it upward
to the north and n o r t h e a s t .
In F i g u r e s 35a, b, and c, t h e r e m a y have been other WNW-ESE
bands f a r t h e r southwest of the station that w e r e not visible due to attenuation by precipitation at r a d a r station. It is i n t e r e s t i n g to note that the
b a n d s w e r e oriented n e a r l y p e r p e n d i c u l a r to the winds just above the
frontal s u r f a c e , and at an angle of about 40° to the frontal surface as s e e n
in a h o r i z o n t a l plane projection. These bands m a y t h e r e f o r e be produced
by w i n d - g e n e r a t e d gravity waves on the frontal s u r f a c e . A m a x i m u m of
precipitation should occur near the c r e s t of the waves and a m i n i m u m n e a r
the lowest points. It was not possible to prove this with the available
weather r e c o r d s .
As the day p r o g r e s s e d , the precipitation b a n d s , as shown on the
r a d a r , began to shift slowly to an E-W orientation ( F i g u r e s 35 d, e, f, g,
and h) and finally shifted to a ENE-WSW orientation and d e c r e a s e d in speed
by 2200 (Figure 35 i, j, k, and 1). This change in orientation a g r e e s with
the change in the winds above the frontal s u r f a c e . By 2100, the winds for
the layer 4000-8000 ft over s o u t h e r n and c e n t r a l Illinois had backed to a
s o u t h e r l y or s o u t h - s o u t h e a s t e r l y d i r e c t i o n and d e c r e a s e d in speed. By late
evening the 4000-8000 ft winds w e r e blowing n e a r l y p e r p e n d i c u l a r to the
s t a t i o n a r y front, and the bands w e r e oriented n e a r l y p a r a l l e l to the front
and p e r p e n d i c u l a r to the winds.
A pilot r e p o r t , 40 m i l e s southeast of Vichy, M i s s o u r i , and 240 m i l e s
southwest of the r a d a r station, gave the top of the o v e r c a s t as 10, 000 ft at 1540.
This a g r e e s with the r a d a r and wind data, indicating that the cloud decks did
not extend to g r e a t h e i g h t s , in which c a s e , they would have been influenced
by the strong w e s t e r l y winds above 10, 000 ft.
F i g u r e 35 i, j, k, and 1 indicates that the bands w e r e moving slowly
or n e a r l y s t a t i o n a r y .
Also, e a r l y in the day the bands a p p e a r e d to move
slower than the winds blowing p e r p e n d i c u l a r to t h e m . If wind-gene rated
g r a v i t y waves on the frontal surface w e r e r e s p o n s i b l e for the banded precipitation, then it is likely that the r i p p l e s w e r e in the nature of standing
waves and drifted d o w n s t r e a m slowing r a t h e r than moving with the wind.
-44As mentioned e a l i e r , the winds above the frontal surface d e c r e a s e d in speed
in the s a m e m a n n e r that the echoes d e c r e a s e d in speed. However, at all
t i m e s the echoes appeared to move slower than the winds.
Conclusions
This case p r e s e n t s the effects of winds in a " s t e e r i n g l a y e r " upon the
echo movement. P a r a l l e l bands of precipitation aire shown to exist approxim a t e l y perpendicular to the winds in the s t e e r i n g l a y e r . These bands a r e
explained by wind-generated gravity waves on the frontal surface, which drift
downwind at a speed l e s s than that of the winds. A changing of orientation of
these precipitation bands' is shown to a g r e e with the slowly backing winds.
This case p a r t i c u l a r l y i l l u s t r a t e s how synoptic and r a d a r data can supplement
one another.
FIG. 35 PARALLEL PRECIPITATION BANDS OF 17
( 1 0 - M I L E RANGE MARKERS UNLESS OTHERWISE
APRIL 1953
INDICATED)
FIG. 35
PARALLEL PRECIPITATION BANDS OF
( 2 0 - M I L E RANGE MARKERS)
17
APRIL
1953
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