CHARACTERISTICS AND DISTRIBUTION OF PRECIPITATION
AREAS OVER THE TROPICAL ATLANTIC
by
ROBERT MICHAEL IWANCHUK
B.S., Kent State University
(1971)
SUBMITTED IN PARTIAL FULFILLMENT
OF TIE REQUIREMENTS FOR THE
DEGREE OF MASTER OF SCIENCE
at the
MASSACHUSETTS
INSTITUTE OF
TECHNOLOGY
May,
1973
Signature of Author
Department of Mletyorology, April 20, 1973
D
Certified
by
a
m
o
My,-197
......................
Thesis Supervisor
Accepted
by
..................
Chairman,
DeDartrental Committee on Graduate Students
Wr1M5MWN
CHARACTERISTICS AND DISTRIBUTION OF ?RECIPITATION
AREAS OVER THE TROPICAL ATLANTIC
by
ROBERT MICHAEL IWANCHUK
Submitted to the Department of Meteorology on April 20, 1973 in
partial fulfillment of the requirements for the degree of Master
of Science.
ABSTRACT
A detailed analysis is made of radar observations from 11-26
July, 1969 during the fourth phase of BOMEX to (1) study the characteristics of individual tropical showers and mesoscale precipitation areas and (2) to observe patterns and distributions of these
areas in disturbed and undisturbed weather conditions. Tropical
precipitation is characterized by numerous showers. Typicaly
in
during undisturbed conditions, showers were less than 80 mi
area, a value which was used in classifying them as opposed to
larger precipitation areas termed mesoscale. The characteristics
and behavior of 82 showers and 34 mesoscale areas were studied for
their entire lifetimes. Durations of showers were usually less
than two hours with a correlation coefficient between maximum area
and duration of 0.6. A diurnal variation in shower numbers was
observed on undisturbed days with a maximum at2 late night' and early
morning hoursaveraged at 106 echoes/56,000 mi . This was two or
three times as many showers as at the early afternoon minimum.
Continuous precipitation areas greater than 80 mi 2 , called small
mesoscale areas, were generally observed during disturbed conditions.
They were usually between 10 2 and 10 3 mi 2 in area with durations of
one to four hours and the sample had a correlation coefficient of
0.7 between maximum area and duration. Practically all mesoscale
areas and about 1/3 of the showers had areas of higher intensity
of precipitation imbedded in them.
Two wave disturbances and two cyclonic vortices were observed.
Widespread showers with a few areas of mesoscale size characterized
the wave disturbances while more concentrated areas containing
numerous cells and larger individual precipitation areas were observed in the cyclonic vortices.
-
Detailed statistics of size distribution, density, and areal
coverage of echoes are presented. The average ratio of the total
precipitation area to the area under observation never exceeded 9%;
values ranged from less than 1.5% during undisturbed (daytime) conditions to 8.7% in one of the disturbances.
Comparison is made between typical precipitation patternsin
the Barbados area and in New England. Tropical disturbances lacked
synoptic-scale areas of precipitation which are found in extratropical storms and contained a much larger number of showers and
smaller areas. Small mesoscale areas in both latitudes had similar
characteristics. Large mesoscale areas (r,-110 3-104 mi 2 ) are found in
both regions, but in the tropics they are clusters of echoes rather
than connected echoes.
Thesis Supervisor:
Title:
Pauline M. Austin
Senior Research Associate
TABLE OF CONTENTS
page
LIST OF TABLES
LIST OF FIGURES
I.
INTRODUCTION
Background and Objectives
A.
B. Outline
II. DATA AND CLASSIFICATION OF ECHOES AND WEATHER CONDITIONS
A. Data
B. Classification of Echoes and Weather Conditions
III. SURVEY OF SHOWERS AND SMALL MESOSCALE AREAS
A. Life Histories of Showers
B. Occurrence of Showers
C. Typical Undisturbed Day
D. Characteristics of Small Mesoscale Areas
IV. ANALYSIS OF INDIVIDUAL DISTURBANCES
A. Method of Analysis
B. 14-15 July
C. 16-18 July
D. 22 July
E. 25-26 July
F. Discussion of Results
V.
DENSITY AND AREAL COVERAGE OF PRECIPITATION AREAS
A. All Situations
B. Undisturbed Conditions
C. Disturbances
D. Echo Height Distribution
5
6
9
9
12
14
14
16
20
20
22
24
26
29
29
30
32
36
39
43
46
46
49
50
52
VI. SUMMARY AND SUGGESTIONS FOR FURTHER STUDY
A. Summary
B. Suggestions for Further Study
54
54
57
TABLES
58
FIGURES
66
ACKNOWLEDGEMENTS
104
REFERENCES
105
LIST OF TABLES
page
1.
Characteristics of the AN/MPS-34 weather radar (from
Hudlow, 1970).
58
2.
Approximate rainfall rates (mm/hr) for given return
powers and ranges.
58
3.
Evaluation of the synoptic condition governing the
radar coverage sector for each three-hour interval.
59
4.
Number of shower echoes observed to last for specified time intervals as a function of the maximum
area achieved.
59
5.
Average number of shower echoes in the area under
observation (56,000 mi 2 ) according to size and various
other factors.
60
6.
Total number of shower echoes observed and the percent
which had imbedded cells at the next two intensity
levels.
60
7.
Results of the survey of small mesoscale areas.
61
8.
Echo heights for two times during the 25-26 July
disturbance.
62
9.
Average number and coverage of precipitation areas ofvarious sizes for the three synoptic categories.
Area under observation:x36,000 mi2 .
63
10.
Same as Table 9 except for undisturbed conditions
(daytime and nighttime).
64
11.
Same as Table 9 except for the four disturbances.
LIST OF FIGURES
page
1.
Illustration of the areal coverage of the radar.
66
2.
Illustration of the off-center PPI scope.
markers at 25-mile intervals.
67
3.
ESSA 9 satellite photograph sequence.
68
4.
Method of tracking shower echoes.
73
5.
Scatter diagram and linear regression line for maximum
area of showers vs. duration.
73
6.
Distribution of durations of 82 showers in survey.
74
7.
Illustration of shower size classification.
74
8.
Representative precipitation maps for a typical undisturbed day.
75
9.
Diurnal variation in echo numbers plus the number of
imbedded areas at levels 2 and 3 for a typical undisturbed day.
76
10.
Comparison of shower motions with wind sounding for
12 July, 0200L.
76
11.
Scatter diagram and linear regression for maximum
area of small mesoscale areas vs. duration.
77
12.
Example of development and motion of small mesoscale
areas with imbedded areas of more intense precipitation.
77
13.
Evolution of the 14-15 July disturbance (from FernandezPartagas and Estoque, 1970),
78
14.
Representative precipitation maps for the 14-15 July
disturbance.
79
15.
Comparison of shower motions with wind sounding for
15 July, 0200L.
80
16.
Relations between the lower tropospheric flow and
the "inverted V" cloud pattern (from Frank, 1969).
81
Range
page
17.
Evolution of the 16-18 July disturbance (from FPE,
1970).
82
18.
Representative precipitation maps for the 16-18 July
disturbance.
83
19.
Comparison of cloud cover photographed by satellite
with the precipitation pattern on radar, 17 July1422L.
84
20.
Representative precipitation maps for the latter
part of the 16-18 July disturbance.
85
21.
Same as Figure 19 except for 18 July/1325L.
86
22.
Evolution of the 22 July disturbance (from FPE, 1970).
87
23.
Representative precipitation maps for the 22 July
disturbance.
88
24.
Evolution of the 25-26 July disturbance (from FPE,
1970).
89
25.
Representative precipitation maps for the 25-26 July
disturbance.
90
26.
950-mb flow at 25 July/0800L (from Leary, 1972).
92
27.
Shower motions at 26 July/
93
28.
950-mb flow at 26 July/0800L (from Leary, 1972).
93
29.
Comparison of cloud cover photographed by satellite
with precipitation pattern on radar, 25 July/1430L.
94
30.
Same as Figure 29 except for 26 July/1334L.
95
31.
Comparison of precipitation pattern with divergence
field on 25 July/0800L.
96
32.
Same as Figure 31 except for 25 July/2000L.
97
33.
Same as Figure 31 except for 26 July/0800L.
98
34.
Cumulative distribution showing the average number
of echoes smaller than the indicated size for the
three synoptic conditions.
99
0800L.
page
35.
Cumulative
distribution showir7 the average per-
99
cent of the total precipitation area covered by
echoes smaller than the indicated size for the
three synoptic conditions.
36.
Cumulative distribution showing the average percent of the total area under observation covered
by echoes smaller than the indicated size for
undisturbed conditions (daytime and nighttime),
mildly disturbed, and disturbed conditions.
100
37.
Same as Figure 35 except for undisturbed conditions
(daytime and nighttime).
100
38.
Same as Figure 35 except for the four disturbances.
101
39.
Same as Figure 36 except for the four disturbances.
101
40.
Same as Figure 38 except with echo size plotted on
a logarithmic scale.
102
41.
Same as Figure 39 except with echo size axis
extended and plotted on a logarithmic scale.
102
42.
Cumulative distribution showing the average percent
of echoes with tops less than the indicated height
for the three synoptic conditions (derived from
Hudlow, 1970).
103
43.
Distribution of echo heights for the three synoptic
conditions.
103
I.
A.
INTRODUCTION
Background and Objectives
Quantitative radar observations have shown that precipitation
areas of several scales and intensities occur simultaneously in all
types of storms.
At one end of the scale is the synoptic precipita-
tion area which is associated with the large-scale upglide motions
which are part of the large-scale circulations.
At the other end
are the small relatively intense showers produced by cumulus convection.
Intermediate between these two extremes of scale and intensity
are mesoscale precipitation areas.
The study of subsynoptic-scale
precipitation areas is important not only in relation to precipitation mechanisms but because they cast some light on the nature of
subsynoptic-scale circulations and their relation to larger-scale
ones.
Measurements of meteorological quantities on the synoptic-scale
are abundant because they can be obtained from networks which have
the density of the regular synoptic reporting stations.
Direct mea-
surement on the subsynoptic-scale of many quantities is not feasible
because of the high density of the required observing network.
Pre-
cipitation, however, is one quantity which can be measured on this
scale by a suitably instrumented radar.
Some studies have been made of precipitation patterns in temperate latitudes.
A fairly systematic analysis of precipitation patterns
in New England has been made by Austin and Houze (1972) and Reed (1972).
10.
The former found that there is fairly extensive cellular cumulus
activity in most storms in New England.
These cells were invariably
found imbedded in small mesoscale areas of precipitation (,-,100 km 2
which have rather well defined characteristics and appear to have a
close relationship with the cells which they contain.
In more wide-
spread storms the small mesoscale areas are imbedded in larger mesoscale areas of lighter precipitation (--000 km2).
The eight New
England cyclones observed by Reed exhibited a marked banded precipitation structure.
Virtually all of the cells and heavy rain were
concentrated into these bands and large mesoscale areas which in turn
were organized into "belts" which maintained a fixed position with
respect to the cyclone, extending from the cyclone center to approximately 500 km ahead of the center.
Browning and Harrold (1969)
analyzed a wave depression over England in great detail.
They observed
bands with cells in them ahead of the warm front and parallel to it
and they also found bands in the warm section which were parallel to
the winds there.
California coast.
Elliot and Hovind (1964) studied four storms on the
Each contained bands of precipitation that were 20
to 40 miles wide and 30 to 60 miles apart (between centers).
These
bands contained all the cells and were trackable for up to three hours
or 100 miles.
The orientation of these bands appeared to be associ-
ated with the wind shear between the convective cloud layer and the
adjacent layer above in a manner similar to that suggested by Malkus
and Riehl (1964) for the occurrence of highly organized cloud rows
over the tropical Pacific Ocean.
The latter have a predominant mode
11.
parallel to the trade winds since the convective cloud layers in the
tropics are mostly imbedded within the trade winds.
However when
they extend up to the stronger she-x zones aloft a crosswind mode
may dominate which is the usual condition in middle latitudes.
Actual radar measurements and analysis of tropical precipitation
on a systematic basis are scarce except for the study of spiral rain
bands in tropical cyclones (see for example, Fujita and Black, 1970).
The major convective rainbands in tropical cyclones are generally
oriented in circular or spiral bands, which converge near the center
of the storm.
The upward motion in these storms is concentrated in
these rainbands and especially in the eyewall cloud and outer rain
bands.
The percent area coverage with radar echoes (i.e., degree of
convective activity) decreases rapidly with height so that above 10
km only a small percent (usually less than 5%) of the area within
200 nautical miles of the storm center is covered with echoes.
From May through July of 1969, the Barbados Oceanographic and
Meteorological Experiment (BOMEX) was conducted in part to examine
the structure of tropical convective systems.
In support of this
objective, the Atmospheric Sciences Laboratory of the U.S. Army
Electronics Command maintained time-lapse photography of an X-band
weather radar scope located on the island of Barbados.
Their goal
was to acquire information concerning the origin, development, movement, size, and intensity of precipitation areas associated with
tropical weather disturbances originating or moving within range of
the radar.
Investigations on some aspects of the weather radar
12.
research during BOMEX was made by Hudlow (1970).
In the present
study, a much more comprehensive analysis is made of the radar observations during the fourth phase of BOMEX, 11-28 July.
The aim is two-
fold, (1) to study the characteristics of individual tropical showers
and mesoscale precipitation areas, and (2) to observe patterns and
distributions of precipitation areas in disturbed and undisturbed
conditions noting any similarities and differences between typical
patterns in tropical and temperate latitudes.
Radar data in the form of photographs of scope displays do not
lend themselves to computer processing, so that although a large
amount of data can be taken, only a limited amount can be subjected
to thorough analysis.
The current need is for quantitative radar data
to be stored in digital form on magnetic tape.
This calls for devel-
opment of scanning routines which will maximize information and minimize redundancy.
By considering the nature of precipitation patterns
and distributions over the tropical Atlantic, this study may serve as
a basis for developing such a detailed measurement schedule.
B.
Outline
The data utilized in this study, the classification systems for
the precipitation areas, and the methods used in formulating them are
discussed in Chapter II.
In Chapter III, characteristics of individual showers and small
mesoscale areas are investigated.
Showers are also studied in a
13.
statistical manner in relation to their diurnal variation, and the
shower pattern of a typical undisturbed day is shown.
Four disturbances which affected the BOMEX region are analyzed
in Chapter IV.
The types and patterns of the precipitation areas are
presented both verbally and through the use of representative figures.
Quantitative analysis of the size distribution and the total
areal coverage of precipitation areas for various synoptic conditions
is in Chapter V.
The final chapter summarizes the results, presents some comparisons between tropical and New England precipitation areas, and gives
recommendations regarding further studies.
14.
II.
A.
DATA AND CLASSIFICATION OF ECHOES AND WEATHER CONDITIONS
Data
The main source of data for this study was radar data in the form
of 16 reels (100 ft per roll) of 35-mm film containing time-lapse photographs of the Plan Position Indicator (PPI).
Description of the radar
and the mode of operation are given by Hudlow (1970) and summarized
here.
12'
The radar was a van-mounted AN/MPS-34 located on Barbados, at 130
N, 590 320 W.
It
was positioned at a vantage point on the island
with the antenna at an altitude of 950 ft above mean sea level thereby
providing an extended radar horizon.
nominal 0
greater.
The elevation angle was at a
permitting detection of targets at ranges of 200 miles or
Characteristics of the radar are listed in Table 1.
Figure 1
illustrates the extent of the areal coverage realized by the radar.
PPI photos depicted an off-center scope (Figure 2) so that a magnification of the prime geographic areas could be realized.
clutter persisted out to 30-40 miles.
na to 7
for a sector of about 100
Ground
A cyclic elevation of the anten-
in azimuth resulted in an almost
total loss of echo information to the west of Barbados.
This was a pre-
cautionary measure to prevent excessive doses of microwave radiation to
residents living west of the radar.
In addition to normal PPI photographs at full receiver gain, an
intensity level sequence was taken every hour by reducing the receiver
gain in known steps.
A total of five gain steps yielded increments of
15.
18 db, 8 db, 8 db, 8 db, and 6 db for steps 1 through 5 respectively.
In this study, level 1 will refer to normal (full receiver) gain,
leve" 2 to gain step number 1 or 1S db less and so on.
Rainfall
rates (R) can be empirically related to the radar reflectivity factor
(Z) of the target.
Some rainfall rates for given return powers and
ranges using the AN/MPS-34 radar constant and an empirical Z-R relationship for Marshall Island drop-size spectra are in
echoes were rarely observed above level 3.
Table 2.
Radar
Therefore these rainfall
rates appear to be extremely low for tropical precipitation which
leads one to suspect an error in radar calibration.
Even if there
should be a sizable error in the calibration, the AN-MPS/34 is a sensitive radar which should detect light rain (1-1
of the area involved.
mm hr
1) over most
Therefore the data are considered adequate
to depict the quantities considered in this study, i.e. number and
size of precipitation areas and the extent to which they contain small
cells of higher intensity.
Height sequences (consisting of PPI photos for each 0.5
0
in
elevation up to the elevation where all echoes disappeared) were taken
twice daily with a polaroid camera although not every day. Computation
of these data by Hudlow (1970) was used in this study.
A meteorological satellite provided photographic coverage of
the data network during BOMEX.
The polar orbiting satellite, ESSA 9,
made one pass per day over the BOMLEX area at an altitude of 910 statute miles.
A sequence of these photographs for selected days over
the period is shown in Figure 3.
Pictures from this satellite were
16.
used to get a general idea of the cloud patterns associated with precipitation areas on radar and to track the movement of disturbances
through the area.
Upper air rawinsonde soundings, in the form of teletypereported winds, taken at Barbados were used to compare echo motions
with the observed winds.
B.
Classification of Echoes and Weather Conditions
A preliminary survey of the radar film was taken early in this
study to obtain a general impression of the sizes and shapes of echoes
observed during the 16-day period.
Particularly noted was the predo-
minance of small rather widely scattered precipitation areas at almost
all times in
the period.
In addition,
larger areas were observed at
times of disturbances, which were identified from other studies of
the BOMEX fourth phase weather phenomena (Fernandez-Partagas and
Estoque, 1970; Janota, 1971; and Leary, 1972).
From the preliminary
survey, it seemed that the largest echoes which appear on undisturbed
days were approximately 10 miles in diameter.
Therefore areas of
this size or smaller are denoted showers while the larger ones, which
typically appear only during disturbed periods, are called mesoscale
areas of precipitation.
However, since the occurrence of elongated
echoes made classification by dimension difficult to implement
(e.g. 10xlO mi vs. 10xl mi), the limit was set at an area of 80 mi2
(approximately the area of a circle 10 miles in diameter).
somewhat from that of Austin and Houze (1972)
It differs
who defined "cells",
17.
presumed to be single cumulus convective elements, as areas smaller
than 50 km2 while any larger ones were called mesoscale areas.
Their
study, however, was concerned witn smaller-scale precipitation aceas
imbedded within larger ones.
Here, where sizes of separate areas
are considered, the application of the term mesoscale only to areas
in excess of 80 m
is convenient; there is no implication that
smaller areas represent single convective cells.
Two studies were made on the showers.
In the first, an analysis
of characteristics and behavior of individual shower described in
2
Chapter III, showers were stratified into size intervals of 0-5 mi,
2
2
6-20 mi, and 21-80 mi . This classification was based on circular
areas with diameters of 2.5, 5, and 10 miles.
The study of shower
2
size distribution in Chapter V used size intervals of 0-10 ni,
2
2
mi, and 21-80 mi.
11-20
The decision to increase the first interval by
2
2
5 mi was due to an overabundance of echoes in the original 6-20 mi
interval.
Consistency of such a classification suggests a further
2
division at 40 mi, thereby doubling the area for each group, but the
time-consuming reality of an additional interval ruled it out.
On the mesoscale, another classification was made.
During
disturbed periods both numerous showers and some larger precipitation
areas occurred.
Sometimes they appeared to be randomly scattered but
at others they became concentrated into a recognizable feature such as
a band or a more or less circular or oval group.
These concentrated
areas moved as a single unit and after persisting for many hours
18.
decayed as a unit although they were made up of individual non-connected echoes.
The individual connected areas larger than showers
were called small mesoscale areas (SMSA's) and a concentrated area
of echoes denoted a large mesoscale area (LMSA).
ally between 100-1000
ni2while
SMSA's were gener-
LMSA's were 10 3-104 mi 2which includes
the clear areas between the non-connected components.
The large
areal extent and long durations of LMSA's made it difficult to track
them for their entire lifetimes.
Except for the lower limit of the SMSA class, these classifications based on the precipitation patterns observed in the Barbados
area are the same as those set up by Austin and Houze (1972) for
New England storms.
They further define a synoptic area of precipi-
tation as greater than 104 km2 and found such areas associated with
each of the cyclonic storiis in their sample.
No such large precipi-
tation areas were found in the BOMEX data sample, nor were connected
areas of LMSA size observed.
As already mentioned, there were notable differences in precipitation patterns between disturbed and undisturbed periods.
The
criteria for classifying a period as being disturbed were such
synoptic-scale phenomena as wave troughs or cyclonic vortices at
the surface, middle and/or upper troposphere.
Disturbed times were
determined from analytic work done by Fernandez-Partagas and Estoque
(1970), hereafter referred to as FPE, and Janota (1971).
They noted
four disturbances which affected the coverage area during the perti-
19.
nent time period.
A two-step method fo2 classifying three-hour time
intervals into synoptic categories was devised.
to classify each day as disturbed
or
The basic scheme was
undisturbed according to the
above mentioned synoptic studies and to further refine this into
three-hour intervals by observing the state of activity on the
radar PPI.
It became apparent that a transitional category was
needed to breach the gap between undisturbed and fully disturbed.
This category was called mildly disturbed.
A certain amount of sub-
jectivity was involved in this differentiation.
Table 3 lists the
results of this classification for the 16-day period.
In the studies of undisturbed periods, the data were sometimes
classified as either daytime or nightime observations.
This classi-
fication was employed to study the diurnal variation of showers which
is characteristic of und'sturbed days (Riehl, 1954).
Two different
methods were used for taking daytime and nightime observations.
In
the first, PPI tracings were selected twice daily at approximately
the same daytime and nightime hours.
These hours were characteristic-
ally times of minimum and maximum echo activity.
In the second method
used in Chapter IIIB, observations taken at all hours were classified
as day or night depending on the local sun time.
Daylight hours in
the area, neglecting twilight, were 0700 to 1700 local time; 2000 to
0500 L were nightime hours.
20.
III.
A.
SURVEY OF SHOWERS AND SMALL MESOSCALE AREAS
Life Histories of Showers
To study the characteristics of individual showers,
82 showers
were observed for their entire lifetimes; specifically, the total
duration and the area at various times were recorded.
Selection was
random and involved a good deal of subjectivity, but care was taken
to cover the entire size range and both disturbed and undisturbed
periods.
No recognition was made, though, as to the synoptic condi-
tions under which these showers were observed.
Echoes were tracked
by tracing them from projections on a film viewer from one PPI sequence to the next as in Figure 4.
Sometimes echoes could not be
tracked for their entire lifetime for one or more of the following
reasons:
1) deformation of an echo causing a division;
2) merging of the original echo with another; and,
3) long elapsed time between successive PPI frames.
The selection was restricted to fairly isolated echoes because of the
high probability of showers merging in densely populated areas.
As
long as a shower was recognizable as the original one through any
mergings and/or divisions, it was retained as part of the sample.
Showers were classified on the basis of maximum size during
their lifetime.
The larger the size of the shower became, however,
21.
the more difficult it was to follow its life history.
Showers-greater
than 30 rd usually contained two or more smaller imbedded echoes of
higher intensity.
This fact made it more probable that a shower of
such size would divide into these components during its lifetime and
even more likely as it was dissipating.
The larger echoes more fre-
quently merged with other echoes making recognition of the original
one difficult.
A more appropriate name for these large showers might
be cell clusters or cell complexes.
A typical shower lifetime might be characterized qualitatively
by the stages of development, division
and disappearance.
and/or merging, dissipation,
The variability of these processes other than
beginning with development and ending with dissipation allows no precise set of rules for governing shower behavior.
The development may
begin as the appearance of an echo where none existed minutes earlier
or by division of one echo into two or more.
loped by the former process were used.
Only echoes which deve-
The rates of development and
dissipation can be quite rapid, a matter of one or two minutes, or
they may last for several minutes.
Shower growth may be either by
the expansion of an echo, the growth of new showers immediately next
to the old one, or the merging of new showers with the original.
Divisions or mergings may be totally absent in the lifetimes of some
showers but the probability of these processes increases for larger
showers.
There appeared to be no preference for showers to divide
and then merge or vice versa.
22.
Comparison of maximum area and duration of showers, summarized
in Figure 5 and Table 4 shows that larger showers usually have longer
lives.
The correlation coefficient was computed to be 0.6.
Table 4
further reveals a substantial range in duration within each size
category exemplifying the variable nature of showers.
The difficulty
in tracking large showers for their total duration is reflected in
their small number in the sample.
The longer durations of the larger
showers implied that these echoes contained more than one cell during
their lifetime.
This was generally observed from the intensity level
sequences and also from the shape of the echo.
The frequency distri-
bution of the durations of the sample showers is given in Figure 6.
Shower motions were observed but differed substantially on
disturbed and undisturbed days.
They are therefore considered
separately for the individual disturbances (Chapter IV) and for undisturbed days (Chapter IIIC).
B.
Occurrence of Showers
This section deals with the statistical aspect of showers in
relation to their day/night variability of occurrence and according
to whether they were taken in disturbed or undisturbed conditions.
PPI tracings of the total precipitation area were taken twice daily,
one at approximately midday and another at late night or early
morning.
These times were chosen because they have been shown to be
statistically the hours of minimum and maximum convection in this
23.
region (Hudlow, 1970).
The area under observation was inside a
square surrounding the radar site and between azimuths 330
as shown in Figure 7.
2
imately 56,000 mi.
and 1700
The total area under observation was approx-
These tracings were taken for all days except
when the radar was not operating.
a transparent overlay grid.
2
2
of 0-5 mi, 6-20 mi, and 21-80
Echo areas were determined using
Echoes were grouped into size intervals
2
ml, and the number of echoes in each
interval was counted and tabulated.
Averages were found for qach size
interval for the 10 daytime and 14 nighttime observations.
Similar
averages were determined for day and night on only undisturbed days
and for undisturbed and disturbed respectively without regard for
time of day.
It should be mentioned that echoes with areas greater
than 80 mi2 appeared on a number of the tracings but were ignored in
this analysis, which is concerned only with showers.
Table 5 gives the results of this survey.
There is a notice-
able diurnal variation in shower numbers with an increase in showers
from day to night.
For all days, this increase is about a factor of
two and becomes a factor of three on undisturbed days only.
Hudlow
has attributed this diurnal variation to the differential cooling
between the sea surface and the overlying layers of moist air during
late night and early morning hours.
For undisturbed days, not only
are there three times as many showers at night as in the daytime but
also there is a tendency for them to be bigger, as seen in the relative
percentages.
There is an increase of abcut 40% in the total number of
showers in disturbed situations as compared with undisturbed ones
24.
(Table 5c).
category.
The most pronounced increase is in the largest size
It should be realized that on disturbed days, there are
additional mesoscale areas which ar2 not included in this count.
An investigation of tropical shower intensities was attempted
based on the same group of observations which were all at times of
when intensity level series were taken.
The number of level 1 echoes
which contained imbedded cells or small areas of levels 2 and 3 was
recorded without any grouping according to synoptic condition or
time of day.
The results are listed in Table 6.
It shows that an
increasing percentage of echoes contain higher intensity levels as
echo size increases.
Because of the lack of range normalization and
uncertainty in the radar calibration, no attempt was made to interpret these results in terms of actual rainfall intensities in the
showers.
C.
Typical Undisturbed Day
Since showers are so characteristic of undisturbed days, a
single day was chosen to examplify the distribution of showers and
the diurnal variation common to this type of day.
The selection of
12 July was based on the fact that it was in the middle of three
relatively undisturbed days at the beginning of Phase IV.
Figure 8
contains four PPI tracings for this day showing the distribution and
variation in echoes.
Figure 9 shows the variation in echo numbers
as well as the number which contained areas at intensity levels 2
and 3.
25.
There was often a tendency for showers to occur in clusters
or only in one quadrant of the radar scope as for example in the
northeast quadrant at 1702L, Figure 8.
in nature.
Such occurrences seem random
The organization of shower echoes into well-defined rows
was not readily apparent during undisturbed days but there was a
tendency for larger showers to become elongated in the direction of
motion.
A typical cloud pattern for undisturbed day is characterized
by the scattered clouds as seen in the square area bounded by longitudes 60 W and 55 W and latitudes 25 N and 20 N on 24 July (Figure 3).
Shower motions were observed early on the morning of 12 July
and were compared with the wind sounding at 0200L from Barbados
(Figure 10).
Shower directions varied by 22
from the ENE, the prevailing wind direction.
to 16 knots.
near 6000 ft.
but all were generally
Speeds ranged from 11
Best agreement in direction and speed is with the wind
This example is typical of other undisturbed days when
shower motions were observed.
Showers generally moved with the low-
level winds but an exact steering level was difficult to determine
because the variation in wind direction in the low troposphere was
small.
On undisturbed days, the easterly current of the trade winds
ranged in depth from 16,000 to 30,000 ft.
Echo heights on undisturbed
days derived from Hudlow's data (5 observing times from 20, 21, and
23 July) show no echoes over 17,500 ft for two daytime observations
and heights up to 40,000 ft for three nightime observations.
The
majority of echoes were under 12,500 ft for the two daytimo observations and less than 14,500 ft at nighttime.
26.
D.
Characteristics of Small Mesoscale Areas
Small mesoscale areas were observed in a manner similar to
that employed in studying the characteristics of showers.
Thirty-
four SMSA's were selected more or less at random but subjectively
to cover a fairly wide size range and different synoptic conditions.
SMSA's were defined as precipitation areas larger than 80 mi2 which
remained connected during their lifetime.
The selection process was
hampered in the same way as for showers by difficulties in distinguishing the original echo under observation through a number of divisions and mergings.
With small SMSA's, it is particularly difficult
because they are "flaky" and have a high likelihood of breaking up.
With the larger ones, this problem was not as pronounced because they
retain their identity more effectively.
The development of SMSA's was by three ways.
the merging of showers in a densely populated area.
One method was
The second pro-
cess was by the expansion of an established cell cluster or complex
2
into an area greater than 80 mi .
This second process is actually'
similar to the first since the cell clusters often form by the mergings of small showers which were originally close together but
separate.
The third process was a kind of "fission" where a SMSA
2
divides into two new areas which are both greater than 80 mi .
The
mature stage of a SMSA was generally characterized by a continual
change of shape and size through mergings, divisions, and internal
deformation.
Many SMSA's, however, maintained their overall shape
27.
such as oval or elongated and were easier to track.
The dissipation
stage was typically seen as a shrinking of the whole area followed
by a break-up or division of the SMSA into many showers.
At otner
times, SMSA's just divided unexpectedly into areas too small to be
called mesoscale areas by the definition in use.
When a division
.2
left some components larger than 80 mi , a subjective decision was
made as to whether the original area was still evident and trackable
or the duration should be ended.
Each SMSA studied was classified by the maximum area attained
in its lifetime.
Tracings were made, wherever possible, at times
with intensity level sequences.
The average number and maximum area
of imbedded echoes at level 2 was determined for each SMSA.
Echoes
were rarely observed above level 3.
As noted from the preliminary survey, mesoscale precipitation
areas occur mostly during disturbed periods.
Therefore of the 34
areas studied, 21 were from disturbed periods with 11 at mildly disturbed times and only 2 on undisturbed days.
The range in maximum area of the sampled SMSA's was 80-1150
.2
mi .
Generally, they were between 100 and 500 mi
Table 7 lists the characteristics of 34 SMSA's.
from 1 hr 10 min to 7 hr 30 min.
2
at their maximum.
Durations ranged
The average maximum area was 363 mi2
and the mean duration was 183 minutes.
Mean sizes and durations are
probably not as significant as distributions because of the rather
small sample and casual sampling technique.
Figure 11 is a scatter
28.
diagram of maximum area vs.
ous range of areas up to about 600 mi
up on four occasions.
There appears to be a continu-
duration.
2
with much larger ones showing
The correlation coefficient between size and
duration was computed to be 0.7 when not considering the four SMSA's
greater than 800 mi 2
The average number of imbedded echoes in each SMSA was 4.8.
2
They ranged in size from 5 to 30 mi2.
Some imbedded areas reflected
the shape of the SMSA, especially the elongated precipitation areas.
This type often had a thin continuous line of imbedded precipitation
or non-connected echoes oriented along the axis of elongation as in
Figure 12.
Generally, imbedded areas were round with an area less
than 10 mi2 but they became larger as the size of the SMSA increased.
29.
IV.
A.
ANALYSIS OF INDIVIDUAL DISTURBANCES
Method of Analysis
Four synoptic-scale disturbances were evident during Phase IV
of BOMEX.
Fortunately, at least a portion of the 2000 azimuth sector
for observation of precipitation experienced precipitation areas
related to each of these storms.
In order to study the types and dis-
tribution of precipitation areas from these disturbances, the radar
PPI was observed for the duration of each storm and selected PPI
traces were taken at enough times to provide an adequate and representative composite of the rain patterns.
A synoptic analysis of the evolution and type of disturbance
is presented for each storm.
This information came essentially from
three sources; a preliminary report on meteorological conditions
during Phase IV of BOMEX by FPE (1970), an analysis of conditions
during the period 13-21 July by Janota (1971), and a case study of the
last disturbance by Leary (1972).
ESSA 9 satellite photographs pro-
vided additional information on associated cloud cover.
The synoptic
analysis is followed by a description of the precipitation pattern.
Whenever possible, shower motions were analyzed in relation to the Barbados rawinsonde sounding.
The motion of mesoscale areas was difficult
to determine because of their constant change in shape and hence were
not matched with the wind soundings.
The analysis of the final dis-
turbance will be in more detail because of the availability of more
data.
30.
The identifying dates for each disturbance are those days on
which they affected the radar coverage area.
B.
1.
14-15 July
Synoptic Situation
This weather system was first detected from satellite pictures
as a broken cloud field near 14 N, 44*W on 12 July and moved to 14 N,
49 W by 13 July.
The earliest history of this travelling disturbance
can be traced back to an African origin (Janota, 1971).
By 14 July,
this disturbance had intensified considerably and appeared on satellite photographs as a large cloud cluster extending over a region 8
in latitude and 5
in longitude.
Throughout this early period, the
disturbance appeared on synoptic analysis charts as a wave trough at
700 mb with the associated cloud cluster to the east of the trough
axis in a manner suggestive of a classical easterly wave.
An upper-
level cyclonic circulation developed at 35,000 ft by 15th/0800L near
0
16 N, 59 W.
Y
The 700-mb perturbation weakened on the 15th and was
difficult to locate by 0800L.
The upper-level cyclone intensified
during the 15th and moved northward out of the BOMEX region.
evolution of this disturbance is shown in Figure 13.
The
Satellite pic-
tures were viewed but were not in a form for presentation in this
study.
2.
Precipitation Pattern
The radar coverage of this disturbance was limited.
Data were
missing from l4th/1000 L, at which time echo activity was beginning
31.
to appear 200 miles to the east of the radar site, until 15th/O0OOL
when the leading edge of the precipitation reached Barbados.
The
resumption of data showed the westward movement of a scattered area
of precipitation which moved out of the eastern sector by 15th/0800L,
(see Figure 14 for representative precipitation maps).
The most notable feature of the precipitation was the scattered
area of echoes which gave the appearance of being in a band.
Such a
band is not readily seen in the selected PPI's but is more inferred
from a knowledge of the orientation of the cloudiness at this time.
The length of this "band" extended through the coverage sector in
both north and south directions.
This is in agreement with the N-S
oriented 700-mb wave axis.
Individual precipitation areas varied in size but none exceeded
2
500 mi2.
2
The larger precipitation areas (greater than 30 mi ) showed
a tendency to be elongated in a NW-SE direction. In terms of numbers,
2Y
echo sizes less than 10 mi2 predominate but cell clusters and SMSA's
contribute more to the total area over which precipitation is occurring.
This aspect of size distributions is covered in more detail
in Chapter V.
The pattern of precipitation was constantly changing within
any mesoscale precipitation area but the overall orientation of the
band of scattered precipitation remained fairly constant.
The 0200L wind sounding on the 15th from Barbados shows a deep
basic easterly current with increasing velocities to about 12,000 ft.
32.
The shower motions observed at this time show a bias of 10
the right of the direction of the low-level winds.
were
0
0
to 20
to
Shower speeds
much larger than on undisturbcd days with the best agreement
with the winds between 7,000 and 9,000 ft.
Larger echoes showed a
tendency to be elongated in a NW-SE direction which is in the direction of the shear vector between the lower-level winds and the winds
at 12-14,000 ft.
C.
16-18 July
1.
Synoptic Situation
As in the 14-15 July disturbance, the first evidence of this
storm was seen on satellite photographs.
0
As early as the 15th, cloud-
On the 16th, an arc-shaped band of
iness could be seen near 50 W.
0
cumuliform clouds was near 56 W.
A rapid intensification of the cloud
mass on the 17th was associated with a 700-mb wave which formed in
situ near 56 W with clouds in the vicinity of 60 W.
ment of 6
The steady move-
longitude per day implied a wave speed of 15 knots.
Fig-
ure 3 presents satellite photographs of the cloud cover for the 16th,
17th, and 18th.
Janota (1971) analyzed the formation of the 700-mb wave as a
moderate perturbation on the easterly jet.
The resultant intensifi-
cation of cloudiness was found to be in a cyclonic vorticity maximum
induced by horizontal shear south of thejet.
Frictionally induced
convergence in the positive vorticity field became better organized
on the 17th and allowed the storm to tap the high moisture content
in the boundary layer.
33.
The major portion of cloudiness precedes the wave axis at 700mb in contrast to the first disturbance where cloudiness was east of
the wave axis.
The shape of the cloud bands or streaks (Figure 3,
17 July) as well as the vorticity maximum (Janota, 1971) resemble the
"inverted V" cloud pattern described by Frank (1969) (see Figure 16).
In this variation of an easterly wave, cloudiness is predominant on
both sides of the wave axis.
The 700-mb wave disappeared within a few hours after 17th/1400L.
FPE show that a new wave formed at 8000 ft by 17th/2000L near 620W in
association with a slowly moving trough seen in their 20,000 ft analyses.
This system progressed northward with an extensive area of
cloudiness associated with it.
The development of this new system is
significant in regards to the precipitation areas observed by the
island radar.
Although this new system moved northward out of the
Barbados area, a remarkable increase in the intertropical convergence
zone (ITCZ) cloudiness occurred over the Atlantic along a line extending east-southeastward from the main cloud mass and through the radar
coverage sector.
2.
Figure 17 shows the evolution of the disturbance.
Precipitation Pattern
The orientation of the cloud system from NE to SW brought the
first noticeable precipitation from this disturbance into the southern
half of the coverage sector by the afternoon of the 16th.
There was
a gradual increase in precipitation to the north as the arc-shaped
34.
line of clouds
roved westward.
Precipitation covered the whole sec-
tor in a scattered fashion by early morning of the 17th.
The precip-
itation pattern was similar to tha; of the 14-15 July disturbanoC
in
the widespread scattered nature of the rain and the fact that it
consisted mostly of showers or shower clusters.
Precipitation areas
became larger in size in the northern part of the sector as the northern portion of the arc-shaped band of clouds moved westward into the
radar coverage sector.
This increase might be attributed to the
advance of the trough line to the rear of the first sign of precipitation or associated cloud band as seen in Figure 16.
This figure
shows that the cloud bands have a cross-flow orientation and are
probably more closely aligned to the lower-level wind shear than to
flow at any one level.
Frank states that frequently the "V's" were
considerably flattened or rounded.
Representative precipitation maps for four times during this
disturbance are presented in Figure 18.
The largest precipitation
areas were observed on the afternoon of the 17th to the north of
Barbados with one SMSA greater than 500 mi
2
.
This major activity
occurred at the time of the passage of the trough axis.
This active
area moved northward out of the sector and was followed by very widely scattered shower echoes.
Figure 19 shows a portion of the ESSA 9 satellite photograph
at 1325L enlarged to the same scale as the PPI trace at this time.
By crude estimation, radar echoes cover less than 1/3 the area
covered by clouds.
This suggests that clouds spread considerably
35.
beyond the actual precipitation areas.
Clouds may be in different
stages of development, some actively precipitating, some probably
not sufficiently developed for precipitation to have
some possibly dissipating.
all the same height.
begun, and
Furthermore, the cloud elements are not
At a range of 63 miles, the AN/MPS-34 beam
center was at 3,000 ft; at 113 miles it was at 7,330 ft; and at 138
miles range it was at 10,500 ft (Hudlow, 1970).
At long ranges only
the taller echoes are being sampled.
The development of the new upper-level trough brought widespread cloudiness to the islands north of Barbados on the 18th and
19th but more significant in the relation to the radar coverage was
the northward penetration of the ITCZ in conjunction with the disturbance to the north.
Figure 3 of satellite photographs on the
18th and 19th reveals this cloud line trailing from the main cloud
mass to the NW.
This band of clouds is reflected in the radar PPI
traces as a persistent area of convective activity which gradually
moved southward during this period, (see Figure 20 for representative PPI's at this time).
2
Echo areas were generally less than 80 mi
2
although some areas were observed up to 400 mi .
The elongation of
echoes in the same direction as the orientation of the cloud band
was clearly evident at times.
A comparison of the radar echoes and
the cloud cover is shown in Figure 21.
The ratio of the echo area
to the cloud cover was estimated to be about 1/4.
The resolution of
satellite photograph enlargement prevented the detection of clouds
36.
associated with the smaller echoes.
A somewhat similar comparison
by Martin and Suomi (1972) did not produce quantitative results, but
they reached the same qualitative ,onclusions.
Their study compared
selected enlarged BOMEX radar images for seven days in July with
nearly simultaneous brightness-contoured ATS-satellite images.
They
noticed almost a one-to-one correspondence between small echoes and
small isolated clouds.
They also found a consistent agreement between
very bright areas and large echoes in a variety of synoptic situations.
The only available information of echo heights (Hudlow, 1971)
indicated that some echoes were greater than 40,000 ft in height on
the 18th.
The majority of echo tops were between 10,000 and 26,000
ft.
D.
22 July
1.
Synoptic Situation
FPE (1970) describe this disturbance as an interesting case
of interaction between the ITCZ and upper-level cyclone development.
The evolution of this system is illustrated in Figure 22.
The early
history appeared to be associated with a small low-level ITCZ cyclonic
vortex.
By 20th/2000L, a 25,000-ft upper-level cyclone was detected
near Barbados.
This system moved slowly northwestward over the
Lesser Antilles chain.
It intensified as indicated by a closed cir-
culation at 16,000 ft by 22nd/0800L.
At the time precipitation was observable by the radar, the
upper and middle-level cyclone was located well to the northwest of
37.
the main area of cloudiness.
It appeared that this main cloudiness
was associated with the ITCZ which had moved northwestward from the
previous day in a northward "bending".
On subsequent days as the
cyclone moved northward, the main cloud mass became completely detached from the ITCZ.
2.
Precipitation Pattern
The position of the ITCZ in association with the upper-level
disturbance on the morning of 22 July permits a good view of the
radar echo distributions within the ITCZ.
The stage of ITCZ develop-
ment differs from that observed on the 18th and 19th.
days the ITCZ was a single well-developed band.
During those
On 22 July the ITCZ
was not a continuous band of cloudiness but was fractured and illdefined with an isolated region of cloudiness southeast of Barbados
being the prominent feature.
This cloud mass appeared to break away
from the ITCZ at a time of dissipation and to move northward in conjunction with the intensifying cyclone to the north.
Throughout the morning of the 22nd, a large mass of echoes was
observed to the south of Barbados.
This area moved west and could
The
not be tracked because of the blackout of the western sector.
concentration of echoes was in marked contrast to the scattered
echo pattern of the two previously discussed disturbances.
Figure 23
shows the representative precipitation patterns for various stages
of the disturbance.
The coverage of precipitation in this disturb-
ance was limited since it brushed by Barbados to the south.
However,
38.
the total area of precipitation was the largest observed in this
One connected area was approximately 3600 mi
study.
2
.
Such an area
forms by the mergings of many SMSA's and showers in a densely populated area.
T1he total areal extent of precipitation in this disturb-
ance made it a good example of a large mesoscale area as described
in Chapter IC.
A LMSA is made up of showers and SMSA's and continu-
ally changes in appearance and size.
Although the precipitation
areas are not connected, the whole pattern moves as one unit.
tion of LMSA's usually exceeds four hours.
Dura-
Their large areal extent
and duration make it difficult to track these areas for their entire
lifetimes.
This LMSA was a particularly good example because the
concentration of precipitation areas contrasted with the generally
clear region to the north.
Its position with respect to the radar
precluded any total areal measurement although in its largest observed
extent it had dimensions of 100 by 150 miles including the gaps in
between individual echoes.
It was observed for 10 hours.
The unusually large total area of precipitation in this disturbance likely results from the well-correlated strip of boundary
layer positive vorticity and convergence, and the ITCZ cloud band.
Such an organized region of convergence was not evident in the preceding
two disturbances (Janota, 1971).
The growth of tropical
disturbances has been suggested to follow a mechanism called conditional instability of the second kind (CISK) by Charney and Eliassen
(1964).
In its simplest form, this hypothesis stresses an in-phase
relationship between cyclonic vorticity and convergence in the
39.
Ekman layer.
In a tropical disturbance, a region of cumulonimbus
cells warms the atmosphere through the release of latent heat; this
hearing lowers the surface pressure slightly which increases the
low-level convergence because of frictional turning in the boundary
layer.
This convergence produces more cumulonimbus which causes
more latent heating thereby lowering the surface pressure even more
and so on (Atkinson, 1971).
In the ITCZ, the established zone of
convergence acts as a mechanism to bolster the CISK cycle and thus
increase the extent of convective activity.
Showers were not tracked because larger areas were the predominant feature of the precipitation pattern.
E.
25-26 July
1.
Synoptic Situation
This disturbance was classified as a warm-core tropical depres-
sion.
The early history of the development of this storm (Figure 24)
was analyzed by FPE in their preliminary report.
As early as 22 July,
this disturbance was observed as a vortical system imbedded in the
0
0
ITCZ near 6 N, 44 W.
A movement to the northwest brought the vortex
center within 50 miles of Barbados on the morning of the 26th.
The
maximum wind observed in this storm was 38 knots with the highest
sustained winds on the order of 30 to 35 knots.
The vortical circu-
lation was found at both 950 and 850 mb on the 25th and 26th (Leary,
1972) but at 700 mb the storm had decreased markedly in intensity.
40.
A pronounced low pressure center was not found at the surface where
the central pressure remained at 1011-1012 mb.
Further intensifica-
tion of this disturbance was inhibited by strong vertical wind shear
near the depression and developing anticyclonic flow behind it
(Leary, 1972).
2.
Precipitation Pattern
The emergence of the storm from an abnormal northward displace-
ment of the ITCZ (see satellite pictures for 25 July, Figure 3) was
initially observed on the radar as a large band of precipitation to
the south (see 25th/0705L of representative PPI's, Figure 25).
The
orientation of this band, ENE-WSW, was aligned with the basic northeasterly flow at low levels (Figure 26).
The dimensions averaged
200 by 25 miles and the band was made up of 3 to 10 SMSA's and numerous showers.
Although the SMSA's and showers were not connected,
this line of activity persisted for up to 9 hours with a slow translation northward.
This band is another illustration of a LMSA.
Individual showers and SMSA's moved WSW during the early history of
the band but gradually their direction backed to WNW.
This shifting
of the direction of echo motions within a short time is attributed
to the tight gradient of horizontal wind shear associated with the
close proximity of the closed vortex circulation.
The banding of radar echoes was noticeable throughout much
of this disturbance (Figure 25, 26th/OOOOL).
Precipitation areas
resembling squall lines formed frequently with dimensions as great
41.
as 150 by 20 miles and durations exceeding 4i hours.
Early on the 26th, the radar echoes became organized in a
spiral pattern similar to those found in fully developed tropical
storms (Figure 25, 26th/0519L).
A tracking of individual shower
motions at approximately 26th/0800L (Figure 27) revealed a definite
vortical circulation.
The 950-mb flow at this same hour (Figure 28)
is in excellent agreement with the observed shower motions.
The precipitation pattern of this disturbance was notable in
several respects.
At the time the vortex center was close to the
radar, the spiral appearance of the radar echoes over a relatively
small area of the scope was striking (Figure 25, 26 July-0519L).
Secondly, the precipitation was not as widespread as in the disturbances of 14-15 July and 16-18 July.
Predominant clear areas either
surrounded the main area of activity or these clear areas were interspersed between groups of echoes.
Enlargements of the ESSA 9 satellite photographs for the 25th
and 26th are presented in Figures 29 and 30 together with the corresponding PPI traces of the radar echo patterns at these times.
In
Figure 29, the band of echoes, described earlier as a LMSA, clearly
agrees with position of the satellite-viewed cloud band.
The actual
orientation and shape of individual cloud elements and radar echoes
are very similar in places.
The crude estimate of the ratio of echo
area to cloud area was less than 1/3.
The intensification of this
storm led to the development of a circular cirrus-cloud shield (Fig-
42.
ure 30), at this time.
The actual area of rain areas was small in
comparison with the cloudiness and this ratio was estimated to be
less than 1/5.
Correlation of shower motions with the observed wind was
complicated by the directional shear of the wind in the horizontal.
This shear within a short distance was often enough to make a wind
sounding at Barbados useless in tracking a shower a small distance
away from the island.
Echo heights processed by Hudlow for two times of polaroid
height sequences on the 25th and 26th are summarized in Table 8.
These echo height counts were derived from that portion of the PPI
display contained between 0
and 125
and from 25 to 200 miles range.
The increase in numbers from daytime (1600L) on the 25th to nighttime (0200L) on the 26 may be partly a diurnal effect but more likely
is due to the nearing of the vortex center.
An attempt was made to investigate the spatial relationship
between convergence and vorticity, and radar precipitation areas
for three times during this disturbance.
Anawalt (1971) looked at
the kinematical structure of the troposphere using BOMEX fourth phase
data and evaluated vorticity and divergence by the Bellamy triangle
method.
He found that there was a significant increase in cyclonic
vorticity and convergence during disturbed conditions which is in
support of the CISK hypothesis.
Anawalt also noticed that there was
an encouraging comparison of satellite-observed regions of maximum
43.
cloudiness and the regions of convergence and positive vorticity.
Similarly one would expect some spatial correlation between such
regions and the contemporary distributions of precipitation areas.
Figures 31-33 depict his analyzed fields of divergence between 1000
and.900 mb for 25th/0800L, 25th/2000L, and 26th/0800L.
includes the corresponding PPI trace.
Each figure
The comparisons indicate that
that precipitation areas tend to occur in regions of convergence but
not necessarily near localized areas of maximum convergence as in
Figure 33.
It is interesting to note that the pattern of precipita-
tion in Figures 31 and 32 is in bands which are oriented along the
belts of convergence in an east-west direction.
A
similar compari-
son of the distribution of precipitation to the divergence field
between 900 and 800 mb did not show as good a relationship.
Leary
(1972) stated that the convergence decreases vertically on both days
at all distances from the vortex center.
This is an indication that
the most important convergence occurred at low levels, particularly
in the boundary layer.
The field of vorticity did not correlate as
well with the area of precipitation at any of the three times of
observations.
F.
Discussion of Results
Essentially two types were observed among the four synoptic-
scale disturbances.
The first two on 14-15 July and 16-18 July were
perturbations in the basic easterly flow evident at middle levels of
the troposphere.
Both developed upper-level cyclonic circulations
44.
but they were not important in affecting the precipitation observed
by the radar since they developed after the initial wave disturbance
passed through the area of radar coverage.
The remaining two dis-
turbances were basically characterized by cyclonic circulations at
middle and upper-tropospheric levels.
Both developed in associa-
tion with the ITCZ with a northward bending being followed by breaking away from the ITCZ and movement on a northwestward track.
The
disturbance of 25-26 July developed a low-level cyclonic vortex
which was classified as a warm-core tropical depression.
The evolu-
tion and structure of this depression appeared to be similar to
those of Atlantic tropical cyclones which develop along the ITCZ,
It is interesting to note that each disturbance was associated with anticyclonic flow in the upper troposphere.
type of flow at 35,000 ft in their analyses.
FPE show this
On occasions when a
corresponding 35,000-ft cyclone developed as in the 14-15 July disturbance, it was located to the north of the anticyclone which was
over the region of disturbed weather.
Similarly, a tendency for
cyclonic flow was observed at 35,000 ft over the region at times of
undisturbed weather.
Malkus and Riehl (1964), in their study of
cloud structure over the tropical Pacific Ocean, noticed that the
upper-level flow patterns were crucial in governing what was observed at low levels.
They hypothesized that the sky type could
always be explained in terms of the vorticity tendency of the air
at high levels (250-150 mb).
If the vorticity tendency was positive
45.
(cyclonic vorticity increasing), upper convergence prevailed, producing subsidence in mid-troposphere and suppressing cloud-bearing
disturbances below; whereas if the upper flow was becoming anticyclonic, outflow aloft was compensated for by convergence below
favoring vigorous cloud growth and intensification of surface disturbances.
Precipitation patterns differed markedly between the two types
of disturbances.
The wave disturbances were composed of showdry-type
precipitation which was widespread but scattered.
The 16-18 July
system appeared more intense and developed than the 14-15 July disturbance.
This can likely be attributed to the weakening wave of
14-15 July while it was affecting the radar area in contrast to the
16-18 July disturbance which was accompanied by an intensifying wave
during its latter stage.
The other two disturbances had more con-
centrated areas of precipitation, and individual precipitation areas
reached larger horizontal extent.
There also existed more organiza-
tion in the distribution of precipitation in these two disturbances,
especially in the spiral structure of the tropical depression.
46.
V.
A.
DENSITY AND AREAL COVERAGE OF PRECIPITATION AREAS
All Situations
The aim of this section of the study was to obtain averages of
the instantaneous density of echoes and their total areal coverage as
a function of echo size.
In all, 72 PPI maps were selected in order
to get a representative sample for each synoptic situation and for
each disturbance.
Twenty maps (PPI tracings) were analyzed for un-
disturbed conditions, 12 for mildly disturbed, and 40 for disturbed
More maps were analyzed for disturbances because the pre-
conditions.
cipitation patterns exhibited greater variability.
Echoes were grouped into size intervals of 0-10 mi
2
81-150 mi ,
2
2
151-500 mi , and greater than 500 mi .
2
,
2
21-80 mi
,
The number of
0
echoes in each size interval was determined for the area from 330
170
on azimuth and from 40 to 150 miles in range.
the average extended to 40 miles in range.
vation was 36,000 mi
included.
2
.
to
Ground clutter on
The total area under obser-
Echoes that were partly inside this area were
The PPI maps were grouped by synoptic condition, the undis-
turbed observations being further divided into daytime and nighttime
conditions and the disturbed observations into the four disturbances.
The complete results of the overall survey for all three synoptic conditions are in Table 9.
echo numbers.
The first four columns deal only with
Immediately observable is the wide range in echo numbers
for all size intervals with the standard deviation relative to the mean
47.
value larger for the big showers than the small ones.
There is a
relative decrease in echo numbers with increasing size as might be
expected and there is also an increase in the average number of
echoes in each size interval going from undisturbed to disturbed
conditions.
There is however a relative decrease from undisturbed
to disturbed in the ratio of the average number in each interval to
.2
the total average number (column 2) for echoes less than 20 mi .
Figure 34 is a graph of the cumulative distribution curves showing
the average number of echoes in the observed area smaller than the
indicated size for the three synoptic conditions.
Mildly disturbed
and disturbed conditions are almost identical in echo numbers for
showers with the overall difference being several additional mesoscale areas (,-4-150 mi 2) in the disturbed cases.
The undisturbed
condition has about 3/4 as many of all sizes up to about 25 mi
and
very few larger ones.
The remaining five columns of Table 8 deal with areal measureIn order to determine the average area of
ments and percentages.
precipitation realized by echoes in a size interval, it was necessary
to estimate an average area for echoes in each interval.
This was
done somewhat subjectively and arbitrarily but any uncertainties
involved should not significantly affect the results of the analysis.
Since no echoes were actually as small as zero, the value 7 mi
2
assigned to the interval 0 to 10 mi2.
values of 14 mi
2
,
50 mi
2
and 125 mi
2
was
For the next three intervals,
were selected,
The reason for
48.
the unusual departure from the midpoint in the last interval was
2
based on an observed tendency for echoes to be closer to 150 mi
tha±i 80 mi
2
.
For echoes in the remaining two size intervals, the
actual areas were determined and then the average area was computed
for all echoes in each size interval and each synoptic category.
The product of the average number of echoes and the mean area in
each size interval determined the average area of precipitation at
an instant for each synoptic condition.
The results of this compu-
tation are in column 6 of Table 9 and the percentage contributed by
areas in each interval to the total average precipitation area is
in column 7.
Figure 35 shows the average contribution of areas
smaller than any given size to the average total area of precipitation.
These results point out the tendency for increased sizes of
precipitation areas from undisturbed to disturbed conditions.
The
fact that all the curves in Figure 35 are steepest at zero and continually flatten out indicates that the small echoes are numerous
enough so that per increment of area they contribute more to the
total precipitation area than do the larger areas.
The average area of precipitation was 2.4% of the total observed
area for undisturbed times, 4.4% for mildly disturbed, and 7.0% for
disturbed times.
It is interesting to note that this ratio increased
by a little less than a factor of two over the previous category.
The cumulative distribution curves of these ratios for each synoptic
category is in Figure 36.
There is a similarity in the curves for
mildly disturbed and disturbed conditions up to echoes of size less
49.
2
than 40 mi .
This suggests that the real distinction between midly
disturbed and disturbed is associated with precipitation areas
greater than 40 mi
2
A similar computation of this ratio by Ruprecht and Bucher
(1970) was made on the Atlantic Expedition 1969 (APEX) with a shipborne weather radar (Selenia Meteor 200, 3 cm wavelength).
For
five weeks (March 4-April 8) the ship was anchored on the equator
at 32*W.
They found the ratio of area covered by echoes to the whole
area scanned by the radar (100 km radius) was in no case larger than
20% and in 98% of all observations it was smaller than 10%.
For the
last two weeks, their ship was continually located in the ITCZ which
may account for their higher maximum value.
B.
Undisturbed Conditions
The 20 observations for this condition were divided equally
into daytime and nighttime conditions.
This study is similar to the
one in Chapter IIIB in that the intent of both is to portray the
diurnal variations of precipitation areas during undisturbed periods.
Here the areal coverage as well as the number of echoes is considered
and also the few mesoscale precipitation areas which occurred were
included.
The results are summarized in Table 10.
Both the aver-
age number of echoes and the areal coverage increase by more than
a factor of two in going from daytime to nighttime conditions.
In all 20 observations, only six areas were observed larger than
50.
80 mi
2
and all six were found during the nighttime.
The overall
distribution of echo sizes between undisturbed daytime and nighttime conditions is similar (Figure 37).
It is the number of echoes
which is the largest varying factor between the two.
The curves
for the ratio of echo area to observed area (Figure 36) illustrate
the similarity not only for mildly disturbed and disturbed conditions up to echo sizes less than 40 mi2 but also the similarity to
them of the curve for undisturbed-nighttime conditions.
The, cumula-
tive ratio for all sizes was 1.4% for daytime and 3.5% for nighttime conditions.
C.
Disturbances
Similar computations were performed for each disturbance and
are presented in Table 11.
The disturbance of 14-15 July had an
average total area of precipitation of 2600 mi2 which was 7.3% of
the total area under observation.
In the eight observations of
this disturbance, twelve precipitation areas from 151 to 500 mi2 had
2
an average area of 312 mi .
Two echoes greater than 500 mi
2
aver-
2
aged 690 mi .
The 16-18 July disturbance was very similar in the size distribution of precipitation areas to the one on 14-15 July.
This is
a reasonable expectation because these two disturbances were similar
in synoptic type and in the characteristic pattern of precipitation.
In Figure 38, the curves of the relative contributions of cumulative
51.
echo sizes to the total average area of precipitation are also
similar.
The steepness of these two curves for small areas indi-
cates that these small areas (less than 80 mi 2) make up more of the
total precipitation than do the larger areas.
The ratio of the
average total precipitation area to the total area scanned by the
radar was 8.5% for 16-18 July as compared with 7.3% for 14-15 July.
The third disturbance of 22 July was unlike the first two
both synoptically and in its precipitation pattern.
In Table 11,
it can be seen that the most striking difference was the dominance
of precipitation areas greater than 500 mi 2 .
size category had an average area of 1182 mi
The six echoes in this
2
.
These observations
did not actually include six individual echoes but were the same
precipitation area at different hours.
This large area on the aver-
age accounted for 44.1% of the average total precipitation area at
any instant, a value which was not exceeded in any other size interval or for any other disturbance.
In a compensating manner, the
contribution of echoes less than 80 mi2 to the average total area
of precipitation was the least for all four disturbances.
It is
possible that the area of precipitation would have been much larger
had the disturbance been further north or the radar further south,
since the precipitation was concentrated in the southern portion of
the scope.
The ratio of the mean area of precipitation to the total
area was 8.7% which was the largest value for all of the disturbances.
52.
The tropical depression of 25-26 July surprisingly had the
smallest mean area of precipitation of all four disturbances.
This
value was 1972 mi2 or 5.4% of the total area scanned by the radar.
The size distribution of precipitation areas in this disturbance
is more similar to that of 22 July than to those of the wave disturbances.
The resemblance in patterns is not as close, however,
as in the case of the two wave disturbances.
Figures 40 and 41 show Figures 38 and 39 replotted with the
cumulative echo sizes on a logarithmic scale in order to illustrate
the behavior of the curves at large echo sizes.
In Figure 41, the
resemblance of the curves for the last two disturbances for sizes
less than 800 mi
2
is very close.
In Figure 40, the slope of the
curves is steeper for smaller echo areas in the cases of the wave
disturbances but steeper at the larger echo sizes for the final two
disturbances reflecting the shift to larger precipitation areas in
the latter.
D.
Echo Height Distributions
Sixteen observations of height sequences analyzed by Hudlow
(1970) were available from Phase IV of BOMEX.
These data listed
the number of echoes that were observed to persist up to a specified
height interval but disappeared at the next height interval.
Of
the 16 observations 5 were from undisturbed conditions, 6 from midly
disturbed, and
5 from disturbed conditions.
The average number of
53.
echoes in each height interval was found for each synoptic condition.
The average cumulative distributions are shown in Figure 42.
There
is a sharp increase between 10,000 and 16,000 ft for undisturbed
conditions reflecting what is apparently the preferred height range
for echo tops on these days.
During undisturbed conditions, 90% of
the echo tops are lower than 20,000 ft while in disturbed conditions
only 58% are below 20,000 ft while the 90% level is at 32,800 ft.
Because of the small sample size, these numerical results should be
regarded with reservation.
More important are the relative values
for different synoptic conditions.
Figure 43 shows frequency histo-
grams illustrating the distributions for the three synoptic categories.
There is a noticeably broader distribution for echo top
heights in disturbed situations as compared with undisturbed ones.
For undisturbed observations, the peak between 10,500 ft and 12,500 ft
is reflected in the increased slope in Figure 42 at these heights.
54.
VI.
A.
SUMMARY AND SUGGESTIONS FOR FURTHER STUDY
Summary
Precipitation as detected by radar in this study was charac-
terized by showers.
Showers
were defined as individual precipita-
2
tion areas less than 80 mi , a value which was the typical upper
limit for precipitation areas during undisturbed conditions.
The
smallest showers ( ( 20 mi 2) were the most numerous, so much so
that per area interval they contributed the greatest proportion of
the total precipitation area.
Of the 82 showers studied, durations
were typically less than two hours.
The correlation coefficient
between the maximum area and durations of showers was found to be
0.6 with a proportionality constant of .13 mi /min.
A diurnal vari-
ation in shower numbers was observed on undisturbed days with a
maximum during late night and early morning hours but the relative
size distributions remained the same for day and night.
There were
over three times as many showers at the maximum than at the minimum.
Precipitation areas of larger horizontal extent were observed
during disturbed conditions.
These continuous areas greater than
80 mi2 were called small mesoscale areas.
3
to 10
They were generally 102
2
mi
in area and had durations of 1 to 4 hours.
A sample of
34 SMSA's had a correlation coefficient between maximum area and
duration of 0.7 and a proportionality constant of 1.7 m 2/min.
Precipitation patterns in four disturbances were observed
-
55.
two being wave disturbances and two cyclonic vortices.
Precipita-
tion in wave disturbances consisted of widespread showers with a
few areas of mesoscale size.
Cyclonic vortices were characterized
by one or two connected precipitation areas of relatively large
size (>500 mi 2) plus a number of smaller mesoscale areas and
numerous cells.
Precipitation frequently tended to be concentrated
into areas containing a few SMSA's and numerous showers and exhibiting a tendency to move as a unit and persist for many hours.
These
concentrated areas were called large mesoscale areas, they exceed
10
4
.2
mi in extent which includes clear areas between individual
echoes.
They had durations in excess of four hours.
It should also
be noted that there was a tendency for a greater percentage of large
showers and mesoscale areas to have higher intensity areas imbedded
in them.
The ratio of the area covered by precipitation to the area
under observation never exceeded 9%.
The average number of echoes
and areal coverage at any given time are:
Synoptic condition
Per cent of area
covered by echoes
Average number of 2
echoes in 36,000 m
Undisturbed day
1.4
39.2
Undisturbed night
3.5
93.9
Mildly disturbed
4.4
94.1
During disturbances
5.4-8.7
72-147
The percent of the area covered by echoes agrees in general with the
56.
values found by Ruprecht and Bucher (1970) over the equatorial
Atlantic.
The ratio of the radar rain area in disturbances to the
cloud cover as observed in the ESSA 9 satellite photograph enlargements was typically low and was crudely estimated to be between 15%
and 30%.
A comparison of tropical precipitation patterns in disturbances and New England storms (Austin and Houze, 1972) revealed two
major differences.
The tropical disturbances lacked a synoptic-
scale area of precipitation while each of the cyclonic storms in
New England contained a continuous synoptic precipitation area.
The
two cyclonic vortices in this study came closest to exhibiting the
widespread nature of a synoptic rain area.
However on a smaller
scale, there were similarities between tropical and high-latitude
precipitation areas.
Large mesoscale areas in both cases had simi-
lar dimensions although they were not connected in tropical storms.
Small mesoscale areas were found in both studies with comparable dimensions and behavior but there were more small ones in the tropics.
both climates imbedded cells were observed in the SMSA's.
The other
major difference was the very large number of showers, especially
small ones, both in disturbances and on undisturbed days over the
tropical Atlantic.
The occurrence of showers on undisturbed days
can be likened to air-mass thunderstorms in New England.
Qualita-
tively speaking there seem to be more convective showers in the
tropics but they do not become as large as those observed in New
In
57.
England.
Since the normal situation over thE tropical ocean is one
of conditional instability, the tropical showers occur much more
frequently than do air-mass showers in higher latitudes.
In fact
Houze (1972) has shown that the extent of convective activity in
temperate latitudes is closely related to the proximity of the
region to a source of tropical maritime air.
B.
Recommendations for Further Study
It is desirable to obtain data in a more manageable form
because detailed analysis of precipitation areas from photographs
is extremely time consuming.
able.
Also more quantitative data is desir-
Digital data and automatic processing methods would be ex-
tremely helpful in meeting these needs.
One of the specific aims
of this study is to provide a basis for planning the taking and
processing of data in GATE (GARP Atlantic Tropical Experiment).
The program will be organized to maximize the information' and minimize the labors of analysis.
Hopefully, descriptive studies such as this, combined with
more quantitative ones when better data are available, will contribute towards future research in the development and testing of
dynamical models.
These models would describe precipitation patterns
in terms of the cumulus convection and mesoscale motions which produce them.
58.
Table 1.
Characteristics of the AN/MPS-34 radar (from
Hudlow, 1970).
Nominal Value
Characteristics
Transmitt-ed power (peak)
180 kw
Wavelength
3.2 cm
Antenna shape
Parabolic
Horizontal Beam width
1 degree
Vertical beam width
1 degree
Antenna gain
26,300 (dimensionless)
Antenna rotation rate
5 rpm
Minimum detectable signal
Linear receiver
-105 dbm
Dynamic range (receiver)
Linear receiver
17 to 20 db
Pulse repetion frequency
186 pulses/sec
Pulse width
5 x 10-6 sec
Sensitivity time control (STC)
Was not used
Range units
Statute miles
Table 2. Approximate rainfall rates (mm/hr) for given return
powers and ranges.
Range (miles)
Intensity
Level 2 (85-87 dbm)
Intensity
Level 3 (75-79)
50
< 0.1
< 0.4
110
< 0.3
.7-1.5
150
< 0.5
1-2
Intensity
Level 4 (69-71)
1 - 1.5
3-4
5-10
59.
Table 3. Evaluation of the synoptic condition governing the
radar coverage sector for each 3-hour interval.
Local Time
00-03
Date
U:
10-12
07-09
04-06
undisturbed
M:
13-15
16-18
D:
mildly disturbed
22-24
19-21
disturbed
Blank spaces due to missing data.
Table 4. Number of shower echoes observed to last for specified
time intervals as a function of the maximum area
achieved.
Maximum area in suare miles
Duration
in minutes
0-10
11-20
41.7
73.3
6-115
25-117
21-30
31-50
51-80
96.8
149.4
234
0-20
21-40
41-60
61-80
81-100
101-120
121-140
141-160
161-180
181-200
>200
Avg.
Range (min)
38-140
Note: Avg. found with exact durations.
109-191
60.
Table 5. Average number of shower echoes in the area under
observation (58,000 mi 2 ) according to size and
various other factors.
a) ror all days according to daytime or nightime observation
Area Interval
Number of
0-5 mi2
Observations Number %
Day
10
Night
14
15.3
27.9
32.8
29.2
b)
6-20 mi2
Number %
23.0
49.8
21-80 mi2
Number %
49.3
53.0
8.4
118.1
Total
%
18.01 46.7
18.8
95.8
100
100
For undisturbed days only
Day
11.4
37.4 114.8
48.4
4.3
14.2
30.5
100
Night
34.0
32.1
53.7
15.0
14.2
106.0
100
57.0
c) For undisturbed and disturbed days
Undis.
10
Dis.
20.6
33.8
32.1
52.7
8.2
13.5
60.9
100
27.0
31.0
42.0
48.3 118.0
20.7
87.0
100
Table 6. Total number of shower echoes observed for all days
and the percent which had imbedded cells at the next
two intensity levels.
0-5 mi2
Number
6-20 mi 2
Number
%
%
100
Level 1
589
Level 2
8
1.3
Level 3
0
0
987
21-80 mi2
100
- Number
%
349
100
256
26
272
78
69
7
134
38
61.
Table 7. Results of the survey of small mesoscale areas.
Date
9
9
9
9
10
14
16
17
17
17
18
18
18
18
18
18
18
19
21
22
22
22
25
25
25
25
25-26
25-26
Avg.
*
Maximum size
(mi2 )
180 sq mi
360
210
420
200
250
230
910
420
410
160
230
120
360
380
90
170
390
80
270
860
80
Duration
(minutes)
>
>
>
>
150 min
165
150
240
> 150
1000
550
1150
320
250
125
110
363
183
560
540
230
300
*
135
185
150
240
270
140
70
75
135
195
230
75
150
130
145
195
90
270
210
240
105
270
300
240
450
210
120
200
150
> 370
Average number of
embedded level 2 areas
4.8
Duration exceeds this value but could not be determined
either because data was interrupted or SMSA moved out of
range.
Could not be determined.
Table 8. Echo heights for two times during 25-26 July disturbance
(from Hudlow, 1970 - Appendix B(2))
Class intervals in thousands of feet
Time
3-7
7/25
1600
7/26
0200
22
7-10.5
10.512.5
12.514.5
14.517.5
17.520.5
20.5-
23.5-
26.5-
23.5
26.5
29.0
3
7
6
8
3
2
29-32
-
32-40
15
>40
Totals
3
53
1
117
63.
Table 9. Average number and coverage of precipitation areas of
various sizes for the three synoptic categories. Area
under observation: 36,000 mi 2 .
1
3
2
4
5
a)
4.4
0)0
-<
0
10
0-10
11-20
21-80
81-150
151-500
7500
43.6
16.7
5.35
.3
.05
0
Total
66.5
0
0
16-92
5-54
0-20
0-40
0-1
23.7
12.2
16.9
.9
.2
0-10
11-20
21-80
81-150
151-500
7500
57.6
22.5
12.3
1.2
.5
0
Total
94.1
26.7
11.0
5.6
.9
1.3
Disturbed condition:
58.7
21.4
16.2
2.0
1.4
.4
100.0
00U
w 0
x
U
65.5
25.2
8.8
.4
.1
0
100.0
17-128
11-46
6-27
0-3
0-4
16-119
3-61
5-44
0-5
0-5
0-2
27.6
3.6
9.2
1.6
1.3
.6
10
w
wa
W 14.
44
54
0
0
W-4 a)
:1 U
0
a)
4-i
-1
>cj
0
5-A4CO
1-4
a)
~a)
averages of 20 observations
Mildly disturbed condition:
Total
P-4
4j
w0)
0CLI-
9
W-4
0 c
10 cc
Undisturbed condition:
8
4i
0-0j
>Ja
0-10
11-20
21-80
81-150
151-500
7500
7
.4
0)E
a4
0
6
305
234
292
37
10
0
34.8
26.6
33.3
4.2
1.1
0
34.8
61.4
94.7
98.9
100.0
100.0
.84
.64
.80
.10
.03
0
.84
1.48
2.28
2.38
2.41
2.41
878
100.0
100.0
2.41
2.41
averages of 12 observations
61.1
23.9
13.0
1.3
.7
0
403
315
612
145
126
0
24.6
19.9
38.3
9.3
7.9
0
24.6
44.5
82.8
92.1
100.0
100.0
1.11
.86
1.68
.40
.34
0
1.11
1.97
3.65
4.05
4.39
4.39
100.0
1601
100.0
100.0
4.39
4.39
averages of 40 observations
58.6
21.4
16.2
2.0
1.4
.4
100.0
411
302
810
254
406
361
16.2
11.9
31.8
10.0
15.9
14.2
16.2
28.1
59.9
69.9
85.8
100.0
1.13
.83
2.22
.69
1.11
.99
1.13
1.96
4.18
4.87
5.98
6.97
2544
100.0
100.0
6.97
6.97
64.
Average number and coverage of precipitation areas
of various sizes for daytime and nightime in
undisturbed conditions.
Table 10.
1
4
3
2
7
6
5
CN
10
9
8
0
'.4
44
o
S40
0 0
IC
N
0
Total
26.7
9.5
2.9
.1
0
0
14-48
5-14
0-7
0-1
Averages of 10 observations
0
0
0
0
0
0
41.5
26.7
29.2
2.6
0
0
499
100.0
10
2
2
.3
Total
93.9
68.1
24.2
7.4
.3
100.0
Nightime condition:
60.5
24.0
8.8
.5
.1
0
25-92
7-54
1-20
0-4
0-1
w) P
30
C14 "
39.2
0-10
11-20
21-80
81-150
151-500
7500
'-4a)
w 0
0
.J1-W
04.)..
$4.)
Daytime conditi
0-10
11-20
21-80
81-150
151-500
7500
~O4j
4
04 r
>44
.<0
0Jr4
21.2
13.6
5.9
1.2
.3
0
207
133
145
125
41.5
68.2
97.4
100.0
100.0
100.0
.57
.36
.40
.03
0
0
100.0
1.36
1.36
.57
.93
1.33
1.36
1.36
1.36
Averages of 10 observations
64.4
25.6
9.4
.5
.1
0
100.0
423
336
440
63
20
0
33.2
26.2
34.4
4.8
1.4
0
33.2
59.4
93.8
98.6
100.0
100.0
1.16
.92
1.21
.17
.05
0
1.16
2.08
3.29
3.46
3.51
3.51
1272
100.0
100.0
3.51
3.51
65.
Average number and coverage of precipitation areas
of various sizes for the four disturbances.
Table 11.
1
3
2
9
8
7
6
5
4
10
v-q
0
0
.C
44.0
0
44
00
~V4
0
0
0>
>44
Total
40-95
14-38
8-26
1-3
0-3
0-1
Total
84.8
33.7
25.6
2.36
.72
.27
41-119
19-61
8-44
0-5
0-2
0-2
Total
42.75
14.9
9.6
2.4
1.7
.7
1. 11
.90
2.57
.77
1.29
.47
1.11
2.01
4.58
5.35
6.64
7.11
100.0
2592
100.0
100.0
7.11
7.11
Averages of 11 observations
57.5
22.8
17.4
1.6
.5
.2
Total
47.3
13.9
10.7
1.38
1.77
.38
75.43
18-95
4-20
5-16
0-4
0-4
0-1
25.1
13.6
3.7
1.3
1.3
.5
19.0
16.2
40.2
9.5
7.5
7.6
19.0
35.2
75.4
84.9
92.4
100.0
1.63
1.29
3.51
.81
.64
.65
1.63
2.92
6.43
7.24
7.88
8.53
3106
100.0
100.0
8.53
8.53
59.4
20.6
13.3
3.3
2.4
1.0
300
208
480
296
488
1400
9.5
6.6
15.1
9.3
15.4
44.1
9.5
16.1
31.2
40.5
55.9
100.0
.82
.57
1.32
.81
1.34
3.84
.82
1.39
2.71
3.52
4.86
8.70
100.0
3172
100.0
100.0
8.70
8.70
25-26 July disturbance:
0-10
11-20
21-80
81-150
151-500
7500
593
472
1280
295
233
236
Averages of 8 observations
15.3
6.9
3.7
2.1
1.7
.6
72.1
0 0)
0 0
0)
ON
;~0)
15.6
28.3
64.6
75.4
93.6
100.0
100.0
10-66
3-24
5-16
0-7
0-5
0-2
4.J4J
~
Ua
15.6
12.7
36.3
10.8
18.2
6.7
22 July disturbance:
0-10
11-20
21-80
81-150
151-500
7500
V
403
329
938
282
468
172
24.5
11.3
9.8
1.7
.8
.6
147.45
4
4j4J
Averages of 8 observations
16-18 July disturbance:
0-10
11-20
21-80
81-150
151-500
7500
0)
~1*4
-4
55.4
22.7
18.1
2.2
1.4
.2
16.9
7.2
5.4
.8
.9
.4
103.75
0
0
C
PW P.
L44-"
14-15 July disturbance:
57.5
23.5
18.75
2.25
1.5
.25
0
4j0
fl
'0 0
.414 0
0-10
11-20
21-80
81-150
151-500
7500
0
0
0
Averages of 13 observations
62.7
18.4
14.3
1.8
2.3
.5
331
195
535
172
403
336
16.8
9.9
27.3
8.7
20.3
17.0
16.8
26.7
54.0
62.7
83.0
100.0
.91
.53
1.47
.47
1.11
.92
.91
1.44
2.91
3.38
4.49
5.41
100.0
1972
100.0
100.0
5.41
5.41
+
4
4-
'K5
L=.I;
O
6RANIER
*ROCKAWAY +
+
0 DISCOVERER
STATUTE MILES
0 MT. MITCHELL
50+
500
700
*OCEANOGRAPHER
Figure 1.
Illustration of areal coverage of the radar.
67.
Figure 2o Illustration of the off-center PPI scope.
25-mile intervals.
Range markers at
68.
16 July - 1323L
17 July - 1422L
Figure 3. ESSA 9 satellite photograph sequence.
69.
18 July - 1325L
19 July - 1424L
Figure 3.
(Cont.)
70.
21 July - 1422L
22 July - 1329L
Figure 3.
(cont.)
71.
23 July - 1424L
24 July - 1331L
Figure 3.
(cont.)
72.
25 July - 1426L
26 July - 1334L
Figure 3.
(cont.)
73.
Figure 4. Method of tracking shower echoes.
50 I-
40
-
30-
0
0
20 -
og
0
o
10
0 .
-o
0
o0
0p
at)
0
1
20
1
40
C
1
60
o
0
0
oc
36
ocoo=age
o
0
0
0000
0
o
1
1
1
80
100
120
DURATION IN MINUTES
1
140
1
160
1
180
200
Figure 5. Scatter diagram and linear regression line
(A = 0.13D + 12.9) for maximum area of showers vs.
duration.
74.
40
80
120
MINUT ES
160
">200
Figure 6. Distribution of durations for 82 showers in survey.
Figure 7. illustration of shower size
classification.
75.
Figure 8. Representative precipitation maps for a typical undisturbed day.
76.
r
120
100
I]
KEY
LIEVEL I
-
80 -
2 60
z
20
..
TIME OF OAY - LOCAL
Figure 9. Diurnal variation in echo numbers plus the number of inbedded areas at
levels 2 and 3 for a typical undisturbed
day (12 July).
150
Is
242*
22- -
20
13015
IS-
611219
14.#102 19
S2~
82
17
z 07
6
72 13
a 213
72 13
62 13
67
17
72 16
2
72 12
1
5209
Figure 10. Comparison of shower motions with
wind sounding from Barbados for 12 July/0200L.
77.
2001-
~01
0
40
08
40
80
120
160
200
240
280
DURATION IN MINUTES
320
360
400
440
Figure 11. Scatter diagram and linear regression line
(A = 1.7D - 0.8) for maximum area of small mesoscale
areas vs. duration.
Figure 12. Example of development and motion of
small mesoscale areas with imbedded areas of more intense precipitation.
16/O0B8Lf
200
0±
±
A-
c::D
\\>
~\
&~
\\\
\\
\
I
I ~I4/2
-
LEGEND
40*
8000'PERTURBATION
--- 35000'CYCLONE
20L 0 08L
POSITIONS:
CLOUDINESS (MORNING)
JUL 13://JUL 14 11JUL15
Figure 13.
50*
60*
Evolution of the 14-15 July disturbance (From Fernandez-Partagas and Estoque, 1970).
79.
Figure 14.
Representative precipitation maps for the 14-15 July disturbance.
80.
24-
92
9'1
16
22
20
92 18
18
16
22 23
80*
'50
27
14
26
122 29
0
92*
--
045
0nW11
C3 83*
27
Z
0200
2230
4
10.8
02 25
0 2 27
2520
0150
25
102 25
10223
4
0223
2
I220
12218
--792
0
87 *
0131
17
15
JULY
92 13
Figure 15. Comparison of shower motions with
wind sounding from Barbados on 15 July/0200L.
81.
I
I
I
I
I
-250N
-20*0
' 7-CU BANDS
5C W
40W
3
W
Figure 16. Relationship between lower tropospheric flow and the "inverted V" cloud pattern (From Frank, 1969).
-A-
A18/20 L
18/08 L
150
17 /20 L
17/08 Z
50
LEGEND
8000' PERTURBATIONS
-- -25000' CYCLONE
POSITIONS: * 20 L o 08L
550
+
100
600
65*
7oo
Figure 17.
Evolution of the 16-18 July disturbance (From FPE, 1970).
83.
Figure 18.
Representative precipitation naps for the 16-18 July disturbance.
84.
-5.
V
,dP
Figure 19. Comparison of cloud cover photographed by satellite with precipitation pattern on radar - 17 July/1422L.
85.
Figure 20. RepresentAtive precipitation maps for the latter part of the 16-18
July disturbance.
86.
S
t
Figure 21. Comparison of cloud cover photographed by satellite with precipitation pattern on radar, 18 July/1325L.
0
23
~~1~
±
A4-
150
21/
LEGEND
- --
16000' SYNOPTIC FEATURES
25000' CYCLONE
POSITIONS:
'\
* 20 L
22 JUL- 1329L
008L
500
||I 23 JUL-1105L
550
650
Figure 22.
Evolution of the 22 July disturbance (From FPE, 1970).
88.
Figure 23.
Representative precipitation maps for the 22 July disturbance.
k%
t4
1510
DISSIPATING
III,
25
o
4-
500
550
24
LEGEND
--
SURFACE CYCLONE TRACK
POSITIONS : 0 20 L 0 08 L
CLOUDINESS
JUL-1334L
///26
25 JUL- 1132L
Figure 24.
Evolution of the 25-26 July disturbance (From FPE, 1970).
90.
Figure 25.
Representative precipitation maps for the 25-26 July disturbance.
91.
Figure 25.
(continued).
92.
Figure 26.
950-mb flow on 25 July/0800L (From Leary, 1972).
93.
Figure 27.
0800L.
Figure 28.
Shower motions on 26 July
950-mb flow on 26 July/0800L (from Leary, 1972).
94,
Figure 29. Comparison of cloud cover photographed by satellite withprecipitation pattern on radar, 25 July/1430L.
95.
Figure 30. Comparison of cloud cover photographed by satellite with precipitation pattern on radar, 26 July/1334L.
96.
Figure 31. Comparison of precipitation pattern with divergence field
on 25 July/0800L.
97.
v.V (10~" sec~ )
Figure 32.
between 1000 and 900 mb.
Same as Figure 31 except for 25 July/2000L.
98.
Figure 33.
Same as Figure 31 except for 26 July/0800L.
99.
110 --
0 100 at 902 mi
2
941 at 250 mi
--
90 -
2
80 66.5 at 200 mi 2
70 ---
U
600-
40 30 -
0 --0----<
O-O
20
UNDISTURBED
MILDLY DISTURBED
DISTURBED
10
0
40
20
100
80
60
160
140
120
ECHO AREA IN SQUARE MILES
Figure 34. Cumulative distribution showing the average number
of echoes smaller than the indicated size for the three synoptic
conditions.
100
90
80
70
50
40-
40
80
120
160
200
240
280
320
ECHO AREA IN SQUARE MILES
Figure 35. Cumulative distribution showing the average percent of
the total precipitation area covered by echoes smaller than the
indicated size for the three synoptic conditions.
100.
O
V
I
I
20
40
I
60
I
I
I
I
I
80
100
120
140
ECHO AREA IN SQUARE MILES
160
1
180
|
200
Figure 36. Cumulative distributions showing the average percent of the
total area under observation covered by echoes smaller than the indicated size for undisturbed conditions (daytime and nightime), mildly disturbed, and disturbed conditions.
loo
90
80
70
Z 60
w 50
40
30
20
1O
0
40
80
120
160
ECHO AREA IN SQUARE MILES
200
Figure 37. Same as Figure 35 except for undisturbed conditions (daytime and nightime).
101.
100
90
80
70
60
I
50
40
30
20
10
0
160
120
80
40
360
320
280
240
200
ECHO AREA IN SQUARE MILES
Figure 38.
20
Same as Figure 35 except for the four disturbances.
40
60
80
100
120
140
160
180
ECHO AREA IN SQUARE MILES
Figure 39.
Same as Figure 36 except for the four disturbances.
200
102.
10
50
100
500
1000
"5000
ECHO AREA IN SQUARE MILES
Figure 40. Same as Figure 38 except with echo size plotted on a
logarithmic scale.
10
.0-*
6 -
--'
0----O
0-O
>---O
2 -
1O
.'
-G'O--
-
50
100
500
1000
14 -15
16 -18
JULY
JULY
22 JULY
25-26 JULY
500C
ECHO AREA IN SQUARE MILES
Figure 41.
Same as Figure 39 except with echo size plotted on a
logarithmic dcale.
103.
100
90
80
70
Z
W 60
U
w 50
a.
40
30
20
10
0
4
12
8
20
16
24
28
32
36
40
THOUSANDS OF FEET
Figure 42. Cumulative distributions showing the average percent of echoes with tops less than the indicated height for the three synoptic conditions (derived from Hudlow, 1970).
20
30
10
ECHO TOP IN THOUSANDS OF FEET
>40
10
20
30
ECHO TOP IN THOUSANDS OF FEET
>40
DISTURBED
15
-
10
10
20
30
ECHO TOP IN THOUSANDS OF FEET
Figure 43.
>40
Distribution of echo heights for the three synoptic conditions.
104.
ACKNOWLEDGEMENTS
I am indebted to several people in helping with the formulation and preparation of this study.
I am especially grateful to
Dr. Pauline M. Austin for her guidance and suggestions during the
development of this thesis.
Appreciation is extended to Mrs. Marie L. Gabbe who typed
the manuscript.
I also thank Steven Ricci who helped in the draft-
ing of the figures; James Mazzarini who did the photographic work;
and Mrs. Barbara Wysochansky who typed the tables.
Appreciation is
also expressed to Mr. Andrew Bunker of Woods Hole Oceanographic
Institution for supplying 10 reels of the BOMEX radar films.
This study was supported by the National Science Foundation
through grant number A-36364X.
105.
REFERENCES
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S.M.
Atkinson, G.D., 1971: Forecasters' Guide to Tropical Meteorology,
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pp.
Austin, P.M., and R.A. Houze, Jr., 1972: Analysis of the structure
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Elliot, R.D., and E. L. -Hovind, 1964: ,On convection bands within
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106.
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Riehl, H., 1954:
pp.
Tropical Meteorology.
McGraw-Hill, New York, 392
Ruprecht, E., and K. Bucher, 1970: Examples of organized shower
structures in the equatorial Atlantic. Symposium on Tropical
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