Echo Tops
Fairly accurate at depicting height of storm tops
Inaccurate data close to radar because there is no beam
angle high enough to see tops.
Often has stair-stepped appearance due to uneven
sampling of data between elevation scans.
Precipitation Estimates
Storm Total
Precipitation
● Total estimated accumulation
for a set amount of time.
● Resets storm total whenever
there is no rain detected for
an hour.
One Hour Precipitation Total
-Updated once per volume
scan.
-Shows accumulated rainfall
for the last hour.
-Useful for determining
rainfall rate of ongoing
convection.
Precipitation Estimate
Advantages
● Great for scattered areas of
rain where no rain gauges are
located
● Provides a graphical ‘map’ of
rainfall for an entire region
● Data can be overlaid with
terrain and watersheds to
predict reservoir and waterway
crests
and
Limitations
● Estimates based on cloud water
levels and not ground level rainfall
● ‘Hail Contamination’ causes
highly inflated values
● High terrain causes
underestimates
● Useful as a supplement, not
replacement for ground truth
information
Interpreting Doppler
Signatures
Display examples provided by:
National Weather Service
Steve Davis - Lead Forecaster
Milwaukee/Sullivan National
Weather Service Forecast Office
Azimuth Resolution Considerations
•
•
•
closer a rotation the more likely it will be identified correctly
rotation smaller than the 0.50 beam width (possible at long ranges)
> rotation is average of all velocities in sample volume
0
Previous 1 beam width improved by super-resolution
Enlarged image along a
radial. Individual
“blocks”
represent one sample
volume. This graphically
shows the radar
resolution.
Azimuth 3
Weak inbound,
weak outbound

Rotation too
small to be
resolved

Azimuth 2

Strong inbound,
strong outbound
Azimuth 1
Rotational couplet identification can be
affected by azimuth resolution.
Range 0
(example)
120 nm

Stronger inbound
than outbound
The Zero Isodop “Problem”
When the radial is
perpendicular to wind
direction, the radar
displays zero velocity This “zero zone” is called
the “Zero Isodop”.
0%
100%
100%
When the wind
velocity is parallel to
the radial, the full
component of the
wind is measured
0%
What percentage
of actual wind
will the radar detect?
00 = 100% - Parallel
150 = 97%
300 = 87%
450 = 71%
600 = 50%
750 = 26%
900 = 0% Perpendicular
Large Scale Winds
Use the Zero Isodop to assess
the vertical wind profile.
The combination shape of
the zero isodop indicates
both veering and backing
winds with height.
Combination
“S” Shape
Backward “S” Shape
“S” shape of the zero
isodop indicates veering
winds with height.
Veering may imply warm
air advection.
Backward “S” shape
of the zero isodop
indicates backing
winds with height.
Backing may imply
cold air advection.
Large Scale Winds
Uniform Flow
Uniform Flow with Jet Core
Straight Zero Isodop indicates
uniform direction at all levels.
Straight Zero Isodop indicates uniform
direction at all levels >> inbound/outbound
max’s show a jet core aloft with weaker
winds above and below.
Example from KMKX 88D
Low level
jet max
January 5, 1994
Steady snowfall
The VAD Wind Profile
(Velocity Azimuth Display)
Small Scale Winds
- Diffluence/Confluence -
Diffluence
Often seen at
storm top level
or near the
ground at close
range to a pulse
type storm
Confluence
would show
colors reversed
Small Scale Winds
- Cyclonic Confluence/Diffluence -
Anticyclonic
confluence/
diffluence
would show
colors
reversed in
each panel.
Cyclonic Confluence
Cyclonic Diffluence
Small Scale Winds
- Pure Cyclonic Rotation -
Anticyclonic
rotation would show
colors reversed
Pure Cyclonic Rotation
Small Scale Velocity Example
Small Scale Velocity Example
Rotation with tornado
Storm Relative Velocity - SRV
vs
Base Velocity
SRV: Subtract estimated velocity of thunderstorm from the
Doppler radial velocity– Make the storm stationary
When diagnosing rotational characteristics, use SRV
motion of the storm masks subtle rotations within the storm
When diagnosing Straight Line Winds (bow echo,
microbursts), use Base Velocity
straight line winds are sum of the winds produced by the storms, plus
storms movement
SRV vs. Base Velocity
- strong rotation -
Storm Relative Velocity
Base Velocity
rotation in tornadic thunderstorm
SRV vs Base Velocity
- subtle rotation -
Base Velocity
Storm Relative Velocity
Janesville F2 tornado. June 25th, 1998 ~ 700 PM
Interesting note: These scans are at 3.40 elevation. The 0.50 elevation
showed little rotational information.
SRV vs Base Velocity
- subtle rotation -
3.40
Base Velocity
Little/no rotation
seen at lowest
elevation
0.50
Storm Relative
SRV vs Base Velocity
Base Velocity
Storm Relative Velocity
SRV vs Base Velocity
- straight line winds -
Base velocity shows max inbound winds
of 55 to 60 kts.
SRV shows max inbound winds of 30
to 40 kts.
Bow Echoes
Detecting and Predicting Downbursts
o Bow echoes are caused by severe downbursts,
accelerating part of a line of thunderstorms ahead
of the rest.
o The strongest downbursts occur under and just
north of the apex of the bow, but can occur
elsewhere too
o Surface winds can exceed 70mph in strong bow
echoes.
o Bow echoes can move at over 50 mph.
o Highest reflectivities and strongest velocities are
found at the apex.
o Look for a tight gradient of reflectivity.
Review Clear-Air Radar

4
10 cm

1/3
UHF
VHF
 4
N
2
6
n a
i 1
6
i i
10 cm UHF
Cn2
 1/3
VHF
Clear-Air Wind Profilers
Wind Profiler Specifications
Frequency
(MHz)
Wavelength
(m)
Maximum
Altitude (km)
Antenna Size
(m)
Target
Band
Designation
50
6
20
100 x 100
Clear Air
VHF
449
0.75
15
15 x 15
Clear Air
and Heavy
Precipitation
UHF
915
~0.3
5-6
5x5
Clear Air
and
Precipitation
UHF
1036
~0.3
5.5-6
5x5
Clear Air
and
Precipitation
UHF