Echo TopsFairly 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.

-Updated once per volume scan.-Shows accumulated rainfall for the last hour.-Useful for determining rainfall rate of ongoing convection.

One Hour Precipitation Total

Precipitation EstimateAdvantages and Limitations

●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

●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 ForecasterMilwaukee/Sullivan National

Weather Service Forecast Office

Range 0 120 nm(example)

Weak inbound,

weak outboundRotation too

small to be resolved

Stronger inbound than

outbound

Strong inbound, strong

outbound

Azimuth 3

Azimuth 2

Azimuth 1

Enlarged image along a radial. Individual

“blocks” represent one sample

volume. This graphically shows the

radar resolution.

•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•Previous 10 beam width improved by super-resolution

Azimuth Resolution Considerations

Rotational couplet identification can be affected by azimuth resolution.

100% 100%

0%

0%

When the wind velocity is parallel to the radial, the full component of

the wind is measured

When the radial is perpendicular to wind direction, the radar

displays zero velocity - This “zero zone” is called

the “Zero Isodop”.

What percentage of actual wind will the radar

detect?

00 = 100% - Parallel150 = 97%300 = 87%450 = 71%600 = 50%750 = 26%900 = 0% -

Perpendicular

The Zero Isodop “Problem”

Large Scale Winds

“S” Shape

“S” shape of the zero isodop indicates

veering winds with height. Veering may

imply warm air advection.

The combination shape of the zero isodop

indicates both veering and backing winds with

height.

Combination

Backward “S” Shape

Backward “S” shape of the zero isodop indicates backing

winds with height. Backing may imply cold air advection.

Use the Zero Isodop to assess the vertical wind profile.

Large Scale Winds

Uniform Flow

Straight Zero Isodop indicates uniform direction at all levels.

Uniform Flow with Jet Core

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 -

Pure Cyclonic Rotation

Anticyclonic rotation would show

colors reversed

Small Scale Velocity Example

Small Scale Velocity Example

Rotation with tornado

Storm Relative Velocity - SRVvs

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 -

0.50

3.40

Base Velocity Storm RelativeLittle/no rotation seen at lowest

elevation

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 EchoesDetecting and Predicting Downbursts

oBow echoes are caused by severe downbursts, accelerating part of a line of thunderstorms ahead of the rest.

oThe strongest downbursts occur under and just north of the apex of the bow, but can occur elsewhere too

oSurface winds can exceed 70mph in strong bow echoes.

oBow echoes can move at over 50 mph.oHighest reflectivities and strongest

velocities are found at the apex.oLook for a tight gradient of reflectivity.

Review Clear-Air Radar

VHFUHF10 cm

1/3

4

VHFUHF10 cm

1/3

4

6 6

1

2N

i ii

n a

2nC

Clear-Air Wind Profilers

Wind Profiler SpecificationsFrequency (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 5 x 5 Clear Air and

Precipitation

UHF

1036 ~0.3 5.5-6 5 x 5 Clear Air and

Precipitation

UHF