Anomalous Propagation Greater density slows the waves more. Less dense air does not slow the waves...

Anomalous Propagation

Greater density slows the waves more.

Less dense air does not slow the waves as much.

Since density normally decreases with height, the radar beam is refracted toward the surface of the Earth.

Refraction

Subrefraction

If the decrease in density with height is more than normal, then the beam bends less than normal and this is called subrefraction.

In this case the beam might shoot over the target and miss the precipitation.

Subrefraction (Cont.)

Superrefraction

If the decrease in density with height is less than normal, then the beam bends more than normal and this is called superrefraction.

In this case the beam bends more toward the surface of the Earth, and it may undershoot the target.

Superrefraction (Cont.)

Ducting

If the decrease in density with height is much less than normal, the beam may bend down to the surface of the Earth in a process called ducting.

If the beam is backscattered to the receiver, it may result in Anomalous Propagation (AP) or “false echoes”.

Ducting (Cont.)

How to read the intensity scale

Clear-Air ScalePrecipitation Mode Scale

Light Precipitation

Very light precipitation

Fog, Clouds, Smoke

Dust (?? Skeptic)

♦Units are decibels of Z (reflectivity).

ExtremeIntenseSevere

Heavy

Moderate

Light

Very light

, Insects, Birds

Hail Detection

•Returns > 55 dBz usually indicate hail.•However, the probability of hail reaching

the ground depends on the freezing altitude.•Usually, a freezing level above 4300 m

(14,000 feet) will not support much hail.•This is because the hail melts before

reaching the ground.•Freezing level can be determined from an

upper air sounding.

Hail?

Max return of 60 dbZ Max return of 65 dbZ

Freezing level was 2100 m Freezing level was 5200 m

Produced golfball sized hail Produced no hail

Hence, hail production depends directly on freezing level.

(7,000 feet) (17,000 feet)

Vertically Integrated Liquid (VIL)■Take a vertical column of the atmosphere:

estimate the amount of liquid water in it.■High VIL values are a good indication of hail.

•The white pixel indicates a VIL of 70.

•This storm produced golfball size hail.

•Drawback: complete scan required (time)

The Hail SpikeAlso called Three-Body Scattering

▪A dense core of wet hail will reflect part of the beam to the ground, which then scatters back into the cloud, and is bounced back to the antenna.

▪The delayed returns trick the radar into displaying a spike past the core.▪Usually, will only result from hail 1 inch in diameter or larger (quarter size).

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.●Totals are in inches●Time range is

sometimes listed on image.●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

●Has helped issue flash flood warnings more efficiently

●Helps fill in the holes where ground truth information is not available

●Much better lead time for warnings

●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

●Lower resolution than reflectivity images

●Useful as a supplement, not replacement for ground truth information

How Doppler Wind Is Displayed Inbound velocities (towards the radar) are shaded blue, with pale shades for light winds and dark shades for strong winds. Outbound velocities (away from the radar) are shaded orange with pale shades (yellow) for light winds and dark shades (red) for strong winds.

Velocity ImageryWarm colors are winds moving away from radome(reds, +)

Cool colors are winds moving toward radome(greens, -)

Tight area of opposing winds (+ and -) can indicate convergence or rotation. Circled area called a couplet. Indicates a possible tornado.

Wind speed is in knots

Detecting Rotation

A velocity couplet may indicate rotation.

X

+ (radar site)

inbound radial velocities

outound radial velocities

Detection Rotation (Cont.)

Not all velocity couplets indicate rotation.

+ (radar site)

inbound radial velocities

outbound radial velocities

Linear (straight line) flow over the radar site.

Bragg Scatter

2sinr

s

Bragg Scatter

Clear-Air Turbulence

Clear-Air Wind Profilers

Radial VelocityPositive TowardSurface

Signal Power

NWS-NOAA Profiler Network

SODAR

SODAR

Parameter Performance

Horizontal wind speed components Range 0-20 m/s, accuracy 0.2 m/s

Horizontal wind speed vectors Range 0 - 25 m/s

Vertical wind speed components Range 0 - 10 m/sec, accuracy 0.1 m/s

Horizontal wind Direction 0 - 359 degrees

Resolution of reading 0.1 m/s

Sampling Height 50 m - 900 m AGL

Environmental Conditions -10C to + 40C, 0 - 100% humidity

Acoustic Frequency 1525 - 2225 Hz (selectable)

Performances of ARPL SODAR

Radio-Acoustic Sounding (RASS)

Ene

rgy

Abs

orbe

d by

Atm

osph

ere

Radar Wavelength

35 GHz

94 GHz

MaximumPropagation

Distance

20-30 km

10-15 km

8 mm3.2 mm

The DOE Cloud Radars

Small Cloud Particles Typical Cloud Particles Very Light Precipitation

Surface

10-km

20-km

Cloud Radar Data from Southern Great Plains

Black Dots:Laser MeasurementsOf CloudBase Height

7:00 pm 7:00 am 7:00 pmtime

Small Cloud Particles Typical Cloud Particles Very Light Precipitation

Surface

10-km

20-km

Cloud Radar Data from Southern Great Plains

Black Dots:Laser MeasurementsOf CloudBase Height

ThinClouds

Insects

7:00 pm 7:00 am 7:00 pmtime