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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.

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anomalous propagation
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.

subrefraction
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.

superrefraction
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.

ducting
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”.

how to read the intensity scale
How to read the intensity scale

Extreme

Intense

Severe

Heavy

Moderate

Light

Very light

Light Precipitation

Very light precipitation

Fog, Clouds, Smoke

Dust (?? Skeptic)

, Insects, Birds

  • Units are decibels of Z (reflectivity).

Precipitation Mode Scale

Clear-Air Scale

hail detection
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.
slide11

Hail?

Max return of 60 dbZ

Max return of 65 dbZ

Freezing level was 2100 m Freezing level was 5200 m

(7,000 feet)

(17,000 feet)

Produced golfball sized hail Produced no hail

Hence, hail production depends directly on freezing level.

vertically integrated liquid vil
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 spike
The Hail Spike

Also 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 tops
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
Precipitation Estimates

Storm TotalPrecipitation

  • 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.
slide16

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
Precipitation Estimate

Advantages 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
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 imagery
Velocity Imagery

Warm colors are winds moving away from radome

(reds, +)

Cool colors are winds moving toward radome

(greens, -)

Wind speed is in knots

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

detecting rotation
Detecting Rotation

A velocity couplet may indicate rotation.

+ (radar site)

outound radial velocities

inbound radial velocities

X

detection rotation cont
Detection Rotation (Cont.)

Not all velocity couplets indicate rotation.

inbound radial velocities

+ (radar site)

outbound radial velocities

Linear (straight line) flow over the radar site.

radial velocity
Radial Velocity

Positive Toward

Surface

slide35

94 GHz

35 GHz

Maximum

Propagation

Distance

Energy Absorbed by Atmosphere

10-15 km

20-30 km

3.2 mm

8 mm

Radar Wavelength

slide37

Cloud Radar Data from Southern Great Plains

20-km

Black Dots:

Laser

Measurements

Of Cloud

Base Height

10-km

Surface

time

7:00 pm

7:00 am

7:00 pm

Small Cloud Particles

Typical Cloud Particles

Very Light Precipitation

slide38

Cloud Radar Data from Southern Great Plains

20-km

Black Dots:

Laser

Measurements

Of Cloud

Base Height

10-km

Insects

Thin

Clouds

Surface

time

7:00 pm

7:00 am

7:00 pm

Small Cloud Particles

Typical Cloud Particles

Very Light Precipitation