anomalous propagation greater density slows the waves more. less dense air does not slow the waves...
Post on 21-Dec-2015
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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
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