Post on 04-Jan-2016
Analysis & Diagnosis 1Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Radar Interpretation Problems
Reflectivity
Analysis & Diagnosis 2Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Acknowledgments
• Phil Chadwick’s Radar Palette Course• MetEd courses- stole a few slides• Polarimetric presentation- staff at King City• Dave Patrick.. Radar and snow rates.• K-stream
Analysis & Diagnosis 3Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Outline Radar Interpretation Problems-Reflectivity
• Attenuation– By Intervening Precipitation– Wet Radome
• Radar Beam filled or not filled • Low topped precip
– Drizzle
• Virga• Bright Band • The sometimes invalid assumptions of the z-r
relationship • Second Trip Echoes
Analysis & Diagnosis 4Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Radar Sources of Error
Attenuation
Analysis & Diagnosis 5Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Attenuation: What lies beyond?
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Attenuation
• The All time best Canadian Example
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Foreground Storm
Wedge of attenuation
Example of Attenuation in Precipitation
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Attenuation
Warm FrontalSupercell
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Attenuation
• north • end• of • derecho?
Analysis & Diagnosis 10Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Correcting Attenuation
Polarimetric Radar
Analysis & Diagnosis 11Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 12Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 13Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Heavy Rain QPE (cont’d)
Rain Accumulation (Z)
Analysis & Diagnosis 14Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Heavy Rain QPE (cont’d)
Rain Accumulation (ZCORR)
Analysis & Diagnosis 15Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Sources of Error - Radar Dome Wetting
Analysis & Diagnosis 16Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Bright Band
• Melting snowflakes are large bright radar targets
• Reflectivity from melting snow is larger than that of the rain below or the snow above as falling snow passes through the melting layer
• Huge impact on quantitative precipitation estimates
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Bright band shows up as ring (or partial ring) of high Z centred on the radar
Bright band
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Bright Band
• Freezing Level ~2.5km AGL
• Either associated with Rain or Freezing Rain
Melting of Snow Flakes starts at 2.5 km height
Analysis & Diagnosis 19Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Melting layer signature:- Z
Analysis & Diagnosis 20Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Melting layer signature:- ρHV
Bright Band Ring
Analysis & Diagnosis 21Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 22Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Vertical Bright Band over WKR January 21st, 2006
rainsnow
0°C
Analysis & Diagnosis 23Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 24Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Radar Beam Overshooting
What real weather the radar can miss
Analysis & Diagnosis 25Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
PPI and CAPPI
1.5km CAPPI
4.0 km CAPPI
0.3 Degree PPI
0.3 Degree PPI
Analysis & Diagnosis 26Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 27Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
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North East South West North
North East South West North
Approximate current lowest beam location BALD
Approximate proposed lowest beam location LOALA
Analysis & Diagnosis 29Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Radar Beam Undershooting
Weather that’s not hitting the ground
Analysis & Diagnosis 30Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Sources of Error - Virga
Typical of strong overrunning WCB with equally strong and dry CCB
Analysis & Diagnosis 31Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Cross section to the southwest
Analysis & Diagnosis 32Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Low-elevation angle Doppler Z PPI scan
Let’s look at the same case observed by the Doppler scans
Low level reflectivities to the west of the radar – rain?
Analysis & Diagnosis 33Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
3.5 Degree Doppler Z PPI scan
Echoes at higher altitude precedeprecipitation at low Levels – virga?
Analysis & Diagnosis 34Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
What the radar misses close in
Analysis & Diagnosis 35Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 36Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Algorithms that fail very close to Radar…Hail
• As you approach the cone of silence the top part of the storm is above the highest scan angle of the radar.
• Height of MAXR and VIL values are unrealistically low.
• URP hail algorithm fails.
Analysis & Diagnosis 37Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
June 16th 2008- 1900Z
Analysis & Diagnosis 38Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
June 16th 2008- 1900Z
Analysis & Diagnosis 39Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
June 16th 2008- 1900Z
150.25/3.25 161/5.75
Z 51.16 55.72
Zcorr 49.52 52.47
Zdr 2.52 2.67
Zdrcorr 2.17 0.53
Rhohv 0.98 0.76
21
1
2
Analysis & Diagnosis 40Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Sources of Error - ECHO TOP - Beamwidth and Beamfilling
Analysis & Diagnosis 41Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Echo Tops
Analysis & Diagnosis 42Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Partial Beam Filling
Analysis & Diagnosis 43Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Radar beam widths and sensitivity
• Not all radars are equal• In Canada• Many are retrofitted Enterprise radars with 1.1°
beam width and 4 m dishes• Some (10) are Andrews radars with 0.65° beam
width and 6 m dishes• Many have obvious issues
– differences in sensitivity between each other
• But many have less obvious, but significant problems that only become apparent with long-term precipitation accumulations
Analysis & Diagnosis 44Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Beam Spreading
At 240 km
3 km diameter
Radar beam
At 30 km
0.5 km diameter
Radar beam
Analysis & Diagnosis 45Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Partial Beam Filling
Analysis & Diagnosis 46Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
One storm 2 radars with different beamwidths
Analysis & Diagnosis 47Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 48Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 49Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
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Second Trip Echoes
to a distant target...first pulse
returning first pulse
second pulse
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Second Trip Echo
actual position of echo beyond range
Echo within range
mapped position of 2nd trip echo
Pulse OnePulse Two
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Second Trip Echo
Elong
ated R
adial
Weaker
Far AwayRandon
Doppler
Changing the PRF changes Location
Check the Satellite
Analysis & Diagnosis 53Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Purple Haze
Analysis & Diagnosis 54Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Reflectivity…Rain Rates and Snow Rates
Snow being depicted using the Rain Z-R Relationship
Snow depicted using the Snow Z-S Relationship
Analysis & Diagnosis 55Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Snow Reflectivity & Snowfall Rate
• Value of a depends on– whether the snow is dry or rimed– snowflake size distribution y-intercept constant
• Large N0 => large number of small flakes compared to large– radar dark matter ??
• Small N0 => small number of small flakes compared to large – snowflake terminal velocity
• dense flakes fall twice as fast
67.13/53/20, )]/([ SVNkZ trimeddry
67.1SaZ
Analysis & Diagnosis 56Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Snow Reflectivity & Snowfall Rate
• For a typical range of values of N0, and dry
snow falling at 100 cm/s and wet snow falling at 200 cm/s
– Here is a graph of Z vs. S in mm/hDry airy snow falling slowly; 0.75 mm/h
Rimed dense snow falling rapidly; 3 mm/h
Factor of 4 difference in snow rate for
same Z !!
Analysis & Diagnosis 57Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Snow Reflectivity & Snowfall Rate
• Plot these empirical Z-S relationships on the theoretical graph …
Spatial dendrites Z = 3300SZ = 3300S1.71.7
Standard Z = 1780SZ = 1780S2.212.21 Plates & columns
Z = 400SZ = 400S1.61.6
Standard eq’n underestimates almost all high
snow rate situations
Analysis & Diagnosis 58Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Percentage Underestimate of Snow Water Equivalent by Standard Z-S Equation - 10+cm Events
-70
-60
-50
-40
-30
-20
-10
0
YVR YXS YYC+4dBZ
YWG YYZ+4dBZ
YOW YFC YQM YHZ YYT Ave All33
Stations
Per
cen
tag
e S
WE
Un
der
esti
mat
e
Pct UnderEst SWE by Std Z-S eq'n - 10+cm Events
Snow Reflectivity & Snowfall Rate
Analysis & Diagnosis 59Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Analysis & Diagnosis 60Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Another hypothesis ZDR vs Z scatter plotsand observed precipitation (snow) • Red squares
– Radar estimate close
• Green– Radar estimate slightly
underdone
• Purple triangles– Radar low by factor of 2-3.
Analysis & Diagnosis 61Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Z and Rainrate
Analysis & Diagnosis 62Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
• Reflectivity Z is empirically related to rain rate R in the form :
Z = aRb
• In Canada we use Marshall-Palmer (stratiform rain) :
Z = 200RZ = 200R1.61.6
• U.S. NEXRAD :
Z = 300RZ = 300R1.4 1.4
• U.S. Tropical :
Z = 250RZ = 250R1.2 1.2
Empirical Z-R Equations
Analysis & Diagnosis 63Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
• Z proportional to drop diameter to 6th power.
• If more big drops than standard DSD radar overestimates rainfall – Attenuation often cancels
this out.
• If more small drops Z underestimates rainfall.
Analysis & Diagnosis 64Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Observed precipitation
Analysis & Diagnosis 65Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Radar Accumulations
Analysis & Diagnosis 66Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
Conclusions
• Radar returns are not always what they seem.• Before making conclusions think about
– -How high the beam is above the ground– How wide is it where you are looking.– What other echoes did it have to go through to get to
the storm you are interested in. – What are the chances the drop size (snowflake)
distribution is different from the “standard”– Does the beam go through a melting layer– Could there be second trip echoes.
• Despite all the above radar does quite a remarkable job of detecting weather.
Analysis & Diagnosis 67Radar Palette Home Radar ArtifactsRadar Palette Home Radar Artifacts
THE END