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Bias and random errors in radar measurements of precipitation and their
scale dependence.
Aldo Bellon, Isztar Zawadzki, and GyuWon Lee
McGill University, Montreal Canada
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Strong attenuation by precipitation at
C-band
Z R
Wet-radome
Attenuation
Each source of error in precipitation Each source of error in precipitation estimates by radar is evaluated separatelyestimates by radar is evaluated separately
Beam broadening
Height increase
How to get Z at How to get Z at groundground !
Radar calibration
AP+GE removal
Optimum Surface Precipitation (OSP)
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High Resolution Sector:15-35 km & 120-320 az.
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Projection of near data into far ranges
h=1.1 1.5
1.9 3.1
40
80
120
160
200
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When observedat a farther range
ERRORS DUE TO EXTRAPOLATION FROM HEIGHT OF MEASUREMENT TO GROUNDERRORS DUE TO EXTRAPOLATION FROM HEIGHT OF MEASUREMENT TO GROUND
BW=0.2 0.5
1.72.0
To determine the errors,values at the measurementheight are compared with the low level ground truth
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Projection of near
range VPRs into far ranges
250 hr of data,
21 events
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Errors in non-
corrected
Errors in corrected
data
Height of lowest elv. angle
ref
ref
R
RRn
NRMS
2/1
2)(1
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Climatological Vertical Profiles
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Errors before Correction with Correction with
correction inner VPR climatological VPR
* * *
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Total Error Summary: Stratified by BB height(Data from 0.50 elevation for r>100km and from 1.5 km CAPPI for r<100km)
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Spatial-scale dependence of errorsSpatial-scale dependence of errors
range
Normalized RMS error after VPR correction as a function of the verification area.(For 1-hr accumulations at the “lowest default height”).
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Time-scale Time-scale dependence dependence
of errorsof errorsNormalized RMS error atthe lowest default height after correction by the VPRas a function of accumulationtime at 10-km resolution.
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Representativeness of VPRRepresentativeness of VPR
Increased NRMS errors for 1-hr accumulations generated with VPR correctionfactors that are appropriate for a different time interval (in order to simulatea different VPR at farther ranges). The uncorrected NRMS and that from theclimatological correction are provided as reference. (At the “lowest default height” and at 10-km resolution).
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ConclusionsConclusionsErrors depend: Height of BB, Verification area,
Length of accumulation, Range
Errors < 20 % if: BB height > 2.5 km, Area > 100 km2
Acc. Time > 45 min. Range < 130 km
Errors due to Ground clutter, DSD variability (R-Z uncertainty) Calibration, Attenuation (C-band) must be added to the VPR errors.
Essential to differentiate between Stratiform/Convection, particularly at far ranges (> 150 km)
The non-homogeneity of the VPR adds may substantiallyincrease the error.