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Jeffrey G. Cunningham, W. David Zittel, Robert R. Lee, and Richard L. IceRadar Operations Center, Norman, OK
Nicole P. HobanUniversity of Missouri, Columbia, Missouri
AMS 36th Conference on Radar Meteorology September 16-20, 2013Breckenridge, Colorado
9B.5
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2
Improve performance of dual polarization quantitative precipitation estimates (QPE) by
reducing absolute systematic ZDR bias
9B Polarimetric Radar ‐ Data Quality / Calibration
Note: Absolute ZDR bias of 0.1 to 0.2 dB produces a 10 - 30% QPE error!
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9B Polarimetric Radar ‐ Data Quality / Calibration 3
1. Develop automated observational methods that evaluate the performance of the current WSR-88D system ZDR calibration method
2. Present fleet-wide statistics from the observational methods
3. Combine these methods to isolate & correct sources of biases
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Light Precipitation
9B Polarimetric Radar ‐ Data Quality / Calibration 4
Data collected on targets with intrinsic ZDR = 0
Stratiform
*After correction applied (developed by A. Ryzhkov)
ZDR* = 0 dB
Boundary LayerDiscontinuity
Moist
ZDR = 0 dB
Dry
Bragg scatter
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9B Polarimetric Radar ‐ Data Quality / Calibration 5
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Light Precipitation
9B Polarimetric Radar ‐ Data Quality / Calibration 6
Data collected on targets with intrinsic ZDR = 0
Stratiform
*After correction applied (developed by A. Ryzhkov)
ZDR* = 0 dB
Boundary LayerDiscontinuity
Moist
ZDR = 0 dB
Dry
Bragg scatter
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9B Polarimetric Radar ‐ Data Quality / Calibration 7
Convective
CaribouKCBW3 Aug ‘1319:22 Z
ZDR
ZDR
Stratiform
CaribouKCBW10 Aug ‘1304:30 Z
Reflectivity
Reflectivity
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9B Polarimetric Radar ‐ Data Quality / Calibration 8
Convective
CaribouKCBW3 Aug ‘1319:22 Z
ZDR
ZDR
Stratiform
CaribouKCBW10 Aug ‘1304:30 Z
Reflectivity
Reflectivity
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9B Polarimetric Radar ‐ Data Quality / Calibration 9
Caribou, ME (KCBW)Z D
R(d
B)
Month/Day
Systematic ZDR Bias
Light Precip (Conv)Light Precip (Strat)
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9B Polarimetric Radar ‐ Data Quality / Calibration 10
Antenna bias changedmanually based on calibration guidance
Antenna bias resetto previous value
Month/Day
Systematic ZDR Bias
Light Precip (Conv or Strat)
Z DR
(dB
)
Boston, MA (KBOX)
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9B Polarimetric Radar ‐ Data Quality / Calibration 11
82 Sites 100 Sites
116 Sites 129 Sites
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9B Polarimetric Radar ‐ Data Quality / Calibration 12
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Light Precipitation
9B Polarimetric Radar ‐ Data Quality / Calibration 13
Data collected on targets with intrinsic ZDR = 0
Stratiform
*After correction applied (developed by A. Ryzhkov)
ZDR* = 0 dB
Boundary LayerDiscontinuity
Moist
ZDR = 0 dB
Dry
Bragg scatter
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9B Polarimetric Radar ‐ Data Quality / Calibration 14
Reflectivity< 10 dBZ
DifferentialReflectivity≅ 0 dB
DifferentialPhase≅ Initial
systemΦDP ≅25°
CorrelationCoefficient> 0.98
Rings at ~15 and 25 km
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9B Polarimetric Radar ‐ Data Quality / Calibration 15
Little Rock, AR (KLZK)15-16 UTC 22 May 2013
Median = 0.94, Mode = 0.75
ZDR (dB)
Bin
Cou
nt
Difference from 0 issystematic ZDR bias
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Month/Day
Z DR
(dB
)
9B Polarimetric Radar ‐ Data Quality / Calibration 16
Systematic ZDR Bias
Light Precip (Conv)Light Precip (Strat) Bragg Scatter
Little Rock, AR (KLZK)
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9B Polarimetric Radar ‐ Data Quality / Calibration 17
Raleigh, NC (KRAX) 16-17 UTC 25 May 2013
Median = -0.50, Mode = -0.50
ZDR (dB)
Bin
Cou
nt
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9B Polarimetric Radar ‐ Data Quality / Calibration 18
Month/Day
Z DR
(dB
)Raleigh, NC (KRAX)
Systematic ZDR Bias
Light Precip (Conv)Light Precip (Strat) Bragg Scatter
Dual polarization components replaced& full calibration done
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9B Polarimetric Radar ‐ Data Quality / Calibration 19
Systematic ZDR Bias
Light Precip (Conv)Light Precip (Strat) Bragg Scatter
Z DR
(dB
)
Month/Day
Caribou, ME (KCBW)
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1. The light precipitation method indicates that 56-59% of cases (54-58% of sites) have a ZDR bias within ±0.2 dB for April-July 2013
2. The Bragg scatter method is a viable alternative to the light precipitation method
3. The light precipitation or Bragg scatter method, when combined with the sunspike method (not shown but described in paper) and data logs, can isolate component problems
9B Polarimetric Radar ‐ Data Quality / Calibration 20
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1. Reduce the variance of ZDR biases across the WSR-88D fleet
a) Maintenance procedures (barn door vs. mouse hole)b) Hardware limitations (connectors, antenna positional
accuracy)
2. Refine observational methodsa) We know our methods do not provide estimates
within 0.1 dB reliablyb) Many observations required to establish biasc) Large changes to bias seen quickly; trends require
many observations to discern3. Geographical / climatological limitations
9B Polarimetric Radar ‐ Data Quality / Calibration 21
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9B Polarimetric Radar ‐ Data Quality / Calibration 22
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9B Polarimetric Radar ‐ Data Quality / Calibration 23
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∆ Receive Path BiasTransmit Path Bias ∆
Methods for estimating
ZDR Calibration Equations
Current engineering method (∆ • Ideally ∆ ∆
Observational (volume scans) • Light Precipitation (∆• Bragg scatter (∆ )
Estimating Sunspikes
• Δ Δ
Total Systematic ZDR Bias
9B Polarimetric Radar ‐ Data Quality / Calibration 24
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9B Polarimetric Radar ‐ Data Quality / Calibration 25
Parameter FilterVCP 11, 12, 121, 211, 212, 221, 31, 32Elevations > 1.0°Range > 20 kmReflectivity 19.0 to 30.5 dBZ, 2 dB StepsCorrelation Coefficient CC > 0.98Melting Layer 1 km below ML HeightSignal-to-Noise Ratio S/N > 20 dB
Adjusted Median ZDR > 200 bins / step٭
Average ZDR Adjusted Median per category
Volumes Averaged 12
Z (dBZ) 20 22 24 26 28 30
ZDR (dB) 0.23 0.27 0.32 0.38 0.46 0.55
٭
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Parameter FilterBig Drop Contamination Test < 40 bins with 40 dBZ or higher
above melting levelStratiform Test
Weak Reflectivity 10 to 30 dBZStrong Reflectivity > 30 dBZSignal-to-Noise Ratio > 20 dBRatio weak to total reflectivity bins (x100)
80%
Duration per Plotted Point Median over 3 hrs
9B Polarimetric Radar ‐ Data Quality / Calibration 26
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9B Polarimetric Radar ‐ Data Quality / Calibration 27
Parameter FilterVCP 21,32Elevations 2.5° & above (batch modes) Range 10 to 100 kmReflectivity -32 < Z < 10 dBZCorrelation Coefficient 0.98 < CC < 1.05Velocity V < -2 or V > 2 m s-1
Spectrum Width W > 0 m s-1
Signal-to-Noise Ratio 0 < S/N < 15 dB
Differential Phase 25° < Φdp < 35°1 Hour Histogram ≥ 35,000 bins
Yule-Kendall IndexSymmetry Test
≤ 0.1
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Zdrtrue = 0 dB
Using Sunspikes in Level II Datato Monitor Zdr Bias
Assumption: Sun is unpolarized
Receiver and Antenna (Ant) Bias OnlyNo Transmit Path (Tx)
0
28
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9B Polarimetric Radar ‐ Data Quality / Calibration 29
Differential Phase (deg)Random values 0-360 deg
Differential Reflectivity (dB)± 2 dB about central value
Correlation Coefficient (/100)< 0.30
Reflectivity (dBZ)-32 to 25 dBZ S/N 10 to 15 dB
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9B Polarimetric Radar ‐ Data Quality / Calibration 30
Parameter Filter
VCP 31,32
Elevations 1.5°, 2.5° (surveillance cut)
Range 20 to 460 km
Signal-to-noise Ratio > 10 dB & < 15 dB
Bin Count > 1000
Volume Scans 3 closest to target elevations morning & evening
Radial Best positional match
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9B Polarimetric Radar ‐ Data Quality / Calibration 31
∗ Δ Δ○ ∆∆ ≅ ∆
Sunspike ZDR Median
= -0.024
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9B Polarimetric Radar ‐ Data Quality / Calibration 32
ZDR (dB)
Sunspike ZDR Median
= -0.024
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9B Polarimetric Radar ‐ Data Quality / Calibration 33
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9B Polarimetric Radar ‐ Data Quality / Calibration 34
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9B Polarimetric Radar ‐ Data Quality / Calibration 35
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9B Polarimetric Radar ‐ Data Quality / Calibration 36
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Total System Bias = Rx Path Bias + Tx Path Bias
Contributions to System ZDR Bias
∆Rx = rxbias + antbias (Rx Path Bias)
∆Tx = txbias + antbias (Tx Path Bias)
Hardware Method
Component in the system ZDR offset
Parameter How it is measured
RCB
Hpow, Vpow Routine receive measurements using CW calibrated signals
R293, R294 Factory measuredR297 Cross-and-straight measurements
TXB
Hps, Vps Routine power sense measurementsR295, R296 Factory measuredR298 Cross-and-straight measurements
SMB Psh, Psv Suncheck measurementsRCB See the first table entry
Melnikov and Zrnic (2013 MOU Report)
(antbias)
37
(rxbias)
(txbias)