Validation of Satellite Precipitation Estimates for Weather and Hydrological Applications
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Validation of Satellite Precipitation Estimates for Weather and Hydrological
Applications
Beth EbertBMRC, Melbourne, Australia
3rd IPWG Workshop / 3rd APSATS, 23 October 2006, Melbourne
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Satellite precipitation estimates -- what do we especially want to get right?
Climatologists - mean bias
NWP data assimilation (physical initialization) - rain location and type
Hydrologists - rain volume
Forecasters and emergency managers - rain location and maximum intensity
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Short-term precipitation estimates• High spatial and temporal resolution desirable
• Dynamic range required
• Motion may be important for nowcasts
• Can live with some bias in the estimates if it's not too great
• Verification data need not be quite as accurate as for climate verification
• Land-based rainfall generally of greater interest than ocean-based
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Some truths about "truth" data
• No existing measurement system adequately captures the high spatial and temporal variability of rainfall.
• Errors in validation data artificially inflate errors in satellite precipitation estimates
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Rain gauge observations
Advantages DisadvantagesTrue rain measurements May be unrepresentative of
aerial valueVerification results biased
toward regions with high gauge density
Most obs made once daily
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Radar dataAdvantages DisadvantagesExcellent spatial and Beamfilling, attenuation,
temporal resolution overshoot, clutter, etc.Limited spatial extent
TRMM PR
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Rain gauge analysesAdvantages DisadvantagesGrid-scale quantities Smoothes actual rainfall Overcomes uneven values
distribution of raingauges
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Verification strategy for satellite precipitation estimates
Use (gauge-corrected) radar data for local instantaneous or very short-term estimates
Use gauge or radar-gauge analysis for larger spatial and/or temporal estimates
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Spatial verification methods
• Visual ("eyeball") verification• Continuous statistics (RMS, correlation, bias, etc)• Categorical statistics (POD, FAR, etc.)
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• Scale decomposition methods • Entity-based methods
"standard"
"scientific" or "diagnostic"
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Step 1: Visual ("eyeball") verificationVisually compare maps of satellite estimates and
observations
Advantage: "A picture tells a thousand words…"
Disadvantages: Labor intensive, not quantitative, subjective
Verifies this attribute?LocationSizeShapeMean valueMaximum valueSpatial variability
Rozumalski, 2000
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Time series of error statistics
24-hr rainfall from NRL Experimental Geostationary algorithm validated against Australian operational daily rain gauge analysis
0.25° grid boxes, tropics only
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Continuous statistics
Mean absolute error||1
1i
N
ii OF
NMAE
Measures average magnitude of error
Root mean square error2
1
)(1i
N
ii OF
NRMSE
Measures error magnitude, with large errors having a greater impact than in the MAE
)(11
i
N
ii OF
NMean Error
Mean error (bias) Measures average error
Correlation coefficient
22 )()(
)( )(
OOFF
OOFFr
Measures correspondence between estimated spatial distribution and observed spatial distribution, independent of mean bias
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Estimated yes no
yes hits misses
no false correctalarms negativesO
bser
ved
Estimated Observed
Falsealarms
Hits
Misses
Correct negatives
Categorical statistics
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Categorical statistics
Probability of Detectionmisseshits
hitsPOD
False Alarm Ratioalarmsfalsehits
alarmsfalseFAR
Threat score (critical success index)
alarmsfalsemisseshitshitsCSITS
Equitable threat score
random
random
hitsalarmsfalsemisseshitshitshitsETS
Bias score misseshitsalarmsfalsehitsBIAS
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Real-time verification example24-hr rainfall from NRL Experimental Geostationary algorithm
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Web links for satellite precipitation validation
• IPWG validation of daily rainfall estimates over many regions - http://www.bom.gov.au/bmrc/SatRainVal/validation-intercomparison.html
• Climate Rainfall Data Center (monthly validation, global) - http://rain.atmos.colostate.edu/CRDC/
• Program for the Evaluation of High Resolution Precipitation Products - http://essic.umd.edu/%7Emsapiano/PEHRPP/
• Validation of 6-hourly and daily Hydro-Estimator and other geostationary estimates over US - http://www.orbit.nesdis.noaa.gov/smcd/emb/ff/validation/validation.html
• TOVAS validation of daily TRMM-based estimates over US - http://disc2.nascom.nasa.gov/Giovanni/tovas/rain.ipwg.shtml
• Forecast verification – Issues, Methods, and FAQ - http://www.bom.gov.au/bmrc/wefor/staff/eee/verif/verif_web_page.html