Date post: | 16-Dec-2015 |
Category: |
Documents |
Upload: | harmony-parkman |
View: | 215 times |
Download: | 1 times |
Validation of Satellite Precipitation Estimates for Weather and Hydrological
Applications
Beth Ebert
BMRC, Melbourne, Australia
3rd IPWG Workshop / 3rd APSATS, 23 October 2006, Melbourne
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
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
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
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
Radar dataAdvantages DisadvantagesExcellent spatial and Beamfilling, attenuation,
temporal resolution overshoot, clutter, etc.Limited spatial extent
TRMM PR
Rain gauge analysesAdvantages DisadvantagesGrid-scale quantities Smoothes actual rainfall Overcomes uneven values
distribution of raingauges
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
Spatial verification methods
• Visual ("eyeball") verification• Continuous statistics (RMS, correlation, bias, etc)• Categorical statistics (POD, FAR, etc.)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
• Scale decomposition methods • Entity-based methods
"standard"
"scientific" or "diagnostic"
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
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
Continuous statistics
Mean absolute error
||1
1i
N
ii OF
NMAE
Measures average magnitude of error
Root mean square error2
1
)(1
i
N
ii OF
NRMSE
Measures error magnitude, with large errors having a greater impact than in the MAE
)(1
1i
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
Estimated yes no
yes hits misses
no false correctalarms negativesO
bser
ved
Estimated Observed
Falsealarms
Hits
Misses
Correct negatives
Categorical statistics
Categorical statistics
Probability of Detectionmisseshits
hitsPOD
False Alarm Ratioalarmsfalsehits
alarmsfalseFAR
Threat score (critical success index)
alarmsfalsemisseshits
hitsCSITS
Equitable threat score
random
random
hitsalarmsfalsemisseshits
hitshitsETS
Bias score misseshits
alarmsfalsehitsBIAS
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