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MULTI-SOURCES PRECIPITATION ESTIMATIONK. Tesfagiorgis, S. E. Mahani, R. Khanbilvardi(NOAA-CREST, CCNY, CUNY, NY-10031)
David Kitzmiller (NOAA-NWS Collaborator) (NOAA NWS/HL, Silver Spring, MD-20910)
1
NOAA-NESDIS CoRP 7th Annual SymposiumFort Collins, CO., August, 2010
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Outline 2
Objectives Data sets Comparison of the different precipitation estimation algorithms
The different bias correction techniques Spatial corrections Results for study cases
Conclusion Future work
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Objectives 3
To improve Satellite Precipitation Estimation (SPE) by selecting appropriate bias correction technique.
To develop a Multi-Sources Rainfall Estimation algorithm to help optimal rainfall estimations. Be capable of extending radar like outputs inside radar gap regions using satellite and the surrounding radar rainfall estimations.
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Data Sets4
Hourly 4kmx4km resolution for the Oklahoma region bounded by
94.50-100o W Longitude 34.50-37.0o N Latitude
Satellite Rainfall Estimations selected from the following NESDIS models AE (Auto-Estimator) GMSRA (GOES Multispectral Rainfall Algorithm)
HE (Hydro-Estimator) SCaMPR-(Self Calibrating Multivariate Precipitation Retrieval)
Blend-(IR/Microwave Blended Algorithm) Radar Rainfall Estimation
Radar Stage IV (ST-IV) Rain-gauge Measurements
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8677 cases, 8677x62x137=73,702,438 pixels considered
Comparison of the Different Rainfall Estimations
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Satellite Rainfall
Estimations
Radar Rainfall Estimation
Yes No
Yes Hits False Alarms
No Misses Correct negatives
Observed yes Observed no
Satellite Rainfall Estimation
Bias Score
False Alarm Ratio
GMSRA 2.71 0.63
HE 1.73 0.46
SCaMPR 2.41 0.68
Auto-Estimator
2.08 0.53
MissesHits
msFalse AlarHits
Score BiasmsFalse AlarHits
msFalse Alar
Ratio Alarm False
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Bias Corrections6
• Field Bias CorrectionGenerally it helps for:– Intensity correction– Frequency correction• Methods of bias corrections:
– Ratios of Mean, Median, Maximum– Mean of Ratio of the corresponding rainy pixels in both Satellite and Radar Rainfall Estimations
– Bias ratio field using Inverse Distance method
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Bias Corrections: continued…
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€
F =RR
SPE
€
F =RRmedian
SPEmedian
€
F =RRmax
SPEmax
We need to calculate the Multiplicative factors (F) for Bias corrections
RR-Radar Rainfalls and SPE-Satellite Rainfall Estimates
Method 1 of Bias correction
The ratio of Max and Mean gave a better output. However, ratio of max is not stable and reliable.
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Bias Corrections: continued…
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• How about Spatial Errors that might have already existed?
• Before working on the Bias Corrections, it is important to make spatial corrections between the satellite and the radar rainfall estimations.
• Spatial Correction using the Method of Least Squares (Brogan 1985):– Apply the method of Hill Climbing to cluster rainy pixels; because the clustered corresponding rainy pixels are easier to pick up
– Pick corresponding points (Rainy Pixels)– Write Least Square equations and apply the method of least squares on these points as shown in equations shown below.
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Spatial Correction
CoefficientsCoefficients InterpretationsInterpretations
Shift in longitude
Scale in longitude
Shear in the longitude
CoefficientsCoefficients InterpretationsInterpretations
Shift in latitude
Scale in latitude
Shear in the latitude
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N
i
iN
ji
Slati
Slonij
Rlat
N
i
iN
ji
Slati
Slonij
Rlon
0 0)()()(
0 0)()()(
01
10
0022
11
2
1
)( )( 1
......
)( )( 1
)( )( 1
)(
......
)(
)(
zzz SlatSlon
SlatSlon
SlatSlon
Rlon
Rlon
Rlon
00
10
01
01
10
0022
11
2
1
)( )( 1
......
)( )( 1
)( )( 1
)(
......
)(
)(
zzz SlonSlat
SlonSlat
SlonSlat
Rlat
Rlat
Rlat
00
10
01
Linear form of the equations with
N=3
R-Radar
S-Satellite
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Spatial Correction 10
Corresponding P
ixel Locations
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Method 2 of Bias Correction
Bias Corrections: continued…
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- Calculate the bias ratio between ST IV and HE- Calulate the bias field using Inverse distance weight technique- Multiply HE by the mean field bias
Method 2 provides a more radar like output both spatially and intesity wise.
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Bias Correction: continued…
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The performance of bias field method for a winter case
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Bias Correction: continued…13
Ensembles of rainfall for a pixel around the center of the study area
•Ensemble generation of bias fields•Instead of 1, 100 realizations
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Bias Correction continued….
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Uncorrected Corrected
Pixels used in the development of the algorithm are not part of the CORR analysis
Case-2006071022 (YYYYMMDDHH)
Case-2006122917 (YYYYMMDDHH)
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Conclusion15
Hydro-Estimator has a better detection capability than the others, so that it is chosen for further studies that will include radar estimations and rain-gauge measurements.
There are cases where the alignment algorithm faces difficulties. When rainfall is very cluttered in radar and continuous in satellite estimations.
In these cases it is difficult to pick up corresponding rainy pixels.
However we can still apply the Bias field generation Algorithm without doing the alignment.
Ensemble generation helps to account other errors (Eg. physical, paralax).
Generation of bias fields can potentially be used to correct satellite estimations in radar gap regions.
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Ongoing Works
Check the performance of the model in other geographical locations.
Implement a technique that will give a multi-sources rainfall algorithm by merging the radar and the satellite estimations.
Produce gridded rain-gauge measuremts using Bayesian Kriging and/or inverse distance method.
Merge the gridded rain-gauge with the combined radar-satellite rainfall estimation.
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Acknowledgements
This study was partially supported and monitored by the National Oceanic and Atmospheric Administration (NOAA) under grant number NA06OAR4810162. The statements contained within this presentation are not the opinions of the funding agency or the U.S. government, but reflect the author’s opinions.
I would like to thank Robert Kuligowski (Ph.D.) for providing all the necessary data.
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