1. Validation of Satellite Precipitation Estimates using High-Resolution Surface Rainfall Observations in West Africa
Paul A. Kucera and Andrew J. Newman
NCAR/RAL
Workshop on the Evaluation of High Resolution Precipitation Products
3-5 December 2007
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2. Operational Summary for NAMMA
Location: Western Senegal, Cape Verde Islands and adjacent ocean.
Time Period: 21 August – 30 September 2006.
A total of 20 precipitation events were observed during the experiment.
A variety of instruments were deployed in Senegal to measure precipitation:
NPOL 10 cm Polarimetric Doppler Radar
A network of high resolution rain gauges (38)
Two video disdrometers
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3. NPOL Radar NASA 10 cm (S-Band) Polarimetric Doppler Weather Radar.
Deployed in Kawsara, Senegal (40 km SE of Dakar).
Scanned with a 15 min temporal and 200 m spatial res to a max range of 150 km.
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4. Rain Gauge Network in Senegal
Large Scale Network
18 gauge sites were deployed to provide areal and range dependent coverage for NPOL.
Provide ground verification data for evaluation of satellite and radar rainfall estimates.
Deployed in June 2005.
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5. Rain Gauge Network in Senegal
Dense Scale Network
20 rain gauge sites were deployed to examine small-scale rainfall variability.
Deployed in August 2006.
Both networks continue to be deployed.
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6. Seasonal Rainfall Variation for 2006
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7. Precipitation Events Observed by NPOL
Event 1: Aug 2006
Event 2: 25 Aug 2006
Event 3: 26 Aug 2006
Event 4: Aug 2006
Event 5: 29 Aug 2006
Event 6: Aug 2006
Event 7: Sep 2006
Event 8: 02 Sep 2006
Event 9: 03 Sep 2006
Event 10: Sep 2006
Event 11: Sep 2006
Event 12: 11 Sep 2006
Event 13: Sep 2006
Event 14: Sep 2006
Event 15: 19 Sep 2006
Event 16: 22 Sep 2006
Event 17: 26 Sep 2006
Event 18: 27 Sep 2006
Event 19: 28 Sep 2006
Event 20: 29 Sep 2006
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8. Time Series of Precipitation Area Observed by NPOL
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9. Radar Rainfall Products
NPOL data were QC’ed to remove ground clutter, second trip, side lobes and other non-meteorological echo.
A + 2 dB bias correction (based on TRMM PR comparison) along with a ZDR correction was applied to NPOL data
Radar rainfall maps were created using NPOL reflectivity mapped to a 5 km horizontal grid at an altitude of 2 km.
Rainfall maps had a temporal resolution of 15 min.
Reflectivity data were converted to rainfall rates using a Z-R derived from co-located disdrometer data:
Z = 148 R1.4
Polarimetric-based algorithm is currently being developed.
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10. Satellite-Rainfall Comparisons
Radar rainfall estimates were averaged to the 0.25 x 0.25 deg grids and accumulated to 3 h resolution.
Comparisons were made with the following algorithms:
TRMM Multi-satellite Precipitation Analysis (TMPA)
CPC Morphing technique (CMORPH)
NRL-Blended (NRLB)
Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN)
HydroEstimator (HE) (Was not analyzed because of problems with the product files)
The high resolution products (0.10 deg x 1 h) were not analyzed in the present study, but is planned for future analysis.
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11. Results
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12. Time Series: RMSE
Algorithms tended to perform slightly better over ocean.
CMORPH had the lowest RMSE on average.
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13. Time Series: POD POD is low for the TMPA algorithm.
There is little difference between ocean and land.
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14. Time Series: HSS The TMPA algorithm had the lowest HSS.
All algorithms are very similar for ocean and land.
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15. Daily Rainfall Comparison
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16. Case Study: 14 September 2006
Large, organized squall line with trailing stratiform.
0000 UTC
0200 UTC
0400 UTC
0600 UTC
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17. Case Study: 14 September 2006
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18. Case Study: 03 September 2006 Isolated Convection Case 1000 UTC
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19. Case Study: 03 September 2006
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20. Statistical Analysis Summary
NRLB TMPA CMORPH PERSIANN
Corr:
Bias:
POD:
FAR:
Cond RMSE:
Equitable TS:
Hedike SS:
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21. Summary
During the field phase of NAMMA, the observed precipitation was episodic. A disturbance passed over Senegal every 1-3 days, larger systems every 3-5 days.
Events ranged from unorganized, isolated convective systems to long-lived, organized MCS’s.
Stratification of statistics indicated that algorithms performed similarly over ocean and land except for a few cases.
Results indicate that the CMORPH algorithm performed the best overall. However, the PERSIANN and NRLB performed very similar. The TRMM 3B42 algorithm had the lowest performance.
The CMORPH algorithm had the lowest bias of 1.18 and PERSIANN had the largest bias with a value of 2.19.
More investigation is needed to better understand the large differences seen in some of the events.
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22. Future Work Short-term: Long-term:
Continue to develop and evaluate NPOL radar rainfall products.
Validation of high(er) resolution precipitation products.
Spatial validation analysis.
Long-term:
Regional validation analysis in West Africa.
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23. Spatial Validation Spatial validation techniques aim to:
Account for uncertainties in timing and location
Account for field spatial structure
Provide information of the errors in physical terms
Provide information that is:
diagnostic
meaningful to users
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24. Radar Network in West Africa
Current Radars
Burkina Faso
Ouagadougou
Bobo Dioulasso
Mali
Bamako
Planned Radars
2 installations in Mali
Upgrade in Senegal
Drawback
Limited rain gauge network for error estimation
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25. Thank You
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