Multi-senseor satellite precipitation estimates on the African continent using combined morphing and histogram-matching techniques Malte Diederich 1, Aynur Bozoglu 2, Clemens Simmer 1, Alessandro Bataglia 1 1 University Bonn 2 EUMETSAT Collaboration between IMPETUS, AMPE, and Precip-AMMA projects

Multisensor Satellite Precipitation Estimates Goals of presented work:  Provide high resolution precipitation estimates (Meteosat-7/MSG resolution) in West Africa (IMPETUS Project)  Create a product that can be merged with and used to dis/reagdregate ground observations  Test and recommend possibilities for upgrades to the EUMETSAT MPE => AMPE  Assess product useability for climatology, hydrology, agriculture (as far as possible) Procedure: Merge information from geostationary IR images and passive microwave platforms  Probability Matching  Morphing  Identify non-raining clouds with SEVIRI – Channels 7 to 10 Examine possibilities for regionalized calibration  Ground validation with interpolated kriged products as well as point measurements

Sensors Geostationary Satellites METEOSAT 7: 30 minutes sampling interval30 minutes sampling interval 5 km resolution at sub-satellite point5 km resolution at sub-satellite point 11 µm IR channel11 µm IR channel Low-earth orbit Satellites Passive Microwave Sensors: TMI (TRMM Tropical Rainfall Measuring Mission) 2A12 ProductTMI (TRMM Tropical Rainfall Measuring Mission) 2A12 Product AMSR-E (AQUA Satellite)AMSR-E (AQUA Satellite) 3 SSM/I (DMSP F13, F14, F15)3 SSM/I (DMSP F13, F14, F15) (3 AMSU-B) (NOAA 15, 16, 17) (Nesdis or simple scattering Algorithm)(3 AMSU-B) (NOAA 15, 16, 17) (Nesdis or simple scattering Algorithm) MSG-1: 15 minutes sampling interval15 minutes sampling interval 3 km resolution at sub-satellite point3 km resolution at sub-satellite point 10 µm IR channel10 µm IR channel Cloud Analysis with some potential for discriminating non-raining cloudsCloud Analysis with some potential for discriminating non-raining clouds

Probability matching of IR and RR: 1. Accumulate co-located passive micro-wave rain estimates and brightness temperature measurements: 200 km, +-4 day accumulation windows, spaced at 10 km and 1 day 2. Match the cumulative distribution function of rain estimates and IR 11 μm brightness temperatures to obtain a look-up table, associating the coldest cloud temperature with the highest rain rate. (histogram-matching) 3. The resulting look-up table translates IR radiances into rain rates Histograms IR Radiance Rain Rate Look-up table IR Radiance Rain Rate

Multisensor Satellite Precipitation Estimate Morphing of PMW rain estimates Calculation of advection vectors by cross-correlating subsequent IR images (10 μm): Size of windows for cross- correlation determines if tops of small clouds or storm systems are tracked. Scan during Overpass 2 h before propagated backward 2 h after propagated forward

Combining Morphing and Histogram Matching Forward and backward propagated PMW scans are merged with propability matching estimates using a weighted averaging system based on time to the last/next PMW scan: Propagated PMW weights decrease linearly with time from overpass, weight reaches 0 at 2 hours from original scan Temporal sampling of microwave-overpasses changes with latitude. At 50 Latitiude, histogram-matching component gives negligable contribution Example of weighting on one day at 10 degree latitude PMW scan Forward propagated PMW Backward propagated PMW Histogram-Matching

Ground Validation 1.Evaluation of Morphing and Probability matching performance. 2.Regional and temporal distribution of systematic errors Data Sets available for validation: Benin: DMN / CATCH / AMMA / IMPETUS monthly accumulations kriged at 0.1° resolution from a dense gauge network, Daily point measurements from 2002 to 2005 Sahel region: AGRHYMET procucts for June-September 2004, AMMA intercomparison excercise 10-day accumulations from filtered synop stations 10-day accumulations from 800 stations kriged at 0.5° resolution : Furthermore: Daily synop data from GPCC for AMPE validation: High density and quality in Europe, scarce and with gaps in Africa Daily Nigerian and South African gauge observations

Ground Validation Benin Monthly sums for the Benin, June-September 2002 Comparisons with a 0.1x0.1 degree Kriging product (IMPETUS) Histogram Matchingwith morphing

Regionaly dependant biases Gradients in air humidity and moisture advection from north to south may lead to altered relation between ice in cloud and surface rain

01 mm 11 mm 21 mm 31 mm 41 mm 51 mm 61 mm False alarm ratio Probablity of detection Skill score Shape of probability density function of daily estimates agreed well with grouund observations, but skill for correct daily prediction diminishes with intensity

Data quality of rainfall measurements in Benin Quality of ground measurements can be estimated from time series of satellite estimates. Some stations display other accumulation time than 6 UTC

Reliability of satellite and gauge cross-validation Satellite/gauge skill at gauge point Gauge/gauge skill as function of distance In addition: Flag single days if ground observation is extremely unrealistic Following validation of gauges in Benin with satellite data: 7% of rain days given by gauges can not have ocured on the associated date At least 6% of non-raining days (between 6% and 20%) given by gauges should have been rainy days

Ground Validation histogram matching onlywith morphing

9.2004Corr.Biasrmsd Morphed + IRS morphed Histo Corr.Biasrmsd morphed Histo Corr.Biasrmsd morphed Histo Lower performance in Europe, but morphing still improves estimates Infra-red screaning: Post-Processing of rain product where semitransparent clouds (SEVIRI Channels 8 and 10) and clear sky (Channel 10) are declared to be no rain areas Lower performance in european test area due to: underestimation of costal precipitation in microwave products Orographic rainfall partially recognized by PMW displaced by merging scheme Very small convective cells not detected by coarse resolution PMW

Future plans Validate other regions of Africa with filtered synop stations (GPCC-input, Nigeria, South Africa) homogeneization of the Passive Micro-Wave estimates with respect to pdf and bias Test other AMSU-B products Optimize weighting system beteween morphing and histogram matching with TRMM radar ground measurements Improve morphing estinates using EUMETSAT cloud type products Conclusions Morphing technique superior to probablility matching It is recommended to recalibrate PMW estimates regionaly, especially coastal areas: Positive bias in Sahel Negative bias in Europe strong bias gradient from moist to dry, especially near coasts Satellite estiamtes are relatively good at detecting rainfall even at point scale, but quantitative skill not so good Even in a relatively dense network like Benin, there are some regions where satellite is better for detecting rain than interpolated gauge products Adding IR screening or classification may inprove morphing estimates

Reminder Vertical reflectivity profile measured by MRR in Benin