Presentation on theme: "Rainfall data for validating satellite rainfall estimates - Precipitation network set up in Africa for AMMA Henri Laurent Marielle Gosset (Benin) Christian."— Presentation transcript:
1 Rainfall data for validating satellite rainfall estimates - Precipitation network set up in Africa for AMMAHenri LaurentMarielle Gosset (Benin)Christian DepraetereThierry Lebel (Niger)IRD/LTHE, Grenoble, FranceAbou AmaniAbdou AliAgrhymet, Niger
2 Ground validation issues - time sampling effect“ground truth”: estimation of areal rainfall and accuracyNeed for a good knowledge of rain fields raingauge networks radarObservations and studies in the AMMA framework (African Monsoon Multidisciplinary Analyses)regional scale – daily rainfall observationsmeso scale – high resolution observations
3 Convective cloud cover – 1 month Reference: all Meteosat imagesTime sampling problemRainfall estimate error (%)1°x1°5°x5°annualJul/AugJun/SepSSM/I407310129TRMM36769223MT256518Sampling: SSM/ISampling: TRMMUsing the dense raingauge network in Niamey (1°x1°), it has been shown that the time sampling error is reduced by % for MT compared to TRMMSampling: MT
5 Raingauge network - Sahel Summary on the kriging method used to create the areal rainfall for validation of satellite rainfall estimates Regression Kriging method used to estimate the mean areal rainfall(grid: 0.5°x0.5°, 1°x1° or 2.5°x2.5°)cumulated oved 10-day, monthly or annual periodsAnisotropy of rainfall fields: the drift has to be taken into account For details on the kriging method, see:Ali et al., J. Appl. Meteo, 2005 (in press)
6 Comparison of different methods for areal rainfall estimate 3 krigging methods taking the drift into accountTheor. errorObserved errorInterpolated values are close,but very different estimations of theoretical error
7 Intercomparison of different satellite products Monthly rainfall, 2.5°x2.5° SahelCMAP (Sat+gauges)GPCP (Sat+gauges)GPCC (gauges)GPI (Sat)SYN (gauges from synoptic network)RMSE (%)
8 Rainfall at regional scale: daily estimation? Tracking from METEOSAT infrared channel19 July 1994Kriging from CILSS daily rain gauges
9 African Monsoon Multidisciplinary Analysis AMMA Long term experimental set up3 meso-scale sites – 2 of which are already well equiped for precipitation measurements.Upper OUEME valleykm2Soudanianrain : mm/yearspecialized in water budget / hydrological processes.data since 1997Gourmakm2Sahelian to SaharianRain : mm/yearsite specialized in vegetation + satellite validation.Few rain gages / Possibility to densify.Niamey square degreekm2Sahelianrain : mm/yearSpecialized in hydrology and the study of land / rain intreactions.Data since 1990 (Hapex –Sahel, Epsat-Niger)
10 Niamey square degree Since 1989: between 30 and 109 raingauges Many works, e.g.:Lebel et al., Water Res. Res., 1992 – Amani et al., Water Res. Res., 1996 – Lebel et al., J. Hydro, 1997 – LeBarbé and Lebel, J. Hydro, 1997 – Amani and Lebel, J. Hydro, 1997 – Amani and Lebel, Sto. Hydro., 1998 – Lebel and Amani, J. Appl. Meteo, 1999 – Mathon et al., J. Appl. Meteo, 2002 – Ali et al., J. Hydromet., 2003SeeVery good knowledge of Sahelian rain fieldsmodelisation, downscaling issuesinterpolation and estimation (kriging)estimation errorNot simply transposable in another region First it is needed to study precipitation fields
11 Raingauge network – Mesoscale sites Available data sets for 2004 Niamey square degree (Niger)33 recording rain gaugesRaw data 5 min, ~ from May to September (depends on the station)Validated data:10-day periods - station and grid (kriging 5 km x5 km)rainfall events (i.e. >30% rainy stations) - station and grid (kriging 5 km x5 km)Oueme (North Benin)35 recording rain gaugesRaw data 5 min, all over the year (with some missing data)Validated data: daily rainfall (stations)Gridded estimates are not available (yet) by lack of climatological knowledge
13 Details: www.lthe.hmg.inpg.fr/catch/xport/ X port Radar developed at LTHE X band – 9.4 GHz diameter 1.8 m – 100 kW polarisation H and V dopplerDetails:Optical Spectrogranulometer, recording data every 1 min on the rain drop size distributions observed at ground level.
14 X-port : Data and objectives Donga / Bénin, 2004-2007 - EOP AMMA Which measurement ?-Reflectivity (power returned to the radar by the precipitation) Amount of precipitation-Polarimetric variables (difference between Horizontal and Vertical signal) median diameter of the drop size distribution attenuation correction-Doppler velocities of hydrometeors.2D - Structure3 D structuremultiparameters
15 Radar - vertical profiles analysis Vertically pointing mode: vertical structure of precipitation(images VPR McGill)Derive statistics of vertical structure in a given climatic regionobserve amount of convective vs stratiform rainquantify occurrence of evaporation improve parameterization feed data base for satellite remote sensing algo.(+ FFT analysis of Doppler spectrum -> evolution of DSD with height)
16 Radar + disdrometer data - Application High resolution 2D fields of precipitation – Homogeneity ? Propagation at ground ? - Down scaling issues - Observation of vertical profile of reflectivity within rain storm – gather profil types and assess variability, useful for inversion of satellite data - DSD analysis + radar polarimetric product : Analyze the time/space variability of Drop size distributions at the ground level.
17 Rainfall types during the rainy season (1999-2003) Ongoing studies on the characterisation of the rainfall events in North Benin using rain gauge dataDevelopment of a rainfall model valid for Sahel and sub-Sahel rain fields. Based on a modelisation of convective cellsRainfall types during the rainy season ( )Identification of Mesocale Convective Systems (rainfall) and determination of their propagation (speed and direction)Depraetere et al., EGU Conference, April 2005, Vienna, Austria
18 Directional chronogram of the rainfall event Computation of optimal direction and speed of the rainfall eventDirectional chronogram of the rainfall eventMeso scale hyetogram derived from the pseudo-chronogramDirectional pseudo-chronogram of the rainfall event
19 Possible collaborations on ground validation issues SummaryPossible collaborations on ground validation issuesrainfall ground truth – estimation and estimation error - regional scale - meso scale possibilities in the West African, sub-Sahelian zone : - Data from the Upper Ouémé meso-scale site - Rainfield modelling /Down scaling issues. - Use of a light X-Band, polarimetric radar for field observation of the precipitating systems
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