Henri Laurent Marielle Gosset (Benin) Christian Depraetere Thierry Lebel (Niger) IRD/LTHE, Grenoble, France Abou Amani Abdou Ali Agrhymet, Niger Rainfall data for validating satellite rainfall estimates - Precipitation network set up in Africa for AMMA L E T H
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Henri Laurent Marielle Gosset (Benin) Christian Depraetere Thierry Lebel (Niger) IRD/LTHE, Grenoble, France Abou Amani Abdou Ali Agrhymet, Niger Rainfall.
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Henri Laurent
Marielle Gosset (Benin)
Christian Depraetere
Thierry Lebel (Niger)
IRD/LTHE, Grenoble, France
Abou Amani
Abdou Ali
Agrhymet, Niger
Rainfall data for validating satellite rainfall estimates - Precipitation network
set up in Africa for AMMA
L ET H
- time sampling effect
-“ground truth”: estimation of areal rainfalland accuracy
Need for a good knowledge of rain fields
raingauge networks
radar
Observations and studies in the AMMA framework(African Monsoon Multidisciplinary Analyses)
- regional scale – daily rainfall observations
- meso scale – high resolution observations
Ground validation issues
Time sampling problem
1°x1° 5°x5°
annual Jul/Aug Jun/Sep annual
SSM/I 40 73 101 29
TRMM 36 76 92 23
MT 25 40 65 18
Rainfall estimate error (%)
Using the dense raingauge network in Niamey (1°x1°), it has been shown that the time sampling error is reduced by 15-25 % for MT compared to TRMM
Convective cloud cover – 1 month
Reference: all Meteosat images
Sampling: SSM/I
Sampling: TRMM
Sampling: MT
Rainfall estimation at regional scale(from daily rainfall)
Intercomparison of different satellite productsIntercomparison of different satellite products
Reference : Regression
Kriging
Reference:
Kriging without drift
CMAP 15.9 21
GPCP 18 25
GPCC 16 27
GPI 42 45
SYN 14.4 24.5
– CMAP (Sat+gauges)CMAP (Sat+gauges)
– GPCP (Sat+gauges)GPCP (Sat+gauges)
– GPCC (gauges)GPCC (gauges)
– GPI (Sat)GPI (Sat)
– SYNSYN (gauges from synoptic (gauges from synoptic network)network)
RMSE (%)
Kriging from CILSS daily rain gauges
Tracking from METEOSAT infrared channel
19 July 1994
Rainfall at regional scale: daily estimation?
AMMA Long term experimental set up3 meso-scale sites – 2 of which are already well equiped for precipitation measurements.
Upper OUEME valley • 15 000 km2
• Soudanian
• rain : 1200-1300 mm/year
• specialized in water budget / hydrological processes.
• data since 1997
Gourma • 30 000 km2
• Sahelian to Saharian
• Rain : 200 - 400 mm/year
• site specialized in vegetation + satellite validation.
• Few rain gages / Possibility to densify.
Niamey square degree • 10 000 km2
• Sahelian
• rain : 450-600 mm/year
• Specialized in hydrology and the study of land / rain intreactions.
•Data since 1990 (Hapex –Sahel, Epsat-Niger)
African Monsoon Multidisciplinary Analysis
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., 2003
See www.lthe.hmg.inpg.fr/catch/
Very good knowledge of Sahelian rain fields
- modelisation, downscaling issues
- interpolation and estimation (kriging)
- estimation error
Not simply transposable in another region
First it is needed to study precipitation fields
Niamey square degree
Raingauge network – Mesoscale sites Available data sets for 2004
Niamey square degree (Niger)
33 recording rain gauges
Raw 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 gauges
Raw 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
Mesoscale Site:-Raingage network, river flow, ground water.- Surface, monitoring of vegetation dynamics.- Meteo/climate stations
puits avec lecteurPluviographe limite de bassin versant
rivièrePuits avec enregistreurLimnigraphe
Upper OUEME valley
L ET H
Optical Spectrogranulometer, recording data every 1 min on the rain drop size distributions observed at ground level.
X port Radardeveloped at LTHEX band – 9.4 GHzdiameter 1.8 m – 100 kWpolarisation H and Vdoppler
Details: www.lthe.hmg.inpg.fr/catch/xport/
2D - Structure
3 D structure
multiparameters
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.
X-port : Data and objectives
Donga / Bénin, 2004-2007 - EOP AMMA
Vertically pointing mode: vertical structure of precipitation
(images VPR McGill)
•Derive statistics of vertical structure in a given climatic region
•observe amount of convective vs stratiform rain
•quantify occurrence of evaporation
improve parameterization
feed data base for satellite remote sensing algo.
•(+ FFT analysis of Doppler spectrum -> evolution of DSD with height)
Radar - vertical profiles analysis
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.
L ET H
Ongoing studies on the characterisation of the rainfall events in North Benin using rain gauge data
•Development of a rainfall model valid for Sahel and sub-Sahel rain fields. Based on a modelisation of convective cells
Rainfall types during the rainy season (1999-2003)
•Identification of Mesocale Convective Systems (rainfall) and determination of their propagation (speed and direction)
Depraetere et al., EGU Conference, 24-29 April 2005, Vienna, Austria
Computation of optimal direction and speed of the rainfall event
Directional chronogram of the rainfall event
Meso scale hyetogram derived from the pseudo-chronogram
Directional pseudo-chronogram of the rainfall event
rainfall 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
Summary
Possible collaborations on ground validation issues