Seasonnal Variations of C- and Ku-bands Basckscattering from Altimeters and Scatterometers over West-Africa
Fatras, Christophe1; Frappart, Frédéric1; Mougin, Eric1; Frison, Pierre-Louis2; Faye, Gayane3; Jarlan, Lionel4
1Geosciences Environnement Toulouse, FRANCE; 2Université Marne-la-vallée, FRANCE; 3Université Cheikh Anta DIOP, SENEGAL; 4Cesbio, FRANCE
The variations of the backscattering responses of land surfaces in West Africa have been studied time wise for different spaceborne sensors: the nadir-looking radar altimeters and the side-looking radar scatterometers at different frequencies (Ku and C bands). We compared ESA's Envisat-RA2 measurements in Ku-band over the period 2003-2010, and the Jason-2 measurements in C-band over the mid 2008-2011 time span for radar altimetry. For radar scatterometry, we used QuikScat in Ku-band and ESA's Metop-ASCAT in C-band data over the corresponding same time-spans. Along-track profiles and their temporal variations are analyzed and their behaviours are related to surfaces properties such as soil types, vegetation cover, and surface hydrology.
Temporal variations of backscattering coefficients are extracted for main land covers along the bio-climatic transect (stone and sand deserts, saharo-sahelian, sahelian, and soudano-sahelian savannahs, tropical-seasonal and tropical forests,) and compared to both rainfall estimates from TRMM and vegetation activity (NDVI) from MODIS. Ranges of variations of the backscattering signals are given for different surfaces and ecosystems along with delays between peaks of rainfall, backscattering responses of radar altimetry and scatterometry, and vegetation activity. The stability of the backscattering is also estimated over deserts and during the dry season over Sahelian environments. Finally, the complementarity of the different spaceborne sensors for continental surfaces monitoring is pointed out.