Comparison of ERS wind-scatterometer and SSM/I data for Sahelian vegetation monitoring

Abstract

ERS wind scatterometer (WSC) and SSNI/I data are compared for monitoring the seasonal variation of herbaceous vegetation over a sahelian region. Temporal evolution of polarization difference brightness temperatures derived from SSM/I data and WSC backscattering coefficient acquired at 45° of incidence angle over four different sites during the period 1992-1993, exhibits a marked seasonality with opposite and symmetrical trends. Observed differences between both signals are mainly attributed to atmospheric effects affecting SSM/I data. The use of a semi empirical model during the 1992 rainy season shows that T temporal evolution is mainly due to the variation of integrated water vapor content of the atmosphere, surface, and air temperature, soil moisture content, and bare soil fraction area. In order to retrieve biomass from SSM/I data, an inversion procedure is performed and compared to previous results obtained with ERS WSC data. The absence of accurate atmospheric data over the Sahel, combined with the sensitivity of the passive model to soil moisture leads to poor results with regard to biomass retrieval from SSM/I data.