Analysis of ERS wind scatterometer time series over Sahel (Mali)

ERS wind scatterometer (WSC) time series are analyzed over different ecoclimatic regions of the African Sahel during the period 1991–1995. At 45° incidence angle, the strong seasonality of σo time series can be directly linked to the successive wet and dry seasons. Moreover, the annual σo dynamic range was found to be strongly correlated to total rainfall. The interpretation of the σo temporal plots is carried out by combining a backscattering model to a grassland growth model. Results highlight the decreasing contribution of the herbaceous component with latitude. However, its contribution is far from negligible and can reach 60% in the Soudano-Sahelian subzone at peak herbaceous mass. Additionally, the tree layer has a negligible effect on the radar signal at the scale of a resolution cell. Finally, a simple parametric backscattering model is calibrated and used in an inversion process. The resolution of the inverse problem is based on a ‘brute-force’ method that consists of exploring all the combinations of parameters of interest. Despite a poor estimation of the temporal variation of the herbaceous mass Bt, the retrieved maximum mass compares well with ground estimates.