SMOS: analysis of perturbing effects over land surfaces
Résumé
Surface soil moisture is a key variable of water and energy exchanges at the land surface/atmosphere interface. But currently there are no means to assess it on a global and timely fashion. The ESA Earth Explorer Opportunity mission Soil Moisture and Ocean Salinity (SMOS) is the first attempt to fill such a gap. SMOS is based upon an L-band 2-D interferometer, an innovative concept of bi-dimensional aperture synthesis method to obtain surface measurement with an appropriate resolution from a tractable (in terms of dimensions) space-borne instrument. Moreover, the sensor has new and very significant capabilities especially in terms of multi-angular view configuration. However as for most of space borne instrument, retrieval of surface parameters/variables will be hampered by several factors. This fact is enhanced for sensors having a coarse spatial resolution. In the specific case of SMOS the ground resolution of 40 km means that the influence of different contributors to the signal has to be accurately assessed and eventually corrected. Finally many pixels will be affected by topography effects. It is thus important to assess exactly which level of topography distorts significantly the brightness temperatures and to which extend so as to be able to either correct soil moisture retrieval for topography effects or flag the data. The goal of this paper is to present the topography effects. The latter has been analyzed in depth by modeling the signal issued from mountainous terrain including moisture and vegetation gradient. The adjacency and shadowing effects were in particular addressed. The second step was to develop a simplified characterization of the topography through a statistical description which is then used to assess exactly the level of topography which has an influence of the signal and from which one has to take it into account in the retrieval process. The potential of SMOS, depending on the view angle configuration and the use of the sole 1.4 GHz is thus investigated over complex targets. These questions are key issues to define the exact range of configurations where SMOS meets the scientific requirements of the mission.