Recent Advances in estimating the L-band microwave signature of natural land surface covers
Résumé
In the framework of the ground segment activities for the SMOS mission, geophysical products (e.g. soil moisture (SM) and vegetation characteristics) will be produced by an operational SMOS Level 2 Soil Moisture retrieval algorithm. The principle of the algorithm is based on an iterative approach, minimizing a cost function computed from the sum of squared weighted differences between measured and modelled brightness temperature (TB) data, for a range of incidence angles. The retrievals provide the best suited parameters driving the direct TB model that minimize the cost function. The selected direct model is the so-called L-MEB (L-band Microwave Emission of the Biosphere) which was used in the first ESA studies aiming at assessing SMOS capabilities from synthetic data set. In the algorithm process, for each incidence angle, the different cover types (vegetated area, open water, urban area, land use, etc.) present within the SMOS footprint are estimated from high resolution land use maps. For low vegetation and forest categories, these maps allow to distinguish between a large number of sub-categories (N > 200) corresponding to grasslands, crops, matorral, tropical & boreal forests, etc. for a variety of climatic and geographic conditions. Based on these refined vegetation classes, corresponding to specific vegetation structure, and on maps of soil properties (for soil texture, roughness and bulk density) parameters driving the L-MEB can be selected and tabulated. Of course L-MEB is not frozen but prone to evolve so as to gradually incorporate significant improvements made in the field. Recently, in the framework of SMOS research activities, advances in understanding and modelling the L-band microwave emissivity of vegetation have been achieved. These results were mainly obtained over natural land covers, over which very few informations were available to date. They could be derived from a substantial number of experimental campaigns carried out throughout Europe, for a variety of vegetation/soil characteristics and climatic conditions; To name but a few, SMOSREX over fallow and bare soil near Toulouse, MELBEX over matorral in the Mediterranean environment of Valencia, Bray and Jülich over, respectively, coniferous (Les Landes) and deciduous forests, etc. The combination of these experimental data with modelling activities over the last years helped to improve significantly our knowledge of the key processes that drive the emission of natural vegetation canopies: effect of water interception by the standing vegetation, attenuation of soil emission by mulch and litter, dependence of vegetation attenuation on configuration parameters (incidence angle, polarization), etc. The most significant recent results obtained over prairies, forests and matorrals are presented and discussed in this communication.