Parameterisation and calibration of L-MEB in the Level-2 SMOS algorithm

ESA's Soil Moisture and Ocean Salinity (SMOS) mission is currently under preparation with a launch early 2008. When operational, it will provide global microwave brightness temperature observations at L-band, in dual polarisation and under a range of viewing angles, both, over land and water surfaces. For the purpose of relating the observed quantities to soil moisture, the L-band Microwave Emission of the Biosphere (L-MEB) model has been developed, specifically for its use for L-band emission over land surfaces. The core of L-MEB is the t-w approach which takes into account vegetation cover effects on the soil moisture signal. The final version of L-MEB is the result of an exhaustive review of "state of the art" modeling approaches in the field of passive microwaves, with the objective of being accurate while remaining simple enough for operational use at global scale. L-MEB is the forward model used in the SMOS level 2 processor, in order to produce geophysical products, e.g. soil moisture (SM) and vegetation characteristics. 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 variety of incidence angles. The retrievals provide the best suited parameters driving the direct TB model that minimize the cost function. 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, scrubs, 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 have to be selected and tabulated. Tests and evaluations of the L-MEB model have been made using experimental data sets, based on airborne measurements (COSMOS / NAFE campaign in Australia (Saleh et al., this issue) and CAROLS in France) and ground-based measurements (SMOSREX over soils and fallow, BRAY '04 and FOSMEX over forests, REBEX over corn, MELBEX I and II over matorral and vineyards in the Mediterranean environment of Valencia (Cano et al., this issue), etc.) This paper will make a synthesis of the calibration of L-MEB obtained during the evaluation of the Level-2 algorithm over these different data sets.