First evaluation of the SMOS observations over the VAS site
Résumé
The SMOS (Soil Moisture and Ocean Salinity) mission was launched on November 2, 2009. Over the land surfaces, simultaneous retrievals of surface soil moisture (SM) and vegetation characteristics are made from the multi-angular and dual polarization SMOS observations by inverting the L-MEB (L-band Microwave Emission of the Biosphere) model. Preliminary analyses evaluating the SMOS observations in terms of Brightness Temperatures (TB) and Level-2 products (SM and vegetation optical depth TAU) were carried out over several sites of all continents. Here, the study is based on SMOS observations and in situ measurements carried out in 2010 over one of the main SMOS calibration/validation sites in Europe: the VAS (Valencia Anchor Station) site in the region of Utiel–Requena, close to Valencia, Spain. The main vegetation types in the region are vineyards (cover fraction ~60%), natural Mediterranean vegetation (pine trees and shrubs ~30%) and orchards. The SMOS observations were analysed in conjunction with those carried out by the L-band ELBARA II radiometer over a vineyard which is representative of the main land use of the VAS site. Retrievals of SM and TAU based on the SMOS observations (Level-2 products) and the ELBARA II instrument were compared and evaluated against in situ measurements and simulations of SM by atmospheric models. A good agreement was found between the time variations in SM over the site retrieved from the SMOS and ELBARA II observations. However, it was found the SMOS Level-2 products underestimated SM by about 0.15 %, even though it is difficult to estimate SM values at large scale that can be considered as a "reference" to evaluate the SMOS SM retrievals. Retrievals computed from SMOS data using a simplified L-MEB version, that did not consider the pixel heterogeneity and the WEF function, produced very satisfying results (R2=66% when compared to retrievals computed from the ELBARA II observations) The time variations in the retrieved TAU computed from the ELBARA II observations could be closely related to those of the NDVI vegetation index, obtained from the MODIS observations in the optical domain. Conversely, the time variations in the SMOS Level-2 TAU product seemed to be strongly affected by microwave interferences over the site.