Errors in SMOS Sea Surface Salinity and their dependency on a priori wind speed

Abstract

The wind speed (WS) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to initialize the retrieval process of WS and Sea Surface Salinity (SSS) obtained by the Soil Moisture and Ocean Salinity (SMOS) mission. This process compensates for the lack of onboard instrument providing a measure of ocean surface WS independent of the L-band radiometer measurements. The SMOS-retrieved WS in the center of the swath (±300km) is adjusted regarding to its a priori estimate. The quality of the SMOS-retrieved SSS (SSSSMOS) is better at the center of the swath than at the edge of the swatch because the larger number of brightness temperature measurements available at the center of the swath reduces the effects of noise and because the greater variety of incidence angles provides more scope for adjusting the WS. This highlights the advantage of using a multi-parameter retrieval with respect to a SSS-only retrieval in which the WS would be entirely prescribed. Systematic inconsistencies between the atmospheric WS modeled using ECMWF and the WS sensed by radiometers are observed. These inconsistencies in the WS are reduced by the retrieval scheme but they still lead to residual biases in the SSSSMOS, especially in the eastern equatorial Pacific ocean if the ECMWF WS is used as an a priori estimate.