Assimilation of SMOS brightness temperatures in the ECMWF IFS

The assimilation of SMOS brightness temperatures (TB) data in Numerical Weather Prediction Systems influences the state of the soil, which in turn affects the exchange of energy and water fluxes between the soil and the near surface atmosphere, with potential implications in the prediction of atmospheric variables. In this paper, the impact of assimilating SMOS TB alone or in combination with screen level observations and ASCAT soil moisture retrievals, on land surface and near-surface atmospheric variables, is assessed. Independent quality controlled in situ soil moisture observations belonging to several networks included in the International Soil Moisture Network were used to validate the quality of both the new soil moisture analyses and the skill to predict soil moisture up to 5 days ahead. The impact on atmospheric variables is indirect and it was evaluated through computation of the forecast skill at different lead times. The analysis period was selected around the boreal summer, a period of the year when evaporatranspiration fluxes are stronger, and when it is therefore expected that the assimilation of remote sensing data provides the largest impact on the state of the soil. The results show that the soil moisture state benefits from the direct assimilation of SMOS TB, especially in better representing the temporal variations of soil moisture in time. The skill on atmospheric variables is mainly driven by the screen level observations. Despite the clear benefits on the soil state, remote sensing data needs to be used with screen level variables to add value on the state of the atmosphere, pointing to inconsistencies in the physical coupling between the land and near-surface components of the ECMWF Earth system.