Treatment of Surface Emissivity for Satellite Microwave Data Assimilation

Observations from satellite microwave instruments have a large information content and are known to improve the quality of global analyses of the atmosphere. However, the use of these observations remains far from being optimal. Indeed, only microwave observations, for which the surface contribute the least, are currently assimilated. This restriction is mainly due to remaining uncertainties about the surface emissivity and the skin temperature. For many reasons, uncertainties about the surface have a larger impact over land than over ocean. Recent assimilation experiments have been conducted in order to assimilate more microwave observations over land. To do so, different methods have been used in order to estimate the land emissivity and/or the land skin temperature directly from satellite observations. This paper describes the methods used to estimate the land emissivity and the skin temperature within the constraints of variational assimilation systems. An overview of the assimilation experiments conducted in many NWP centres is also given together with their main results.