Variational surface analysis from screen level atmospheric parameters

Abstract

A variational method to retrieve soil parameters from screen level parameters is investigated.A one-dimensional model including vertical diffusion and land-surface schemes is used. Asensitivity study evaluates the influence of soil moisture and temperature in 2 layers on thesurface fluxes and the 2m parameters. The mean soil water content appears to be the mostimportant soil parameter for modelling surface fluxes and 2m parameters. During stable conditions, the mean soil temperature is also important for a good forecast of screen level parameters. The screen-level observations are found to be informative on soil moistures during the day andon soil temperatures during the night. The characteristics of the variational assimilation methodare presented in the framework of identical twin experiments. The general shape of the costfunction with a 24-h assimilation period reveals the ambiguity in the repartition of water in thetwo soil reservoirs and the presence of secondary minima. The ability of this method to initializesoil parameters is tested with various soil water contents and vegetation fraction covers. Finally, some results show the influence of the frequency of the observations and the length of theassimilation period on the quality of the analysis.