Variational analysis of effective soil moisture from screen-level atmospheric parameters: Application to a short-range weather forecast model

A variational analysis scheme for soil moisture that assimilates analysed f ields of screen-level atmospheric temperatures and relative humidities in t he domain of the operational regional weather forecast model of the Deutsch er Wetterdienst, is investigated over a five-day period in March 1994. To reduce the computational load implied by the variational analysis scheme , a stand-alone system comprising the soil module plus a simplified atmosph eric boundary-layer description has been isolated from the full forecast mo del. The coupling between the two systems is mediated via boundary conditio ns and source terms. The retrieved soil moisture values are consistently much lower than those i n the operational reference run. They can be regarded as tuned effective pa rameters providing optimum lower-boundary conditions for the atmosphere. Th e improved quality of the resulting sensible- and latent-heat fluxes can be substantiated by better atmospheric forecasts. Numerical experiments give estimates of the influence on analysed soil mois tures of the misspecification of radiative forcing. Soil moisture values de rived from observations of atmospheric relative humidity alone are more sta ble with respect to radiation errors than those derived from atmospheric te mperature observations.