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 fields of screen‐level atmospheric temperatures and relative humidities in the domain of the operational regional weather forecast model of the Deutscher 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 atmospheric boundary‐layer description has been isolated from the full forecast model. the coupling between the two systems is mediated via boundary conditions and source terms. The retrieved soil moisture values are consistently much lower than those in the operational reference run. They can be regarded as tuned effective parameters providing optimum lower‐boundary conditions for the atmosphere. the 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 moistures of the misspecification of radiative forcing. Soil moisture values derived from observations of atmospheric relative humidity alone are more stable with respect to radiation errors than those derived from atmospheric temperature observations.