Modelling the surface processes and the atmospheric boundary layer for semi-arid conditions

A simple parameterization of land-surface processes is tested for application in semi-dry areas. The surface scheme is included in the physical package of the Meteo-France atmospheric model ARPEGE. Its calibration using the EFEDA91 data set was necessary prior to mesoscale integrations of selected days of the field experiment. In order to model correctly the water budget and the energy balance over vineyards under dry soil conditions, the surface scheme was improved by parameterizing the vapor phase transfer in the soil and using two distinct roughness lengths for momentum and heat transfer. The introduction of the roughness length for heat is included in a surface boundary layer scheme, avoiding the classical iterative calculations of the heat transfer coefficient. Favourable results of tests of these modifications were found both for a time period of one month in stand-alone mode using an observed atmospheric forcing and for a diurnal cycle using a single column atmospheric model fully interactive with the surface scheme. Predictions of the surface temperature and of the sensible heat flux are significantly improved when a value of the roughness length for heat twenty times lower than the roughness length for momentum is used. Finally, the sensitivity of the predictions of the vertical development of the convective boundary layer to the value of the roughness lengths is examined.