MODELING THE SURFACE PROCESSES AND THE ATMOSPHERIC BOUNDARY-LAYER FORSEMIARID CONDITIONS

A simple parameterization of land-surface processes is tested for appl ication in semi-dry areas. The surface scheme is included in the physi cal package of the Meteo-France atmospheric model ARPEGE. Its calibrat ion using the EFEDA91 data set was necessary prior to mesoscale integr ations of selected days of the field experiment. In order to model cor rectly the water budget and the energy balance over vineyards under dr y soil conditions, the surface scheme was improved by parameterizing t he vapor phase transfer in the soil and using two distinct roughness l engths for momentum and heat transfer. The introduction of the roughne ss length for heat is included in a surface boundary layer scheme, avo iding the classical iterative calculations of the heat transfer coeffi cient. Favourable results of tests of these modifications were found b oth for a time period of one month in stand-alone mode using an observ ed atmospheric forcing and for a diurnal cycle using a single column a tmospheric model fully interactive with the surface scheme. Prediction s of the surface temperature and of the sensible heat flux are signifi cantly improved when a value of the roughness length for heat twenty t imes lower than the roughness length for momentum is used. Finally, th e sensitivity of the predictions of the vertical development of the co nvective boundary layer to the value of the roughness lengths is exami ned.