BARE-GROUND SURFACE HEAT AND WATER EXCHANGES UNDER DRY CONDITIONS - OBSERVATIONS AND PARAMETERIZATION

A simplified land-surface parameterization is tested against bare-soil data collected during the EFEDA experiment conducted in Spain in June 1991. A complete data set, made up of soil properties as well as hydrological and atmospheric measurements, is described and discussed. The 11-day data set is characterized by very dry conditions and high surface temperatures during the day. Large values of sensible and soil heat fluxes and small values of surface evaporation (almost-equal-to 1 mm/day) were observed. This data set was modelled, leading to the following conclusions: (i) In the model, the parameterization provides values of the soil thermal properties and subsequently of the predicted soil heat fluxes which are overestimated when compared with the observations. (ii) Following the literature, a value of the ratio between the roughness lengths for momentum Z(o) and heat Z(oh) of close to 10 for fairly homogeneous areas of bare soil and vegetation is used. This value leads to a fair prediction of the surface temperature. If the roughness lengths were taken to be equal, as is often assumed in atmospheric modelling, a poorer prediction results. (iii) Finally, the vapor phase transfer mode is found dominant close to the surface and a modified parameterization including this effect is proposed. It allows a fair prediction of both surface evaporation and near-surface water content.