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 hydr ological and atmospheric measurements, is described and discussed. The 11-day data set is characterized by very dry conditions and high surf ace temperatures during the day. Large values of sensible and soil hea t fluxes and small values of surface evaporation (almost-equal-to 1 mm /day) were observed. This data set was modelled, leading to the follow ing conclusions: (i) In the model, the parameterization provides value s of the soil thermal properties and subsequently of the predicted soi l heat fluxes which are overestimated when compared with the observati ons. (ii) Following the literature, a value of the ratio between the r oughness lengths for momentum Z(o) and heat Z(oh) of close to 10 for f airly homogeneous areas of bare soil and vegetation is used. This valu e leads to a fair prediction of the surface temperature. If the roughn ess lengths were taken to be equal, as is often assumed in atmospheric modelling, a poorer prediction results. (iii) Finally, the vapor phas e transfer mode is found dominant close to the surface and a modified parameterization including this effect is proposed. It allows a fair p rediction of both surface evaporation and near-surface water content.