DAYTIME EVOLUTION OF RELATIVE-HUMIDITY AT THE BOUNDARY-LAYER TOP

Data from the Hydrological and Atmospheric Pilot Experiment-Modelisati on du Bilan Hydrique (HAPEX-MOBILHY) field program and results from a one-dimensional model of the soil and atmospheric boundary layer are a nalyzed to study the daytime evolution of the relative humidity at the boundary layer top. This evolution is thought to control the developm ent of boundary layer clouds. This study examines the dependence of bo undary layer relative humidity on soil moisture, large-scale vertical motion, and the moisture content and temperature stratification above the boundary layer. The response of the boundary layer relative humidi ty to external forcing involves competing mechanisms and the net effec t on relative humidity is difficult to predict without complete analys is of the relative humidity tendency equation. As one example, drier s oil leads to smaller boundary layer specific humidity but also leads t o cooler temperatures at the boundary layer top due to greater boundar y layer growth. When the latter effect dominates, the relative humidit y at the boundary layer top is greater over drier soil. In contrast, d rier soil leads to lower relative humidity at the boundary layer top w hen the air above the boundary layer is strongly stratified or quite d ry. These and other nonlinear interactions are posed in terms of a det ailed analysis of the budget equation for boundary layer top relative humidity.