Daytime Evolution of Relative Humidity at the Boundary Layer Top

Abstract

Data from the Hydrological and Atmospheric Pilot Experiment-Modélisation du Bilan Hydrigue (HAPEX-MOBILHY) field program and results from a one-dimensional model of the soil and atmospheric boundary layer are analyzed to study the daytime evolution of the relative humidity at the boundary layer top. This evolution is thought to control the development of boundary layer clouds. This study examines the dependence of boundary 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 humidity to external forcing involves competing mechanisms and the net effect on relative humidity is difficult to predict without complete analysis of the relative humidity tendency equation. As one example, drier soil leads to smaller boundary layer specific humidity but also leads to cooler temperatures at the boundary layer top due to greater boundary layer growth. When the latter effect dominates, the relative humidity at the boundary layer top is greater over drier soil. In contrast, drier soil leads to lower relative humidity at the boundary layer top when the air above the boundary layer is strongly stratified or quite dry. These and other nonlinear interactions are posed in terms of a detailed analysis of the budget equation for boundary layer top relative humidity.