Field Measurements of the Amount of Surface Layer Air versus Height in the Entrainment Zone

Abstract

The structure of the atmospheric entrainment zone, an interfacial layer between the convective boundary layer and the stable air aloft, is studied using coincident high resolution aircraft and lidar observations obtained during Boundary Layer Experiment?1983 in Oklahoma. Humidity as measured by a fast-response Lyman alpha humidiometer is used as a tracer to estimate the amount of surface-layer origin air reaching various heights in the entrainment zone. Two approaches are taken to describe the humidity structure of the entrainment zone. The first approach models the frequency distributions of the three types of air in the entrainment zone: unmixed free atmosphere (dry); unmixed surface layer air (moist); and a mixture of these two. The resultant modeled frequency distributions of specific humidity capture the following observed features: Low in the convective boundary layer, surface layer air is frequently observed with little mixture air and no free atmosphere air. Higher in the convective boundary layer, near the middle of the entrainment zone, the proportion of unmixed free atmosphere and mixture air increases while the proportion of unmixed surface layer air decreases. Approaching the top of the entrainment zone, unmixed surface layer and mixture air proportions decrease to zero leaving only unmixed free atmosphere air. These results are critical for the successful forecasts of fair-weather cumulus. The second approach uses the linear mixing character of air with different specific humidities and yields vertical profiles of the proportion of surface layer air present. These profiles are described well by the cumulative distribution function of asymmetric double exponential functions.