INFLUENCE OF BOWEN-RATIO ON BOUNDARY-LAYER CLOUD STRUCTURE

An investigation of the influence of the ratio of surface sensible hea t flux to latent heat flux, the Bowen ratio, on the structure of bound ary-layer clouds is carried out utilizing numerical large eddy simulat ions (LES). The role of cloud-top radiational cooling, cloud-top tempe rature and moisture jump conditions, and wind shear are included in a secondary way. Although no detailed comparisons have been made, the LE S results appear to be qualitatively consistent with the Atlantic Stra tocumulus Transition Experiment, the recent field study on marine boun dary-layer cloud structure. Some conclusions that follow from an exami nation of these LES results are the following: First, there is a highl y bimodal character to the cloud ceiling frequency within a very low B owen ratio boundary layer. The updrafts tend to produce a lower cloud ceiling than the surrounding environment with its weak downdrafts. Sec ond, a very low Bowen ratio with the aid of some boundary-layer shear makes the development of persistent microcell cloud circulations possi ble within the boundary layer. Third, when the surface latent heat flu x is the dominant factor in the dynamics of the boundary layer, the ap proach to a conditionally stable lapse rate results in the potential f or subsequent decoupling. Last, the maximum partial cloud fraction is very well represented by the relation suggested by Sommeria and Deardo rff for a Gaussian probability distribution for the range of condition s studied.