Defining Marine Boundary Layer Clouds with a Prognostic Scheme

Abstract

A prognostic cloud scheme is described based on Tiedtke's cloud parameterization for large-scale meteorological models, which uses prognostic equations for both mean liquid water content and cloud fraction. The present work parameterizes the relevant physical processes in the framework of convective circulation in the cloud-topped marine boundary layer. Both the steady-state and downstream solutions of the scheme are studied under the conditions of marine boundary layers. The results show that the predicted cloud fraction and mean liquid water content are strongly regulated by the strength of mass-flux detrainment and relative humidity in the environmental downdrafts in the upper part of the cloud layer. Thus, the intensity of the turbulent mixing is important in defining marine boundary layer clouds in the scheme. When coupled with a boundary-layer model that has a mass-flux representation of convective mixing, the cloud scheme gives reasonable downstream variations of marine boundary layer clouds. The sensitivities of the cloud scheme to some parameters are discussed in the context of the boundary layer model.