Modeling study of the cloud-topped marine boundary layer

Abstract

A coupled convective-radiative, boundary-layer model of marine stratocumulus clouds is presented. The model, which slightly generalizes Lilly's (1968) cloud­ topped mixed-layer model, has as dependent variables the cloud-top height, the cloud-base height, mixed-layer equivalent potential temperature and total water mixing ratio, the turbulent fluxes of equivalent potential temperature, total water mixing ratio, and virtual potential temperature, the cloud-top jumps of equivalent potential temperature and total water mixing ratio, the cloud-top temperature, and the net radiative flux divergence at cloud top and in the mixed layer. The model has been programmed for both the steady-state and time dependent cases using two different closure assumptions. Closure method one is a weighted average of Lilly's (1968) maximum and minimum entrainment assumptions. Clo­sure method two assumes the ratio of the integral buoyant energy dissipation to the buoyant energy production to be constant. Experiments wee performed to deter­mine which closure method produced the best results, and to simulate the observed diurnal thinning of the marine stratocumulus clouds. Results indicate that closure method two provides a more realistic representation of the observed marine boundary layer than does closure method one for both the steady-state case and the time dependent case. Results from the diurnal cycle simulations indicate that it is possible to simulate the observed thinning trend of the stratocumulus cloud layer although the magnitude of the thinning is less than observed. Results also indicate that thin clouds are more susceptible to thinning than are thick clouds.