The Physics of the Marine Stratocumulus-Capped Mixed Layer

Abstract

In order to simulate the stratocumulus-capped mixed layer, a one-dimensional stratocumulus model is developed. This model consists of five major points: 1) a one-dimensional (1D) option of the CSU Cloud/Mesoscale Model, 2) a partially diagnostic higher-order turbulence model, 3) an atmospheric radiation model, 4) a partial condensation parameterization, and 5) the drizzle process. This model is tested against the observed structure of the marine stratocumulus layer reported by Brost et al. In this paper we also investigate the interactions among the following physical processes: atmospheric radiation, cloud microphysics, vertical wind shear, turbulent mixing, large-scale divergence, the sea surface temperature and the presence of high-level clouds above the capping inversion. The model simulated fields were found to be in generally good agreement with observations, although the amount of cloud liquid water predicted was too large. This may have been a result of employing a wind profile that exhibits somewhat weaker shear than observed, since the sensitivity experiment with an unbalanced wind similar to that observed produced liquid water contents similar to the observed values. It is also found that drizzle precipitation greatly alters the liquid water content of the cloud and the rate of radiative cooling. This then feeds back into the turbulence structure of the cloud. For the case with large-scale subsidence and the presence of high-level clouds above the capping inversion, the effect of cloud top radiative cooling is found to become less important. Longer time integrations (up to 6 hours) revealed a 15 to 20 min periodicity in cloud top entrainment. The length of the period of oscillation was regulated by the magnitude of shear and the presence of drizzle. Complete removal of shear and drizzle processes resulted in the elimination of sporadic entrainment. Finally, sensitivity experiments were also conducted to examine the role of shortwave radiation. It is found that the influence of shortwave radiation on the cloud layer varies with the intensity of overlying large-scale subsidence and the moisture content of the airmass overlying the capping inversion.