Marine Stratocumulus Convection. part II: Horizontally Inhomogeneous Solutions

Abstract

Solutions of the horizontally inhomogeneous version of the coupled, convective-radiative, cloud-topped mixed-layer model described in Part I of this study are presented. Both numerical and approximate analytical methods are used to investigate the downstream variations which occur as boundary-layer air flows through regions of varying sea surface temperature and large-scale divergence. Six numerical experiments are performed. In the first two experiments boundary-layer air flows through regions of constant large-scale divergence but of increasing or decreasing sea surface temperature. In the cold advection case the boundary layer warms, moistens and deepens in time, while the turbulent fluxes increase. In the warm advection case the boundary layer cools, drys and becomes shallower, while the turbulent fluxes decrease. In addition the cloud base descends and there is a tendency to form a surface fog. In the third and fourth experiments boundary-layer air flows through regions of constant sea surface temperature but of increasing or decreasing large-scale divergence. In these two integrations essentially no model variable changes except cloud top. Cloud top slowly rises if divergence is decreasing and slowing falls if divergence is increasing. The adjustment time for cloud top is long and thus the boundary-layer depth may be far from its horizontally homogeneous steady-state value. The distinct difference between the adjustment time for the thermodynamic properties of the mixed layer and the adjustment time for cloud top is illustrated in a fifth experiment. The sixth experiment simulates the wintertime flow of cold air off the Asian continent across the warm Kuroshio Current. Although the surface flux of water vapor is very large, the boundary-layer mixing ratio is fairly constant, i.e., the boundary layer deepens so rapidly in the downstream direction that the mixing of dry air across cloud top maintains a relatively dry boundary layer.