Stratus Clouds over the Southeast Pacific: Representation and Evaluation in a Regional Model

The fourteen-model results evaluation by the Preliminary VAMOS Ocean-Cloud-Atmosphere-Land Study (VOCAL) Model Assessment (PreVOCA) indicated that most regional models under-predict the low-cloud fraction during the month of October of 2006 over the southeast Pacific near the Chilean coast. The diurnal cycle of liquid water path (LWP) was also underestimated by most of the regional models at the stratus buoy site (20S, 85W) when verified against satellite observations. Recent observations and modeling studies suggested that drizzle (light precipitation) may have significant impacts on cloud coverage, structure and optical properties.

In this study we examine impacts of drizzle processes on the stratocumulus structure and evolution. We use regional mesoscale model (COAMPS) coupled with an advanced cloud microphysics parameterization to simulate the marine boundary layer observed dur-ing VOCALS-REx field program. Different simulations are conducted to evaluate re-sponse of the marine boundary layer structure to different aerosol environments. We par-ticularly pay attention to the relationship among drizzle, cloud amounts, radiative fluxes and turbulence dynamics in the boundary layer. Some preliminary results have shown that the boundary layer tends to be decoupled due to the evaporative cooling in the sub-cloud layer when drizzle is present, and become more convectively unstable. As a result, more shallow cumulus clouds are more likely to be developed.