The goal of this study is to improve the accuracy in the mesoscale numerical predictions of the structure and evolution of summertime nocturnal marine stratocumulus over the Eastern Pacific Ocean. High-resolution Mesoscale Model 5 (MM5) simulations have been carried out with the finest horizontal resolution of 1 km and 53 vertical levels. A series of sensitivity tests was performed to obtain accurate baseline model simulations. The tests included various initial and boundary conditions, model grid setup, selection of physical parameterization parameters, and four-dimensional data assimilation. The model simulations of the cloudy marine atmospheric boundary layer for 10 and 11 July 2001 were verified against the aircraft observations collected during the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS II) experiment, as well as using buoy observations archived by the National Buoy Data Center.

The baseline model simulation showed that the predicted inversion base is in good agreement with the observations. However, the model cannot accurately simulate the extreme strength of the inversion as measured by the aircraft. The skill of the MM5 model in predicting the evolution and structure of marine stratocumulus will be discussed, focusing the analysis on inversion structure and strength, diurnal marine PBL morphology, turbulence transfer, entrainment processes, and the production of drizzle. Preliminary results of assimilating aircraft and satellite data to improve initial conditions over the ocean will also be discussed.