A Regional Simulation of Marine Boundary-Layer Clouds

Abstract

A regional version of a two-layer model is used to simulate marine boundary-layer clouds over an area (20°–42°N and 117°–145°W) of the eastern North Pacific. The large-scale conditions required for the model are provided by European Centre for Medium-Range Weather Forecasts analyses and the Comprehensive Ocean-Atmosphere Data Set. The simulated cloud-top height and fractional cloudiness are compared with satellite data. The model predicts a realistic pattern of cloud-top height, although the heights appear to be overestimated in the southwest and underestimated along the eastern border of the domain. The overall simulated cloudiness is qualitatively comparable to the satellite derived, but the model predicts excessive cloudiness over cold ocean surfaces and insufficient cloudiness over an area centered at 30°N, 130°W. The possible causes for these discrepancies are discussed. Budget studies show that horizontal advection contributes substantially to the cloud-top height and cloud-layer moisture budgets. Thus, one may need to consider the effects of horizontal advection when validating steady-state solutions of a one-dimensional model. The simulated cloudiness is shown to be sensitive to large-scale subsidence, moisture above the clouds, and drizzle. An increase in the large-scale divergence by 2 × 10−6 s−1 decreases average cloudiness for the area by 30%. An increase in the moisture at 350 mb by 2 g kg−1 increases the cloudiness by 20%. The suppression of drizzle in the model increases the cloudiness by 34% and significantly changes its pattern.