A REGIONAL SIMULATION OF MARINE BOUNDARY-LAYER CLOUDS

A regional version of a two-layer model is used to simulate marine bou ndary-layer clouds over an area (20-degrees - 42-degrees-N and 117-deg rees - 145-degrees-W) of the eastern North Pacific. The large-scale co nditions required for the model are provided by European Centre for Me dium-Range Weather Forecasts analyses and the Comprehensive Ocean-Atmo sphere Data Set. The simulated cloud-top height and fractional cloudin ess are compared with satellite data. The model predicts a realistic p attern of cloud-top height, although the heights appear to be overesti mated in the southwest and underestimated along the eastern border of the domain. The overall simulated cloudiness is qualitatively comparab le to the satellite derived, but the model predicts excessive cloudine ss over cold ocean surfaces and insufficient cloudiness over an area c entered at 30-degrees-N, 130-degrees-W. The possible causes for these discrepancies are discussed. Budget studies show that horizontal advec tion contributes substantially to the cloud-top height and cloud-layer moisture budgets. Thus, one may need to consider the effects of horiz ontal advection when validating steady-state solutions of a one-dimens ional model. The simulated cloudiness is shown to be sensitive to larg e-scale subsidence, moisture above the clouds, and drizzle. An increas e in the large-scale divergence by 2 x 10(-6) s-1 decreases average cl oudiness for the area by 30%. An increase in the moisture at 850 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.