INFLUENCE OF SEASONAL-VARIATION IN-CLOUD CONDENSATION NUCLEI, DRIZZLE, AND SOLAR-RADIATION, ON MARINE STRATOCUMULUS OPTICAL DEPTH

A modeling study was performed to investigate the factors that control the seasonal cycle in the optical depth of marine stratocumulus cloud at a Southern Hemisphere site. Cloud optical depth is primarily influenced by cloud depth and cloud droplet concentration, the latter being controlled by more or less known fluctuations in cloud condensation nucleus (CCN) concentration. The study primarily focussed on the factors influencing cloud depth. The results show that there are two important factors controlling cloud depth, namely solar radiation and drizzle. Seasonal variations in insolation act to enhance cloud depth in winter compared to summer, while drizzle acts in the opposite direction. Drizzle intensity is calculated to be higher in winter when the concentration of CCN is low and cloud droplet effective radius is large. Subcloud evaporation of drizzle droplets reduces entrainment rate, while increased precipitation at the surface depletes cloud water. Thus, because seasonal cycles in cloud depth due to variations in insolation and drizzle are of opposite phase the result is a partial cancelation of their separate influences on cloud optical depth.