Aerosol-Cloud Relationships in Marine Stratocumulus

Marine stratocumulus clouds (marine Sc) are generally optically thick and shallow, exerting a net cooling influence on climate. Perturbations in aerosol affect cloud droplet number concentration and size distribution, and thereby have an influence on precipitation, entrainment, cloud thickness and adiabaticity. We report here on a numerical study of the dynamical responses of marine Sc to aerosol changes using the Weather Research and Forecasting (WRF) model with bin-resolved cloud microphysics scheme as a three-dimensional large eddy simulation model. High-resolution simulations of idealized case of nocturnal marine Sc are carried out with a horizontal domain of 3.2 km by 3.2 km and a vertical domain of 1.6 km. Aerosol number concentration ranging from 100 cm^-3 to 2500 cm^-3 are considered.

Previous studies indicate that the marine Sc cloud thickness response to an aerosol perturbation is largely determined by competing effects of suppressed precipitation (moistening/cooling effect) and enhanced entrainment (drying/warming effect); as a result, the cloud may thicken or thin in response to an aerosol perturbation. We focus on marine Sc cloud thickness effects as well as entrainment-induced adiabaticity influences for both drizzling and nondrizzling cloud. The aim is to isolate these various effects and obtain a first-order estimate of their relative importance.