ASSESSING THE MECHANISMS GOVERNING THE DAYTIME EVOLUTION OF MARINE STRATOCUMULUS USING LARGE-EDDY SIMULATION

Abstract

The individual mechanisms responsible for governing the evolution of afternoon cloud properties were explored for a case of thin stratocumulus off the coast of California by applying mixed-layer theory to Large-Eddy Simulation (LES) output. The development of a cloud-base tendency equation permitted the determination of the relative importance of mechanisms governing the evolution of boundary-layer liquid-water static energy () and total water mixing ratio (). The Control simulation performed admirably in comparison to observed estimates of liquid water content, vertical velocity variance, and radiative fluxes sampled by the CIRPAS Twin Otter aircraft. The cloud response to various environmental forcing scenarios was investigated through a suite of sensitivity tests, including variations in subsidence velocity, temperature/moisture tendencies, surface fluxes, wind shear near the inversion, and radiative forcing. In the Control simulation, rising cloud-base tendencies were related to entrainment warming and drying and shortwave absorption, while lowering cloud-base tendencies were associated with longwave cooling. Although there was substantial solar heating during the afternoon, the entrainment fluxes remained active throughout the analysis period. A reversal of cloud-base tendency was often observed in the simulations, as the reduction in shortwave warming later in the afternoon allowed for the recovery of the cloud. The evolution of cloud-base tendency is found to be insensitive to the net radiative flux divergence for most of the simulations (LWP ranging from ~10-50 g m-2). Error analysis suggests our method of entrainment flux calculation could be improved by a more complete understanding of entrainment layer physics.