MODELING OF STRATOCUMULUS CLOUD LAYERS IN A LARGE-EDDY SIMULATION-MODEL WITH EXPLICIT MICROPHYSICS

A new large eddy simulation (LES) stratocumulus cloud model with an ex plicit formulation of microphysical processes has been developed, and the results from three large eddy simulations are presented to illustr ate the effects of the stratocumulus-topped boundary layer (STBL) dyna mics on cloud microphysical parameters. The simulations represent case s of a well-mixed and a radiatively driven STBL. Two of the simulation s differ only in the ambient aerosol concentration and show its effect on cloud microphysics. The third simulation is based on the data obta ined by Nicholls, and the simulation results from this case are contra sted with his measurements. Cloud-layer dynamical parameters and cloud droplet spectra are in reasonably good agreement with observations. A s demonstrated by the results of three large eddy simulations presente d in the paper, the cloud microphysical parameters are significantly a ffected by cloud dynamics. This is evidenced by the sensitivity of the cloud drop spectra itself, as well as by that of the integral paramet ers of the spectra, such as mean radii and droplet concentration. Expe riments presented here also show that cloud microstructure is signific antly asymmetric between updrafts and downdrafts. Mixing with dry air from the inversion may significantly enhance evaporation and result in cloud-free zones within the cloud. As a result of mixing, the cloud l ayer is very inhomogeneous, especially near its top and bottom. The au thors analyze in detail the fine structure of the supersaturation fiel d and suggest an explanation for the formation of the model-predicted supersaturation peak near the cloud top. The LES results suggest that super saturation may have a sharp increase in near-saturated parcels t hat undergo forced vertical displacement at the cloud-layer top. The m ain forcing mechanism that may supply the additional energy for the fo rced convection in the case presented is from propagating gravity wave s. Although radiative cooling may also result in increased convective activity at cloud top, the sensitivity tests presented here suggest th at, at least in these simulations, this effect is not dominant.