a Study of the Cloud/radiation Interaction Using a Second-Order Turbulence Closure Radiative/convective Model.

A high resolution one-dimensional version of a second order turbulence radiative/convective model, developed at Los Alamos National Laboratory, was used to study the structure and diurnal cycle of marine stratocumulus clouds with respect to their interaction with radiation and their response to external forcings. The fidelity of the model to the underlying physics was assessed by comparing model simulations to data taken at San Nicolas Island during the intensive field observation marine stratocumulus phase of the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE IFO), conducted during July 1987. The model was able to reproduce the observed diurnal cycle of the liquid water content, cloud base, and mean and turbulence variables fairly well. It was found that the modeled stratocumulus cloud thins during the day because of the phenomenon of "decoupling" which is caused by the warming of the cloud layer relative to the subcloud layer. A number of sensitivity runs were designed to examine the evolution of the marine stratocumulus cloud capped boundary layer when subjected to changing conditions of large scale divergence, initial inversion strength, effective drop size radius, sea surface temperature, and wind speed. In general, the strength of the decoupling effect was found to be strongly linked to the overall shortwave heating in the cloud layer through the thickness of the cloud and the magnitude of the liquid water content. Another general feature of the simulations is that the inversion height for a well-established quasi-equilibrium boundary layer tended to be fairly insensitive to changes in external forcings. The boundary layer height did not change appreciably except when the cloud layer tended to dissipate.