Regional Simulations of Marine Boundary Layer Clouds During ASTEX

A regional boundary layer model was used to simulate the steady state and time evolution of marine boundary layer clouds during the Atlantic Stratocumulus Transformatlon Experiment (ASTEX). The model used is a two-layer mass-flux model which can simulate both stratiforrn and cumuliform clouds in the marine boundary layer. The model can predict thermodynamic structures, turbulent fluxes of moisture and heat in the boundary layer, cloud fractional coverage and reflectivity of the boundary layer cloud.

The model was applied to the region of 20deg.N-40deg.N and 15deg.W-40deg.W for the period of ASTEX. The analysis of European Center For Medium-Range Weather Forecast was used to provide the large-scale conditions required to drive the model. Two simulations were made. One was a steady-state simulation, the other was a time evolution of the boundary layer cloud for 4 days.

Both simulations showed that boundary layer heights increased, and cloud fraction decreased toward the south. The simulated boundary layer also tended to be more decoupled toward the south in the sense that the cloud base stable layer became more strengthened. The temporal simulation showed strong diurnal variabilities of cloud fraction and the cloud reflectivity, which is consistent with general observations of diurnal variation of these clouds. The simulated surface moisture flux during the night was larger than that during the daytime, since the decoupling due to the solar warming traps moisture in the subcloud layer. The surface buoyancy flux, however, did not change significantly during the simulated period. The simulated vertical structure was also compared with the mean profile derived from soundings made during ASTEX.