Impacts of Large-Scale Environments on Mesoscale Cellular Structure in Marine Boundary Layer Clouds

Recent observations and modeling studies have shown that aerosol-cloud-drizzle interaction plays an essential role in some observed closed-to-open cell transitions in marine stratocumulus clouds. This study aims to understand how large-scale conditions (such as sea surface temperature, low tropospheric stability, and water vapor above the boundary layer) may affect the closed-to-open cell transition given a similar aerosol change in the environment. COAMPS-LES (Coupled Ocean/Atmosphere Mesoscale System-Large-Eddy Simulation) coupled with Feingold's two-moment cloud parameterization is used in this study. The model is configured to cover 100kmx100kmx2km domain with the resolution of 150mx150mx2km. A set of simulations have been made over various large-scale conditions. When SST is moderately high (about20º C), a change of background CCN from 100 cm-3 to 35 cm-3 initiates the transition from closed-cell to open-cell structure. The mesoscale ring structure of convective elements is clearly defined with the radius being about 10-30 km. The fundamental forcing in the transition is the precipitation initiated by low aerosol number concentration. The evaporative cooling produces strong downdrafts in the sub-cloud layer and divergence at the surface, which organizes the convergence lines to support updrafts and sustain the open-cell structure. When SST is 14° C, the stronger lower tropospheric stability limits the cloud-top entrainment. The relative humidity is relatively high and the cloud base is low in the marine boundary layer, which limit the evaporation. Consequently, the evaporatively driven downdrafts are not sufficiently strong to support robust updrafts. Therefore, the transition for lower SST is not as clearly defined as those for the moderately high SST. It is also found that at high SSTs ( 25º C), solid stratocumulus clouds are transformed to scattered shallow cumulus clouds due to intensified cloud-top entrainment without any presence of drizzle. However, the organization of the convective elements is significantly weaker than in the drizzling case, since the evaporatively driven mesoscale downdrafts are no longer available.