Large-Eddy Simulation of Mesoscale Dynamics and Entrainment Around A Pocket of Open Cells Observed in VOCALS RF06

Large-eddy simulations of a pocket of open cells (POC) based on VOCALS Regional Experiment (REx) Research Flight 06 are analyzed and compared with aircraft observations. A doubly-periodic domain 192 km×24 km with 125 m horizontal resolution and 5 m vertical resolution near the capping inversion is used. The POC is realized in the model as a fixed 96 km wide region of reduced cloud droplet number concentration (Nc) based on observed values; initialization and forcing are otherwise uniform across the domain. The model reproduces aircraft-observed differences in boundary-layer structure and precipitation organization between a well-mixed overcast region and a decoupled POC with open-cell precipitating cumuli, although the simulated cloud cover is too large in the POC. A sensitivity study in which Nc is allowed to advect following the turbulent flow gives nearly identical results over the 16 hour length of the simulation (which starts at night and goes into the next afternoon). The simulated entrainment rate is nearly a factor of two smaller in the less turbulent POC than in the more turbulent overcast region. However, the inversion rises at a nearly uniform rate across the domain because powerful buoyancy restoring forces counteract horizontal inversion height gradients. A secondary circulation sets up that diverts subsiding free-tropospheric air away from the POC into the surrounding overcast region, counterbalancing the weaker entrainment in the POC with locally weaker