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Large-eddy simulation of mesoscale dynamics and entrainment around a pocket of open cells observed in VOCALS-REx RF06

By A. H. Berner, C. S. Bretherton and R. Wood

Abstract

Large-eddy simulations of a pocket of open cells (POC) based on VOCALS Regional Experiment (REx) NSF C-130 Research Flight 06 are analyzed and compared with aircraft observations. A doubly-periodic domain 192 km × 24 km with 125 m horizontal and 5 m vertical grid spacing 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 (<i>N</i><sub>c</sub>) 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 <i>N</i><sub>c</sub> is allowed to advect following the turbulent flow gives nearly identical results over the 16 h length of the simulation (which starts at night and goes into the next afternoon). <br><br> 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 develops in the model 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 subsidence

Topics: Environmental sciences, GE1-350, Geography. Anthropology. Recreation, G, DOAJ:Environmental Sciences, DOAJ:Earth and Environmental Sciences, Physics, QC1-999, Chemistry, QD1-999
Publisher: Copernicus Publications
Year: 2011
DOI identifier: 10.5194/acp-11-10525-2011
OAI identifier: oai:doaj.org/article:3a4645d587f9472e9a491b35de2370dd
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