Atmos. Chem. Phys. Discuss., 11, 13317-13353, 2011
© Author(s) 2011. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Large-eddy simulation of mesoscale dynamics and entrainment around a pocket of open cells observed in VOCALS RF06
A. H. Berner, C. S. Bretherton, and R. Wood
Department of Atmospheric Science, University of Washington, Seattle, Washington, USA

Abstract. 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 h 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 subsidence.

Citation: Berner, A. H., Bretherton, C. S., and Wood, R.: Large-eddy simulation of mesoscale dynamics and entrainment around a pocket of open cells observed in VOCALS RF06, Atmos. Chem. Phys. Discuss., 11, 13317-13353, doi:10.5194/acpd-11-13317-2011, 2011.
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