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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, R. Wood
Atmospheric Chemistry and Physics ()

Abstract

Large-eddy simulations of a pocket of open cells (POC) based on VOCALS\nRegional Experiment (REx) NSF C-130 Research Flight 06 are analyzed and\ncompared with aircraft observations. A doubly-periodic domain 192 km x\n24 km with 125 m horizontal and 5 m vertical grid spacing near the\ncapping inversion is used. The POC is realized in the model as a fixed\n96 km wide region of reduced cloud droplet number concentration (N(c))\nbased on observed values; initialization and forcing are otherwise\nuniform across the domain. The model reproduces aircraft-observed\ndifferences in boundary-layer structure and precipitation organization\nbetween a well-mixed overcast region and a decoupled POC with open-cell\nprecipitating cumuli, although the simulated cloud cover is too large in\nthe POC. A sensitivity study in which N(c) is allowed to advect\nfollowing the turbulent flow gives nearly identical results over the 16\nh length of the simulation (which starts at night and goes into the next\nafternoon).\nThe simulated entrainment rate is nearly a factor of two smaller in the\nless turbulent POC than in the more turbulent overcast region. However,\nthe inversion rises at a nearly uniform rate across the domain because\npowerful buoyancy restoring forces counteract horizontal inversion\nheight gradients. A secondary circulation develops in the model that\ndiverts subsiding free-tropospheric air away from the POC into the\nsurrounding overcast region, counterbalancing the weaker entrainment in\nthe POC with locally weaker subsidence.

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