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Cloud-Resolving Simulations of Boundary-Layer Cloud Regimes with a Third-Order Turbulence

Cheng, A.(a,b) and Xu, K.-M.(a), Atmospheric Sciences, NASA Langley Research Center (a), Hampton University (b)
Twelfth Atmospheric Radiation Measurement (ARM) Science Team Meeting

LES (large eddy simulation) models can explicitly resolve large turbulent eddies, which contain m ost of the turbulent kinetic energy and do most of the transport in the boundary layer. These edd ies have to be parameterized in cloud-resolving models (CRMs), which have much coarser resolution . A sophisticated turbulent parameterization is needed in order to produce adequate simulations o f cloud processes in CRMs. Most CRMs use a one- and a half-order prognostic turbulent kinetic ene rgy closure. Third-order turbulence closure is only used in the CRM originally developed at the U niversity of California-Los Angeles (UCLA; Krueger 1988) and currently used in several institutio ns. Associated with this type of turbulence closure is the subgrid-scale condensation (SGSC) para meterization (Sommeria and Deardorff 1977). This study examines the importance of the SGSC parameterization in the simulation of different bo undary-layer cloud regimes, using a 2-D CRM with a third-order turbulence closure. Four boundary- layer cloud regimes have been simulated with and without the SGSC parameterization. These four bo undary-layer regimes are based upon observations from the following field experiments: Atlantic S tratocumulus Transition EXperiment, Barbados Oceanographic and Meteorological Experiment, Atmosph eric Radiation Measurement and First ISCCP Regional Experiment. Preliminary results indicate that the importance of SGSC is cloud regime dependent; i.e., an SGSC parameterization is absolutely n ecessary for simulating trade cumuli, but not for simulating the single-layer stratocumulus regim e. An implication of these results for cloud resolving modeling is that an SGSC parameterization should be used to overcome the inadequate resolution of CRMs in order to minimize the degradation of model performance as resolution becomes coarser. Sommeria, G. and J. W. Deardorff, 1977: Subgrid-scale condensation in models of nonprecipitating clouds. J. Atmos. Sci., 43, 344-355.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).