Model Impacts of Entrainment and Detrainment Rates in Shallow Cumulus Convection

A. P. Siebesma Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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A. A. M. Holtslag Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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Abstract

A mass flux parameterization scheme for shallow cumulus convection is evaluated for a case based on observations and large eddy simulation (LES) results for the Barbados Oceanographic and Meteorological Experiment (BOMEX). The mass flux scheme is embedded in a one-column model with prescribed large-scale forcings. Comparing the findings of the latter with the LES results, it is found that the mass flux scheme is too active. As a result, the scheme is mixing too much heat and moisture between the cloud layer and the inversion layer, giving rise to erroneous moisture and temperature profiles for the trade wind region. This is due to an underestimation of the lateral exchange rates. LES results show that for shallow cumulus cloud ensembles (lateral) entrainment and detrainment rates are typically one order of magnitude larger than values used in most operational parameterization schemes and that the detrainment rate is systematically larger than the entrainment rate. When adapting them enhanced rates, the mass flux scheme produces realistic mass fluxes and cloud excess values for moisture and heat and is therefore capable of maintaining the stationary state as observed during BOMEX.

Abstract

A mass flux parameterization scheme for shallow cumulus convection is evaluated for a case based on observations and large eddy simulation (LES) results for the Barbados Oceanographic and Meteorological Experiment (BOMEX). The mass flux scheme is embedded in a one-column model with prescribed large-scale forcings. Comparing the findings of the latter with the LES results, it is found that the mass flux scheme is too active. As a result, the scheme is mixing too much heat and moisture between the cloud layer and the inversion layer, giving rise to erroneous moisture and temperature profiles for the trade wind region. This is due to an underestimation of the lateral exchange rates. LES results show that for shallow cumulus cloud ensembles (lateral) entrainment and detrainment rates are typically one order of magnitude larger than values used in most operational parameterization schemes and that the detrainment rate is systematically larger than the entrainment rate. When adapting them enhanced rates, the mass flux scheme produces realistic mass fluxes and cloud excess values for moisture and heat and is therefore capable of maintaining the stationary state as observed during BOMEX.

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