MODEL IMPACTS OF ENTRAINMENT AND DETRAINMENT RATES IN SHALLOW CUMULUSCONVECTION

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 Experim ent (BOMEX). The mass flux scheme is embedded in a one-column model wi th prescribed large-scale forcings. Comparing the findings of the latt er 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 b etween the cloud layer and the inversion layer, giving rise to erroneo us moisture and temperature profiles for the trade wind region. This i s due to an underestimation of the lateral exchange rates. LES results show that for shallow cumulus cloud ensembles (lateral) entrainment a nd 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. W hen adopting these enhanced rates, the mass flux scheme produces reali stic mass fluxes and cloud excess values for moisture and heat and is therefore capable of maintaining the stationary state as observed duri ng BOMEX.