A simple parameterization for detrainment in shallow cumulus

Recently there has been a regained interest in the parameterization for entrainment in cumulus convection. Unfortunately little attention has been paid to the parameterization of the detrainment process although this counterpart of the cloud mixing process is equally important for obtaining realistic mass flux profiles in cumulus convection. A new simple but flexible parameterization for the detrainment process in shallow convection is presented. Using LES results it will be shown that the magnitude of the fractional detrainment for shallow convection is decreasing with increasing depth of the cloud layer. Consequently a simple detrainment formulation will be presented that takes this dependancy into account. The only free parameter in this detrainment formulation is the percentage of the mass flux at cloud base that is still present half-way the cloud layer. This free parameter turns out to depend on the environmental thermodynamic structure of the cloud layer. Results of this parameterization for a wide range of shallow cumulus convection cases with a single column model will illustrate the strength of this new detrainment parameterization.