A cumulus parameterization based on the generalized convective available potential energy

Abstract

This paper reports tests of a cumulus parameterization in which the reference state associated with the generalized convective available potential energy (GCAPE) is chosen as the end-state of the convective adjustment. The GCAPE is defined as the enthalpy difference between the given state and the reference state and represents the total potential energy available for conversion into convective kinetic energy in a given sounding. The reference state is the unique state in which the system enthalpy is minimized; it is also a statically neutral or stable state. By assuming that convection drives the atmosphere from the given state toward the reference state, we can use Nitta's diagnostic method to determine the cloudbase mass flux in a prognostic model. The adjustment timescale is finite and varies, depending on the intensity of the large-scale forcing, so this is a "relaxed" scheme. The effects of detrainment are parameterized in a very simple way. After the cloudbase mass flux has been obtained, Nitta's method and Johnson's simple downdraft model are used to determine the feedback of cumulus convection on the large-scale temperature and moisture fields. We have performed a semiprognostic test of the new adjustment scheme using the GATE Phase III data. The results are in fair agreement with observations.