A Multiparcel Model for Shallow Cumulus Convection

Abstract

A new parameterization for cumulus convection is formulated, that consists of an ensemble of small, rising parcels. Large eddy simulation (LES) results are used to parameterize the lateral mixing of such a parcel: for the mixing process a relaxation timescale is defined and its value is determined by investigating individual LES clouds. The timescale is found to be nearly independent of cloud depth, which implies that the entrainment rate is inversely proportional to the vertical velocity. As a consequence, a dynamical feedback mechanism is established: the parcel dynamics influence the mixing rate, which, together with the environmental properties, feeds back on the parcel properties and therefore on the parcel dynamics. The multiparcel model is validated with LES fields. The characteristics of the buoyant part of the clouds are reproduced: the decreasing fractional cover and increasing liquid water content with height, the vertical dynamics and mass flux, and the conserved properties and the marginally buoyant state. The model also produces the variability typical for shallow cumulus.