An eddy‐diffusivity/mass‐flux parametrization for dry and shallow cumulus convection

Abstract

Recently, a new consistent way of parametrizing simultaneously local and non‐local turbulent transport for the convective atmospheric boundary layer has been proposed and tested for the clear boundary layer. This approach assumes that in the convective boundary layer the subgrid‐scale fluxes result from two different mixing scales: small eddies, that are parametrized by an eddy‐diffusivity approach, and thermals, which are represented by a mass‐flux contribution. Since the interaction between the cloud layer and the underlying sub‐cloud layer predominantly takes place through strong updraughts, this approach offers an interesting avenue of establishing a unified description of the turbulent transport in the cumulus‐topped boundary layer. This paper explores the possibility of such a new approach for the cumulus‐topped boundary layer. In the sub‐cloud and cloud layers, the mass‐flux term represents the effect of strong updraughts. These are modelled by a simple entraining parcel, which determines the mean properties of the strong updraughts, the boundary‐layer height, the lifting condensation level and cloud top. The residual smaller‐scale turbulent transport is parametrized with an eddy‐diffusivity approach that uses a turbulent kinetic energy closure. The new scheme is implemented and tested in the research model MesoNH. Copyright © 2004 Royal Meteorological Society