A dual mass flux framework for boundary layer convection. Part II: Clouds

A dual mass flux framework for boundary layer convection. Part II: Clouds
Date Published
ECMWF-ARM Report Series
Document Number
R. Neggers

This paper relates the extension of the Eddy Diffusivity Mass Flux (EDMF) framework for turbulent transport into the statistical modelling of boundary layer clouds. The advection-diffusion decomposition that defines EDMF is projected onto the distribution as used in the statistical cloud model. Each EDMF component is thus assigned its own independent PDF, resulting in an updraft PDF and a diffusive PDF. The bimodal distribution is configured and integrated in conserved variable space, the position and orientation of each PDF determined by its unique nature. The parameterization of the associated bimodal decomposition of variance introduces close ties to the transport scheme. While the grid-box mean variance is reconstructed using a prognostic variance budget, the variance of the updraft component is parameterized as a function of the spread among various resolved model updrafts. Individual model components and the scheme as a whole are evaluated in detail against large-eddy simulations of a number of prototype Trade-wind cases. The results show that the structure of cloud fraction, condensate and variance is reproduced. The diffusive PDF acts to represent stratiform clouds, while the advective PDF represents cumuliform clouds in conditionally unstable layers. This allows representation of complex scenarios in which both cloud forms occur, such as the transitional Trade-wind regime featuring cumulus rising into stratocumulus.