Incorporating Effects of Subgrid Surface Heterogeneity in a Multi-Plume EDMF Parameterization

The horizontal grid spacing in atmospheric general circulation models is of order 10km-100km, depending on the application. Surface models often represent significant heterogeneity at smaller scales, but communication from the surface is typically aggregated such that the atmosphere sees only a single representative value, and any direct influence of heterogeneity on the atmosphere is neglected. In contrast, large eddy simulations and real-world observations indicate that surface heterogeneity plays a role in both boundary layer turbulence and the triggering and properties of atmospheric convection. Here we incorporate effects of surface heterogeneity in a multi-plume mass flux scheme, following two approaches. In the first, updrafts are explicitly distributed over all surface tiles with a positive buoyancy flux. Each updrafts initial thermodynamic and kinetic properties are calculated using the tile-based fluxes. In the second, a statistical approach is used to define a distribution of updraft properties based on the tile-level fluxes. The impacts of heterogeneity on convective triggering, depth, and boundary layer turbulence are documented in a set of single column experiments. The approaches are shown to convey surface heterogeneity into the boundary layer and to increase updraft activity during marginal time periods.