Representing effects of surface heterogeneity in a multi-plume eddy diffusivity mass flux boundary layer parameterization

Abstract

Earth system models (ESMs) typically represent surface heterogeneity on scales smaller than the atmospheric grid, while land–atmosphere coupling is based on grid mean values. Here we present a general approach allowing subgrid surface heterogeneity to influence the updraft thermodynamic properties in a multi-plume mass flux parameterization. The approach is demonstrated in single column experiments with an eddy diffusivity–mass flux (EDMF) boundary layer scheme. Instead of triggering based on grid mean surface values, updrafts are explicitly assigned to individual surface tiles with positive buoyancy flux. Joint distributions of near-surface vertical velocities and thermodynamic variables are defined over individual surface tiles, and updraft properties are drawn from the positive tails of the distributions. The approach allows updraft properties to covary with surface heterogeneity, and updrafts from different tiles maintain distinct properties to heights of several hundred meters. Mass flux contributions to subgrid variances are increased near the surface, but impacts on mean state variables are relatively small. We suggest that larger impacts might be obtained by adding a specialized plume to represent the effects of secondary circulations.