A Parametrization of the Convective Boundary Layer with Subgrid Orography

Année de publication

2026

Current parameterizations of shallow convection ignore subgrid orography, assuming a flat surface at the mean elevation. Consequently, they often fail in mountainous regions to reproduce realistic profiles and surface temperatures and to simulate the transition to deep convection. We have produced a set of idealized large-eddy simulation (LES) of dry convection with different orographies, including flat cases, to highlight the importance of the subgrid orography on the evolution of the mean profiles and to characterize the coherent structures contributing to the vertical mixing: the thermal updrafts and the slope winds. From that process analysis of the LES, we have developed a generalization of eddy-diffusivity mass-flux (EDMF) parameterizations for daytime mountainous boundary layers, which unifies the anabatic slope winds and the thermal updrafts within the mass-flux framework. The 1D parameterization includes a subgrid orography following an area-height distribution of the atmosphere on which we define the vertical profiles of prognostic variables. It reproduces the daytime evolution of the mean temperature profile, as well as the characteristics of anabatic winds and thermal updrafts simulated in the LES. Further developments are needed to take into account moist processes, nighttime katabatics, and the interactions with synoptic winds. To be used in a 3D atmospheric model, this parameterization should be combined with a dynamical core that handles porous mountains, necessitating an adaptation of current models that are based on terrain-following grids. With these additional developments, this parameterization could improve the skill of coarse-gridded (>1 km) atmospheric models over mountainous regions.</div>

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