A Scale-Adaptive Turbulence Model for the Dry Convective Boundary Layer

Abstract

A scale-adaptive model is developed for the representation of dry convective boundary layer (CBL) turbulence in numerical models operating at O(100) m to O(1) km horizontal resolution, also known as the model gray zone of the CBL. The new model is constructed based on a planetary boundary layer (PBL) scheme and a large-eddy simulation (LES) closure that are both turbulence kinetic energy–based parameterizations. Scale adaptivity is achieved by ‘‘blending’’ the PBL scheme with the LES closure through an inverse averaging procedure that naturally accounts for vertical variations of the dominant turbulent length scales, hence the gray zone range. High-resolution wide-domain LES benchmark cases covering a broad range of CBL bulk stability are filtered to gray zone resolutions, and analyzed to determine the averaging coefficients. Stability dependence of the dominant length scales is revealed by the analysis and accounted for in the new model. The turbulence model is implemented into a community atmospheric model, and tested for idealized cases. Compared to two established gray zone models, the new model performs equally well under strongly convective conditions, and is more advantageous for the weakly unstable and near neutral CBL.