Locating the Turbulent Gray Zone in High-Resolution Numerical Weather Prediction

Abstract:

The turbulent gray zone, or terra incognita, is a range of grid resolutions where the grid is too coarse to use a large eddy simulation (LES), but too fine to use a one-dimensional planetary boundary layer (PBL) scheme. The presence of the gray zone is a problem for numerical weather prediction (NWP) practitioners, both in a research and operational capacity. Generally, the gray zone is considered to span from O(100 m) to O(1 km), however these limits are just approximations. The inadequacies of turbulence models in the gray zone has been shown to have a considerable influence on resolved-scale dynamics. As computational resources become more available, higher resolution atmospheric models are inevitable, so it is important that guidelines exist for choosing an appropriate grid-resolution. Here we consider the behavior of LES closures at intermediate resolutions and deduce some basic criteria for a properly-resolved atmospheric LES. Idealized scenarios for convective and sheared atmospheric cases are considered first, followed by a real case in the Southern Great Plains.