Subgrid Turbulence Models in the Boundary-Layer Gray Zone

Forecasting localised weather conditions such as fog, poor air quality, high surface temperatures and thunderstorms remains a significant challenge for numerical weather prediction (NWP). A key part of the challenge involves the representation of small scale processes, including turbulence in the atmospheric boundary layer. High resolution NWP promises to deliver better localised forecasts, but will increasingly rely on adapting existing or developing new parametrisations to work in the so-called “gray zone” where boundary-layer turbulence and other small-scale processes are partially resolved. This study compares several subgrid models using large-eddy simulations (LES), and evaluates their performance as potential candidates for the gray-zone regime. Methods of anaylsis include comparing sub-filter stress contributions from the models, turbulence statistics and organised structures against results from high-resolution LES.