Momentum Transport in Boundary-Layer Wind Climate Simulations and Its Parametrization By the Higher-Order Closure Scheme Cloud Layers Unified By Binormals (CLUBB): Challenges and Opportunities

Accurately representing sub-grid momentum flux is a critical aspect of ensuring the reliability of climate simulations of boundary-layer winds. The prevalent utilization of first-order momentum flux parametrizations in leading climate models, characterized by downgradient diffusion methods coupled with distinct cumulus momentum transport schemes, is simplistic. In contrast, higher-order turbulence parametrizations, which directly prognose momentum flux, offer enhanced versatility and generality, closely aligning with the fundamental governing equations.

This study explores the sensitivity of boundary-layer wind climate simulations using the AM4-GFDL global model. We compare the effects of employing a first-order, diagnostic sub-grid momentum flux parametrization against the higher-order scheme Cloud Layers Unified by Binormals (CLUBB) which directly prognoses sub-grid turbulent fluxes. Moreover, we demonstrate how the boundary-layer wind turning angle can be effectively used as a metric to evaluate the impact of changing the sub-grid momentum flux parametrization on the boundary-layer wind structure. Through this analysis, we highlight the potential of the higher-order scheme CLUBB to significantly enhance our understanding and modelling capabilities of the structure of boundary-layer winds in climate integrations.