Enhancing global wind climate simulations in the GFDL-AM4 model by unifying planetary boundary layer and cloud turbulence parametrizations with the higher-order scheme CLUBB

The higher-order turbulence scheme, Cloud Layers Unified by Binormals (CLUBB), is known for effectively  simulating the transition from cumulus to stratocumulus clouds within leading atmospheric climate models.  Here we investigate an underexplored aspect of CLUBB: its capacity to simulate near-surface winds and the Planetary Boundary Layer (PBL), with a particular focus on its coupling with surface momentum flux, modelling of turbulent lengthscale, and direct prognosis of turbulent momentum flux. First, using the GFDL atmospheric climate model (AM4), we examine two distinct coupling strategies, distinguished by their handling of surface momentum flux during the CLUBB’s stability-driven substepping performed at each atmospheric time step.  The static coupling maintains a constant surface momentum flux, while the dynamic coupling adjusts the surface momentum flux at each CLUBB substep based on the CLUBB-computed zonal and meridional wind speed tendencies. Our 30-year present-day climate simulations (1980-2010) show that static coupling overestimates 10-m wind speeds compared to both control AM4 simulations and reanalysis, particularly over the Southern Ocean (SO) and other midlatitude ocean regions. Conversely, dynamic coupling corrects the static coupling 10-m winds biases in the midlatitude regions, resulting in CLUBB simulations achieving there an excellent agreement with AM4 simulations. Furthermore, analysis of PBL vertical profiles over the SO reveals that dynamic coupling reduces downward momentum transport, consistent with the found wind-speed reductions. Instead, near the tropics, dynamic coupling results in minimal changes in near-surface wind speeds and associated turbulent momentum transport structure. Then, implementing a more generalized calculation of the turbulent length scale leads to an overall degradation of winds but improves root mean square error and bias of low cloud amount in key regions of stratocumulus to cumulus transition. Remarkably, we show that updating CLUBB formulation of momentum flux from diagnostic to prognostic, to permit countergradient momentum fluxes, improves nearly everywhere global winds, while retaining the benefits of the more generalised lengthscale formulation in accurately capturing the low cloud amount. Finally, the wind turning angle serves as a valuable qualitative metric for assessing the impact of changes in surface momentum flux representation on global circulation patterns.