The Effect of Coupling Between CLUBB Turbulence Scheme and Surface Momentum Flux on Global Wind Simulations.

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. This study investigates 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. 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. Notably, 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. Plain Language Summary: The Cloud Layers Unified by Binormals (CLUBB) scheme offers a promising way to model the complexities of cloud behavior, but its impact on winds and global circulation has been less explored. In our study, we investigate how different ways of representing the complex coupling between surface drag and the lowest kilometer of the Earth's atmosphere affect global wind speeds and circulation. We specifically examine two distinct approaches: a static approach, which feeds a constant surface drag to CLUBB, and a dynamic approach, which adjusts the surface drag based on the winds updates computed by CLUBB. Over a present‐day climate, we find that static coupling tends to produce excessively large wind speeds in certain regions, like the Southern Ocean and parts of the North Atlantic and North Pacific. Instead, dynamic coupling produces excellent near‐surface wind speeds in these regions, and also over the rest of the globe. Moreover, we discover that dynamic coupling reduces the downward turbulent transport of momentum, highlighting the enhancements in near‐surface wind speeds found with this approach are physically consistent. Lastly, we use the change in wind direction with height to qualitatively evaluate how the two coupling methods affect global circulation patterns. Key Points: Dynamic coupling between Cloud Layers Unified by Binormals (CLUBB) and surface momentum flux enhances global winds climate simulations bringing CLUBB in line with control atmospheric climate modelIn midlatitude regions, the dynamic coupling enhances the boundary‐layer momentum transport compared to the static couplingThe wind turning angle turns out a useful qualitative metric, linking changes in surface momentum flux to the changes in global circulation [ABSTRACT FROM AUTHOR]