The effect of coupling between CLUBB turbulence scheme and surface
momentum flux on global wind simulations
Abstract
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.