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.

This study investigates how climate sensitivity depends upon the spatial pattern of radiative forcing. Sensitivity experiments using a coupled ocean-atmosphere model were conducted by adding anomalous incoming solar radiation over the entire globe, Northern Hemisphere mid-latitudes, Southern Ocean, and tropics, respectively, with both positive and negative perturbation considered. The varied forcing patterns led to highly divergent climate sensitivities, with extratropical forcing inducing significantly more global-mean temperature change compared to tropical forcing. This dependence is particularly strong over the Southern Hemisphere, where the climate is nearly twice as sensitive to Southern Ocean forcing as tropical forcing. This dependence of climate sensitivity on the location of radiative forcing stems from covariations between lapse rate feedback, cloud feedback and tropospheric stability. These results contrast with the conventional SST-pattern effect in which tropical surface temperature changes regulate the climate sensitivity, and has important implications for geoengineering and understanding the mechanisms of paleoclimate change.

Most global climate models with convective parameterization have trouble in simulating the observed diurnal cycle of convection. Maximum precipitation usually happens too early during summertime, especially over land. Observational analyses indicate that deep convection over land cannot keep pace with rapid variations in convective available potential energy, which is largely controlled by boundary-layer forcing. In this study, a new convective closure in which shallow and deep convection interact strongly, out of equilibrium, is implemented in atmosphere-only and ocean-atmosphere coupled models. The diurnal cycles of convection in both simulations are significantly improved with small changes to their mean states. The new closure shifts maximum precipitation over land later by about three hours. Compared to satellite observations, the diurnal phase biases are reduced by half. Shallow convection to some extent equilibrates rapid changes in the boundary layer at subdiurnal time scales. Relaxed quasi-equilibrium for convective available potential energy holds in significant measure as a result. Future model improvement will focus on the remaining biases in the diurnal cycle, which may be further reduced by including stochastic entrainment and cold pools.

Advances in high-performance computing have enabled large-eddy simulations (LES) of turbulence, convection, and clouds, but their potential to improve parameterizations in global climate models (GCMs) is only beginning to be harnessed. We design an experimental setup in which LES can be driven by large-scale forcings from GCMs. This can be done anywhere in the atmosphere, and we use this setup to create a library of LES of clouds across tropical and subtropical regimes. The LES are used to simulate the transition from stratoculumus to shallow cumulus over the East Pacific. The results are not very sensitive to the choice of the host GCM driving the LES. The setup is also used to simulate clouds under climate change. The LES simulate a positive but weak shortwave cloud feedback. The LES library expands the datasets available for calibrating parameterizations in GCMs.

The modern “wet” tropics are dominated by the Intertropical Convergence Zone, however “dry” tropics likely occurred in Earth’s history. It is unclear how the tropics change between wet and dry climates because recent progress has focused on modern and warmer climates. We show the tropical hydrological cycle undergoes a wet-to-dry regime transition when surface wetness is decreased in a general circulation model. The dry regime occurs when precipitation is suppressed by negative evaporation. The regime transition is dominated by near-surface relative humidity, in contrast to our traditional understanding which assumes changes in relative humidity are small. We show near-surface relative humidity changes are controlled by re-evaporation of stratiform precipitation. The moistening effect of re-evaporation is non-local: re-evaporation happens near the lifting condensation level and moisture diffuses downward to the near-surface. Our results provide a first step toward understanding tropical hydrological cycle changes between wet and dry climates.

Climate models project that tropical warming is amplified aloft in response to increased CO$_2$. Amplification aloft is expected following moist adiabatic adjustment and the Clausius-Clapeyron relation. Here, we show that moist adiabatic adjustment overpredicts the multi-model mean temperature response at 300 hPa by 12.9–25.3\% across the model hierarchy. We show that overprediction is influenced by at least three mechanisms: large-scale circulation, direct effect of CO$_2$, and convective entrainment. Accounting for the large-scale circulation and the direct effect of CO$_2$ reduces overprediction by 5.7\% and 3.8\% respectively, but does not eliminate it. To test the influence of entrainment, we vary the Tokioka parameter in aquaplanet simulations with and without a large-scale circulation. When varying the climatological entrainment rate in the aquaplanet, overprediction varies from 6.7–17.9\%. The sensitivity of overprediction to climatological entrainment rate in the aquaplanet configured in radiative-convective equilibrium agrees well with the predictions of zero-buoyancy bulk-plume models.