Marine stratocumulus clouds cover nearly one-quarter of the ocean surface and thus play an extremely important role in determining the global radiative balance. The semipermanent marine stratocumulus deck over the southeastern Atlantic Ocean is of particular interest, because of its interactions with seasonal biomass burning aerosols that are emitted in southern Africa. Understanding the impacts of biomass burning aerosols on stratocumulus clouds and the implications for regional and global radiative balance is still very limited. Previous studies have focused on assessing the magnitude of the warming caused by solar scattering and absorption by biomass burning aerosols over stratocumulus (the direct radiative effect) or cloud adjustments to the direct radiative effect (the semidirect effect). Here, using a nested modeling approach in conjunction with observations from multiple satellites, we demonstrate that cloud condensation nuclei activated from biomass burning aerosols entrained into the stratocumulus (the microphysical effect) can play a dominant role in determining the total radiative forcing at the top of the atmosphere, compared with their direct and semidirect radiative effects. Biomass burning aerosols over the region and period with heavy loadings can cause a substantial cooling (daily mean −8.05 W m⁻²), primarily as a result of clouds brightening by reducing the cloud droplet size (the Twomey effect) and secondarily through modulating the diurnal cycle of cloud liquid water path and coverage (the cloud lifetime effect). Our results highlight the importance of realistically representing the interactions of stratocumulus with biomass burning aerosols in global climate models in this region. Significance Each year biomass burning (BB) aerosols emitted in southern Africa during the fire season (July–October) transport westward over the southeast Atlantic Ocean and interact with underlying stratocumulus decks. Previous studies showed that BB aerosols greatly perturb the top of atmosphere radiation balance by scattering and absorbing solar radiation and by altering cloud properties via changing the lower tropospheric stability (direct and semidirect effects). Using a state-of-the-art model in combination with satellite observations, we found that BB aerosols that are entrained into the clouds function as cloud condensation nuclei, and increase the brightness of stratocumulus clouds (indirect effect). In these models, this indirect effect dominates the direct and semidirect effects of BB aerosols over this region.

An advanced aerosol treatment, with a focus on semivolatile nitrate formation, is introduced into the Community Atmosphere Model version 5 with interactive chemistry (CAM5‐chem) by coupling the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) with the 7‐mode Modal Aerosol Module (MAM7). An important feature of MOSAIC is dynamic partitioning of all condensable gases to the different fine and coarse mode aerosols, as governed by mode‐resolved thermodynamics and heterogeneous chemical reactions. Applied in the free‐running mode from 1995 to 2005 with prescribed historical climatological conditions, the model simulates global distributions of sulfate, nitrate, and ammonium in good agreement with observations and previous studies. Inclusion of nitrate resulted in ∼10% higher global average accumulation mode number concentrations, indicating enhanced growth of Aitken mode aerosols from nitrate formation. While the simulated accumulation mode nitrate burdens are high over the anthropogenic source regions, the sea‐salt and dust modes respectively constitute about 74% and 17% of the annual global average nitrate burden. Regional clear‐sky shortwave radiative cooling of up to −5 W m⁻² due to nitrate is seen, with a much smaller global average cooling of −0.05 W m⁻². Significant enhancements in regional cloud condensation nuclei (at 0.1% supersaturation) and cloud droplet number concentrations are also attributed to nitrate, causing an additional global average shortwave cooling of −0.8 W m⁻². Taking into consideration of changes in both longwave and shortwave radiation under all‐sky conditions, the net change in the top of the atmosphere radiative fluxes induced by including nitrate aerosol is −0.7 W m⁻².

Dust aerosol plays an important role in the Earth System. As a natural aerosol, dust aerosol is often calculated interactively in global climate models and temporal variations of dust emission in the past century are far less constrained compared to those of anthropogenic aerosol emissions. Here we evaluate dust emission in East Asia simulated by 15 climate models participating in the Coupled Model Intercomparison Project Phase 5. The results show that none of the models can reproduce the observed decline of dust event frequency during 1961–2005 over East Asia. The models tend to simulate either much less decline or even increase of dust emission. The discrepancy is mainly ascribed to weaker or opposite trends of surface wind speeds and precipitation in the models. These results cast a doubt on the interpretation of long‐term variations of dust‐affected fields in climate models and highlight the need for further improvements of the models.

The maximum supersaturation (S_x) in clouds is a key parameter affecting the cloud's microphysical and radiative properties. We investigate the S_x of the marine boundary layer clouds by combining airborne and surface observations in the Eastern North Atlantic. The cloud droplet number concentration (N_c) in the least diluted cloud cores agrees well with the number concentration of particles larger than the Hoppel Minimum (HM) (N_>HM) below clouds, indicating that the HM represents the average size threshold above which particles are activated to form cloud droplets. The S_x values derived from surface observations vary from 0.10% to 0.50% from June 2017 to June 2018, with a clear seasonal variation exhibiting higher values during winter. Most of the S_x variance (∼60%) can be explained by the cloud condensation nuclei (CCN) concentration and updraft velocity (w), with the CCN concentration playing a more important role than w in explaining the variation of S_x. The influence of CCN concentration on S_x leads to a buffered response of N_c to aerosol perturbations. The response of N_c to low aerosol concentration during winter is further buffered by the high w. The global Community Earth System Model (CESM) simulated S_x values in the Azores have a positive bias compared to measured S_x, likely due to overestimated w and underestimated CCN concentration. The CESM simulated S_x exhibits higher values further north over the North Atlantic Ocean, which is attributed to stronger w. The suppression of S_x by aerosol is also evident in regions with high CCN concentrations.

The reliability of climate simulations and projections, particularly in the regions with complex terrains, is greatly limited by the model resolution. In this study we evaluate the variable‐resolution Community Earth System Model (VR‐CESM) with a high‐resolution (0.125°) refinement over the Rocky Mountain region. The VR‐CESM results are compared with observations, as well as CESM simulation at a quasi‐uniform 1° resolution (UNIF) and Canadian Regional Climate Model version 5 (CRCM5) simulation at a 0.11° resolution. We find that VR‐CESM is effective at capturing the observed spatial patterns of temperature, precipitation, and snowpack in the Rocky Mountains with the performance comparable to CRCM5, while UNIF is unable to do so. VR‐CESM and CRCM5 simulate better the seasonal variations of precipitation than UNIF, although VR‐CESM still overestimates winter precipitation whereas CRCM5 and UNIF underestimate it. All simulations distribute more winter precipitation along the windward (west) flanks of mountain ridges with the greatest overestimation in VR‐CESM. VR‐CESM simulates much greater snow water equivalent peaks than CRCM5 and UNIF, although the peaks are still 10–40% less than observations. Moreover, the frequency of heavy precipitation events (daily precipitation ≥ 25 mm) in VR‐CESM and CRCM5 is comparable to observations, whereas the same events in UNIF are an order of magnitude less frequent. In addition, VR‐CESM captures the observed occurrence frequency and seasonal variation of rain‐on‐snow days and performs better than UNIF and CRCM5. These results demonstrate the VR‐CESM's capability in regional climate modeling over the mountainous regions and its promising applications for climate change studies.

We incorporate the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) module in the Community Earth System Model version 2 (CESM2) Community Atmosphere Model Version 6 with interactive chemistry (CAM6‐chem), and couple it with the four mode version of the Modal Aerosol Module (MAM4). The MOSAIC module is used to simulate the thermodynamics of the gas‐aerosol mass exchange, with a special focus on simulating nitrate aerosol. By comparing against ground and satellite observations, we found that the MOSAIC/MAM4 scheme performs reasonably well in simulating spatiotemporal distributions of aerosols, including nitrate aerosol. We conducted a series of model experiments with and without nitrate aerosols, and examined the radiative effect (RE) associated with nitrate aerosols in 1975, 2000, and 2010, and accessed the radiative forcing (RF) of nitrate aerosols between the present day and pre‐industrial periods. Comparing with the nitrate aerosol RE, we predicted relatively small RF of anthropogenic nitrate aerosol from aerosol‐radiation interactions (RF_ari: −0.014 W m⁻²) and large RF from aerosol‐cloud interactions (RF_aci: −0.219 W m⁻²). Regional signatures of nitrate RE/RF are noticeable and important: for instance, very small changes in RE_ari in Europe and USA, but 2.8–3 times increases in RE_ari in India and China from 1975 to 2010, while RE_aci/RF_aci in China is a warming effect due to the competing effect between sulfate and nitrate aerosols as cloud condensation nuclei.