Simulating whole atmosphere dynamics, chemistry, and physics is computationally expensive. It can require high vertical resolution throughout the middle and upper atmosphere, as well as a comprehensive chemistry and aerosol scheme coupled to radiation physics. An unintentional outcome of the development of one of the most sophisticated and hence computationally expensive model configurations is that it often excludes a broad community of users with limited computational resources. Here, we analyze two configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)) with simplified “middle atmosphere” chemistry at nominal 1 and 2 degree horizontal resolutions. Using observations, a reanalysis, and direct model comparisons, we find that these configurations generally reproduce the climate, variability, and climate sensitivity of the 1 degree nominal horizontal resolution configuration with comprehensive chemistry. While the background stratospheric aerosol optical depth is elevated in the middle atmosphere configurations as compared to the comprehensive chemistry configuration, it is comparable between all configurations during volcanic eruptions. For any purposes other than those needing an accurate representation of tropospheric organic chemistry and secondary organic aerosols, these simplified chemistry configurations deliver reliable simulations of the whole atmosphere that require 35% to 86% fewer computational resources at nominal 1 and 2 degree horizontal resolution, respectively.

Controls on pristine aerosol over the Southern Ocean (SO) are critical for constraining the strength of global aerosol indirect forcing. Observations of summertime SO clouds and aerosols in synoptically varied conditions during the 2018 SOCRATES aircraft campaign reveal novel mechanisms influencing pristine aerosol-cloud interactions. The SO free troposphere (3-6 km) is characterized by widespread, frequent new particle formation events contributing to much larger concentrations (≥ 1000 mg-1) of condensation nuclei (diameters > 0.01 μm) than in typical sub-tropical regions. Synoptic-scale uplift in warm conveyor belts and sub-polar vortices lifts marine biogenic sulfur-containing gases to free-tropospheric environments favorable for generating Aitken-mode aerosol particles (0.01-0.1 μm). Free-tropospheric Aitken particles subside into the boundary layer, where they grow in size to dominate the sulfur-based cloud condensation nuclei (CCN) driving SO cloud droplet number concentrations (Nd ~ 60-100 cm-3). Evidence is presented for a hypothesized Aitken-buffering mechanism which maintains persistently high summertime SO Nd against precipitation removal through CCN replenishment from activation and growth of boundary layer Aitken particles. Nudged hindcasts from the Community Atmosphere Model (CAM6) are found to underpredict Aitken and accumulation mode aerosols and Nd, impacting summertime cloud brightness and aerosol-cloud interactions and indicating incomplete representations of aerosol mechanisms associated with ocean biology.

Climate models struggle to accurately represent the highly reflective boundary layer clouds overlying the remote and stormy Southern Ocean. We use in-situ aircraft observations from the Southern Ocean Clouds, Radiation and Aerosol Transport Experimental Study (SOCRATES) to evaluate Southern Ocean clouds in a cloud-resolving large-eddy simulation (LES) and two coarse resolution global atmospheric models, the CESM Community Atmosphere Model (CAM6) and the GFDL global atmosphere model (AM4), run in a nudged hindcast framework. We develop six case studies from SOCRATES data which span the range of observed cloud and boundary layer properties. For each case, the LES is run once forced purely using reanalysis data (‘ERA5-based’) and once strongly nudged to an aircraft profile (‘Obs-based’). The ERA5-based LES can be compared with the global models, which are also nudged to reanalysis data, and is better for simulating cumulus. The Obs-based LES closely matches an observed cloud profile and is useful for microphysical comparisons and sensitivity tests, and simulating multi-layer stratiform clouds. We use two-moment Morrison microphysics in the LES and find that it simulates too few frozen particles in clouds occurring within the Hallett-Mossop temperature range. We modify the Hallett-Mossop parameterization so that it activates within boundary layer clouds and we achieve better agreement between observed and simulated microphysics. The nudged GCMs achieve reasonable supercooled liquid water dominated clouds in most cases but struggle to represent multi-layer stratiform clouds and to maintain liquid water in cumulus clouds. CAM6 has low droplet concentrations in all cases and underestimates stratiform cloud-driven turbulence.

Southern Ocean (SO) clouds are critical for climate prediction. Yet, previous global climate models failed to accurately represent cloud phase distributions in this observation-sparse region. In this study, data from the Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES) experiment is compared to constrained simulations from a global climate model (the Community Atmosphere Model, CAM). Nudged versions of CAM are found to reproduce many of the features of detailed in-situ observations, such as cloud location, cloud phase and boundary layer structure. The simulation in the latest versions of the model has improved its representation of SO clouds with adjustments to the ice nucleation and cloud microphysics schemes that permit more supercooled liquid. Initial comparisons between modeled and observed hydrometeor size distributions suggest that the modeled hydrometeor size distributions are close to observed distributions, which is remarkable given the scale difference between model and observations. Comparison to satellite observations of cloud physics is difficult due to model assumptions that do not match retrieval assumptions. Some biases in the model’s representation of SO clouds and aerosols remain, but the detailed cloud physical parameterization provides a basis for process level improvement and direct comparisons to observations. This is critical because cloud feedbacks and climate sensitivity are sensitive to the representation of Southern Ocean clouds.

The COVID19 pandemic led to dramatic changes in economic activity in 2020. We use estimates of emissions changes for 2020 in two Earth System Models (ESMs) to simulate the impacts of the COVID19 economic changes. Ensembles of nudged simulations are used to separate small signals from meteorological variability. Reductions in aerosol and precursor emissions, chiefly Black Carbon (BC) and sulfate (SO$_4$), led to reductions in total anthropogenic aerosol cooling through aerosol-cloud interactions. The average overall Effective Radiative Forcing (ERF) peaks at +0.29$\pm$0.15 Wm$^{-2}$ in spring 2020. Changes in cloud properties are smaller than observed changes during 2020. Impacts of these changes on regional land surface temperature range up to +0.3K. The peak impact of these aerosol changes on global surface temperature is very small (+0.03K). However, the aerosol changes are the largest contribution to COVID19 emissions induced radiative forcing and temperature changes, dominating ozone, CO$_2$ and contrail effects.

The widespread presence of meteoric smoke particles (MSPs) within a distinct class of stratospheric aerosol particles has become clear from in-situ measurements in the Arctic, Antarctic and at mid-latitudes. We apply an adapted version of the interactive stratosphere aerosol configuration of the composition-climate model UM-UKCA, to predict the global distribution of meteoric-sulphuric particles nucleated heterogeneously on MSP cores. We compare the UM-UKCA results to new MSP-sulphuric simulations with the European stratosphere-troposphere chemistry-aerosol modelling system IFS-CB05-BASCOE-GLOMAP. The simulations show a strong seasonal cycle in meteoric-sulphuric particle abundance results from the winter-time source of MSPs transported down into the stratosphere in the polar vortex. Coagulation during downward transport sees high latitude MSP concentrations reduce from ~500 per cm3 at 40km to ~20 per cm3 at 25km, the uppermost extent of the stratospheric aerosol particle layer (the Junge layer). Once within the Junge layer’s supersaturated environment, meteoric-sulphuric particles form readily on the MSP cores, growing to 50-70nm dry-diameter (Dp) at 20-25km. Further inter-particle coagulation between these non-volatile particles reduces their number to 1-5 per cc at 15-20km, particle sizes there larger, at Dp ~100nm. The model predicts meteoric-sulphurics in high-latitude winter comprise >90% of Dp > 10nm particles above 25km, reducing to ~40% at 20km, and ~10% at 15km. These non-volatile particle fractions are slightly less than measured from high-altitude aircraft in the lowermost Arctic stratosphere (Curtius et al., 2005; Weigel et al., 2014), and consistent with mid-latitude aircraft measurements of lower stratospheric aerosol composition (Murphy et al., 1998), total particle concentrations also matching in-situ balloon measurements from Wyoming (Campbell and Deshler, 2014). The MSP-sulphuric interactions also improve agreement with SAGE-II observed stratospheric aerosol extinction in the quiescent 1998-2002 period. Simulations with a factor-8-elevated MSP input form more Dp>10nm meteoric-sulphurics, but the increased number sees fewer growing to Dp ~100nm, the increased MSPs reducing the stratospheric aerosol layer’s light extinction.

Measurements from the Solar Occultation For Ice Experiment (SOFIE) are used to characterize meteoric smoke and meteor influx in both hemispheres. New smoke extinction retrievals from sunrise measurements in the Northern Hemisphere (NH) are presented, which complement the previously reported sunset observations in the Southern Hemisphere (SH). The sunrise observations are in good agreement with simulations from the Whole Atmosphere Community Climate Model (WACCM), for both the seasonal and height dependence of smoke in the mesosphere. The SOFIE - WACCM comparisons assumed that smoke in the mesosphere exists purely as Fe-rich olivine. This is justified because olivine is detected optically by SOFIE, it has the same elemental abundance as incoming meteoroids, and it is anticipated by theory and laboratory experiments. Treating mesospheric smoke as olivine furthermore brings closure in terms of the ablated and total meteoric influx determined here from SOFIE and a recent and independent investigation based on models and observations. SOFIE observations from 2007 - 2021 indicate a global ablated meteoric influx of 7.3 +/- 2.0 metric tons per day (t/d), which corresponds to a total influx (ablated plus surviving material) of 25.0 +/- 7.0 t/d. Finally, SOFIE indicates less smoke in the polar winter SH compared to NH winter. Finally, the results indicate stronger descent in the NH polar winter mesosphere than in the SH winter. This hemispheric asymmetry is indicated by smoke and water vapor results from both SOFIE and WACCM.

This study uses cloud and radiative properties collected from in-situ and remote sensing instruments during two coordinated campaigns over the Southern Ocean between Tasmania and Antarctica in January-February 2018 to evaluate the simulations of clouds and precipitation in nudged-meteorology simulations with the CAM6 and AM4 global climate models sampled at the times and locations of the observations. Fifteen SOCRATES research flights sampled cloud water content, cloud droplet number concentration, and particle size distributions in mixed-phase boundary-layer clouds at temperatures down to -25 C. The six-week CAPRICORN2 research cruise encountered all cloud regimes across the region. Data from vertically-pointing 94 GHz radars deployed was compared with radar-simulator output from both models. Satellite data was compared with simulated top-of-atmosphere (TOA) radiative fluxes. Both models simulate observed cloud properties fairly well within the variability of observations. Cloud base and top in both models are generally biased low. CAM6 overestimates cloud occurrence and optical thickness while cloud droplet number concentrations are biased low, leading to excessive TOA reflected shortwave radiation. In general, low clouds in CAM6 precipitate at the same frequency but are more homogeneous compared to observations. Deep clouds are better simulated but produce snow too frequently. AM4 underestimates cloud occurrence but overestimates cloud optical thickness even more than CAM6, causing excessive outgoing longwave radiation fluxes but comparable reflected shortwave radiation. AM4 cloud droplet number concentrations match observations better than CAM6. Precipitating low and deep clouds in AM4 have too little snow. Further investigation of these microphysical biases is needed for both models.