PNNL

Abstract

The E3SMv2 model, like various other global climate models that include representations of aerosol-cloud inter- actions, uses an empirically chosen lower bound on the simulated in-cloud cloud droplet number concentration (CDNC) to help constrain the effective radiative forcing of anthropogenic aerosols, ERFaer. This study identifies where ultra-low CDNCs (i.e., in-cloud values lower than 10 cm-3) occur in E3SMv2 simulations and which of the occurrences have 10 the strongest impact on ERFaer. Process-level analyses are presented to reveal characteristics of the cloud droplet formation, transport, and removal processes associated with impactful ultra-low CDNCs. Simulations performed with present-day emissions show 15 that ultra-low CDNCs occur most frequently over the mid- and high-latitude oceans in both hemispheres, while the occurrences are also frequent in polluted continental regions despite the high aerosol concentrations. Sensitivity experiments reveal that ultra-low CDNCs in the lower troposphere 20 (with air pressure higher than 600 hPa) are the main contributors to the sensitivity of E3SMv2’s global mean ERFaer to the lower bound of CDNC. The Northern Hemisphere middle latitudes and the Arctic region show the largest absolute changes of annual mean ERFaer when the lower bound is imposed, while the Arctic region features the largest relative changes in the annual mean ERFaer. Ultra-low CDNCs that strongly affect the global mean ERFaer are typically associated with cloud fractions larger than 0.9 and are found in the main body of large-scale cloud systems. Such cases occur under strong water vapor condensation and weak turbulence, and the statistical distributions of sub-grid vertical velocity feature relatively small skewness, suggesting the clouds are likely low-level stratus. A comparison of cloud droplet number budgets associated with different CDNC ranges suggests that the ultra-low CDNCs 35 are caused by weak sources, especially lack of aerosol activation, rather than strong sinks. Composite mean cloud condensation nuclei concentrations diagnosed at 0.1% supersaturation suggest that in southeast China, the simulated cloud droplet nucleation process is likely the main culprit of ultra-low CDNCs, while in the North Pacific storm track, the Arctic region, and southeast United states, cloud droplet nucleation and aerosol concentrations are both possible contributors to the simulated ultra-low CDNCs. Sensitivity experiments show that in lower-troposphere grid boxes with large 45 cloud fractions and weak turbulence, boosting the grid-box mean characteristic updraft velocity used in the aerosol activation parameterization or enhancing the turbulent mixing of cloud droplet number can increase the simulated CDNC but also undesirably increase the magnitude of the global 50 mean ERFaer. Overall, our study suggests that mid- and high- latitude low-level stratus occurring under weak turbulence is a cloud regime worth further investigating for the purpose of identifying and addressing the root causes of ultra-low CDNCs and strong ERFaer in E3SM.