Response in Clouds and Atmospheric Circulations Simulated by a Multiscale Modeling Framework to Rapid Adjustment induced by CO2 Increase

The increase of CO₂ concentration induces inhomogeneous surface and atmospheric heating over land and ocean, causing rapid adjustments that alter the cloud formation, Earth’s radiative energy budget, hydrological cycle, and atmospheric circulation in a time scale of days to weeks, and long-term response in air temperature warming. The rapid adjustment is an important component in climate change, its mechanism and quantitative assessment is an engaging topic for current climate research. In this study, the rapid adjustment is assessed from a sensitivity experiment that fixes sea surface temperature and doubles CO₂ concentration in a Multi-scale Modeling Framework (MMF) with a third-order turbulence closure in its cloud-system resolving model (CRM) component. This model replaces all cloud parameterizations in a conventional global climate model with two-dimensional CRMs. The sensitivity experiment is compared to the control experiment with the standard CO₂ concentration. It produces more enhanced radiative heating at 700 hPa and the surface than a similar MMF without the advanced turbulence closure, resulting in destabilization and more clouds in the low troposphere, as well as stabilization and fewer clouds in the middle troposphere. Although the cloud fraction increases over most land regions at all vertical levels as anticipated for the circulation change due to stronger land surface heating, the stronger subsidence caused by such circulations is only significant for the stratocumulus regimes off the west coasts of the continents with climatological stable low tropospheric stratification and strong subsidence. The reason for the differences between the two MMFs is that stratocumulus clouds and their response to stronger subsidence are properly simulated in the MMF with the advanced turbulence closure. Detailed analysis in terms of the large-scale circulation and stability regimes and mechanisms for the rapid adjustment will be presented at the meeting.