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

Description

The increase of CO2 concentration induces inhomogeneous surface and atmospheric radiative heating over land and ocean, causing rapid adjustments that alter the surface and troposphere temperature, cloud formation, Earth’s radiative energy budget, hydrological cycle, and atmospheric circulation in a time scale of days to weeks. 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, which replaces all cloud parameterizations in a conventional global climate model. The sensitivity experiment is compared to the control experiment with the standard CO2 concentration. The increase of low cloud fraction due to doubled CO2 concentration is significantly larger in this study than a similar MMF without the advanced turbulence closure. Such enhancement of low cloud fraction is highlighted in the subtropical regimes under strong large-scale subsidence, where the persistent stratocumulus clouds are more sensitive to the increased lower-tropospheric stability and boundary-layer moistening than other types of maritime low clouds. The reason for the differences between the two MMFs is that stratocumulus clouds are better simulated in the MMF with the advanced turbulence closure. Detailed analysis on changes of clouds due to the atmospheric dynamics associated with the rapid adjustment are presented.