The response of deep convection to solar geoengineering

Solar geoengineering proposes to reduce the surface warming caused by increased concentrations of greenhouse gases through a manipulation of the incoming solar radiation. The response of the climate system to various geoengineering scenarios has been investigated using Global Climate Models (Kravitz, Caldeira, et al. 2013, Kravitz, Rasch, et al. 2013), including the impacts on tropical precipitation, stability and radiative fluxes. However, similar simulations in high-resolution limited domain models are largely limited to shallow convection (Schneider, Kaul, and Pressel 2020).

This study focuses on the effect of solar geoengineering on tropical deep convection. We performed a set of idealized simulations in a cloud-resolving model (Khairoutdinov and Randall 2003) with increasing CO₂ concentrations. Solar geoengineering was represented simply by fixing the sea surface temperature to 300 K across all experiments. This setup resulted in the expected decrease in radiative cooling, and thus reduced evaporation and precipitation. A slight decrease in anvil cloud cover was observed, but cloud top temperature remained nearly constant, supporting the fixed anvil temperature (FAT) hypothesis (Hartmann and Larson 2002), more so than the tropopause temperature. The shortwave and longwave cloud radiative effects both decreased in magnitude due to the reduced cloud fraction. Additionally, the longwave heating effect was reduced further due to a lower effective emission temperature of the clear sky, resulting in a smaller difference between the radiation emitted by clear and cloudy sky.

These results partially agree with results from GCMs and they offer insight into how tropical clouds might respond to solar geoengineering. Plans for future work include a more realistic representation of solar geoengineering (e.g. interactive SST with a reduced solar constant) as well as improved high cloud microphysics.