Convection, Clouds and Sea Surface Temperature

University Of Washington, Seattle WA

Investigators

Abstract

Clouds have competing effects on Earth's energy budget, as clouds warm the Earth by trapping outgoing infrared radiation but they also cool by reflecting sunlight back to space. For deep convective clouds in the tropics the balance between these competing effect is very close, so that their net radiative effect is near zero. The reasons for the close cancellation are not known, and it is possible that global warming may tip the net cloud radiative effect to favor warming or cooling, enhancing or dimishing the warming effect of greenhouse gas increases. Work under this award examines the balance between cloud heating and cooling with a focus on differences in cloud radiative effects at different stages of the cloud lifecycle, and on the relationship between tropical clouds and sea surface temperatures (SSTs). The work on cloud lifecycle effects is motivated by the PI's recent examination of satellite data suggesting that the high ice clouds accompanying deep tropical convection are self-sustaining through an interaction between radiation and turbulence. The near-zero radiative effect of convective clouds would then result from a combination of the net cooling effect of the high ice clouds and the net warming effect of the deep precipitating clouds that generate them. The work on SST-cloud interactions is motivated by the relationship between SSTs and the tropospheric overturning circulations that connect them to both deep and shallow clouds. Preliminary results show an unexpected increase in the strength of the overturning circulations as SSTs warm, and the project explores the reasons for the strengthening and its implications for cloud cover. The work is conducted through analysis of observational data from satellites and other sources and idealized modeling experiments. The work has broader impacts due to the role of cloud radiative effects in determining the amount of warming that will occur as a result of increases in greenhouse gas concentrations. The project also considers the effect of greenhouse warming on tropical SST contrasts, an important consideration as the regional impacts of tropical climate change are strongly influenced by SST contrasts. The project also supports two graduate students and contribute to the development of educational materials including a third edition of the Principal Investigator's textbook on global physical climatology. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.