Clouds play a central role in Earth's climate as they regulate both the amount of sunlight reaching the Earth and the amount of infrared radation the Earth emits to space. But the climatic effect of clouds is an aggregate effect over a bewildering variety of cloud types, structures, and sizes. In principle this aggregate effect can be calculated on a cloud-by-cloud basis, simulating all clouds in detail and then adding up their individual contributions. But such simulations are impractical and in any case do not provide a satisfactory explanation for cloud behaviors. An alternative approach is to look for ways in which the fine-scale properties of clouds are determined, at least approximately, by the bulk properties of the large-scale atmosphere. This approach has the advantage of focusing on cloud properties which are likely to be influenced by climate change and may act to amplify or moderate the amount of warming caused by greenhouse gas increases.
In recent work the PIs have developed a theory in which the stratification of the large-scale atmosphere exerts a thermodynamic constraint on the fine-scale complexity of tropical convective clouds. The theory assumes a quasi-equilibrium state in which convective potential energy generated by solar heating at ground level is dissipated by horizontal turbulent mixing between rising air in clouds and surrounding clear air, which has lower potential energy. Since the sides of a cloud constitute a boundary through which mixing occurs across a potential energy drop, the theory implies a relationship between the length of cloud perimeter on a surface of atmospheric potential energy (specifically saturated static energy) and the stability of the atmosphere to overturning motions at that level. In quasi-equilibrium this relationship further implies that the number of clouds with a given perimeter length is inversely proportional to that perimeter length. Also, the mass flux through the sides of a cloud is related to the potential energy difference between the cloud and its environment. This relationship can be used to show that clouds of high convective potential energy are exponentially more rare than clouds of lower potential energy (a form of the Boltzmann distribution).
Work under this award tests the theory's predictions regarding the perimeter and energy dependence of cloud occurrence using satellite observations and model simulations. Further work addresses the extent to which the relationship between cloud number and perimeter length can be used to infer the number of clouds with a given horizontal area (possibly following Korcak's law), as cloud area has a direct connection to the climatic effects of clouds. Finally, the implications of the theory for changes in tropical convective clouds in response to climate change will be explored. Atmospheric stability is expected to increase in a warmer climate, and the relationship between stability and perimeter length suggests that warming will increase cloud number and total perimeter. But some work is required to reconcile this result with other thermodynamically-based arguments for changes in tropical cloud cover with warming.
The work has societal relevance given concerns regarding the severity of climate change and its impacts on people and the environment. Clouds are perhaps the greatest source of uncertainty in efforts to determine the amount of warming produced by increases in greenhouse gas concentrations, and the relationships between fine-scale cloud properties and large-scale atmospheric conditions examined here could help to reduce this uncertainty. The project includes participants at the University of Lille and thus promotes international research collaboration. It also provides support and training to two graduate students and one undergraduate, thereby building the future workforce in this research area.
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.
