On the interactions between clouds and atmospheric circulation in the tropics and midlatitudes
Wall, Casey James
MetadataShow full item record
Cloud radiative feedbacks are the largest source of uncertainty in climate projections. It has been shown that narrowing this uncertainty will require a better understanding of the two-way interactions between clouds and the atmospheric circulation. Understanding cloud-circulation interactions and constraining cloud-climate feedbacks are therefore important and urgent goals in climate research. These topics are investigated in this thesis. In Part A, satellite observations are used to study the interactions between clouds and atmospheric circulation over the Southern Ocean. Atmospheric motions modify the boundary-layer stratification, inversion strength, and large-scale vertical motion, and in doing so, modulate clouds. Surface heat fluxes also significantly modulate shallow clouds. The ability of climate models to simulate these processes is investigated. Climate models consistently struggle to accurately represent shallow clouds in subfreezing environments. The implications of these model biases for uncertainty in cloud-climate feedbacks is discussed. In Part B, cloud-circulation interactions are investigated over the warm and convective tropical oceans. In these regions, the average shortwave and longwave cloud radiative effects are individually large but nearly cancel at the top of the atmosphere. It has been hypothesized that this cancellation is caused by two-way interactions between clouds, atmospheric circulation, and sea surface temperature (SST). This hypothesis is investigated using a variety of satellite observations and climate model simulations. First, observations from polar-orbiting satellites are used to investigate the relationships between large-scale circulation and cloud properties. Next, a cloud-tracking algorithm is applied to geostationary satellite observations and is used to study the evolution of clouds, the ambient environment, and the underlying SST over the life cycle of convective storms. Finally, a climate model in global radiative-convective equilibrium configuration is used to investigate the impact of cloud-circulation-SST interactions on tropical climate. All three studies are consistent with the hypothesis that cloud-circulation-SST interactions cause the close balance in cloud radiative effects over warm tropical oceans. It is hypothesized that this mechanism could constrain cloud-climate feedbacks in the warm and convective tropics.
- Atmospheric sciences