Observing aerosol impacts on Arctic clouds

Abstract

Aerosol-cloud interactions represent a significant source of uncertainty in anthropogenic climate forcing, particularly in the Arctic where clouds critically influence the surface energy budget. The Arctic is undergoing rapid environmental transformations, and it is critical to understand how these changes affect cloud radiative properties to accurately project future regional climate shifts.

This thesis uses satellite data to develop constraints for aerosol-cloud interactions. Although satellites often struggle to obtain reliable measurements of Arctic clouds, this work shows that through careful filtering, the retrieval biases can be reduced. This work finds that the aerosol impacts on cloud liquid water path vary significantly across the region, predominantly controlled by lower tropospheric stability. As the stability is strongly influenced by the surface temperatures, this suggests a temperature-dependent aerosol effect. This could lead to a reduced cooling effect of clouds in a warmer, more polluted Arctic.

This work develops a novel method for creating pseudo-Lagrangian trajectories to study the temporal development of clouds as they move from the sea ice edge across the open ocean. By controlling for the initial conditions using reanalysis data, this work finds that the breakdown in the cloud field is strongly related to precipitation, which in turn is affected by the presence of aerosols through precipitation suppression.

Additionally, this thesis finds that the air mass history is strongly linked with the formation of ice along these trajectories. The predominance of mixed-phase clouds at temperatures as high as -13C suggests that the availability of biological ice nucleating particles plays a central role. Overall, this thesis highlights the need for a more detailed understanding of aerosol properties, sources and sinks in the region as a key target to improve our representation of Arctic clouds in climate models.