PNNL

Abstract

Cloud lifecycles are influenced by interactions among clouds and mixing with cloud-free environments. Cloud shapes dictate the length of cloud boundaries, which affects these interactions. This study uses a large eddy simulation to examine the relationships between the shallow cumulus cloud shapes, processes at their boundaries, and lifecycles. To characterize the shape of clouds, two metrics are newly defined: one reflects the overall horizontal cloud shape, and the other defines the cloud edge irregularity. Simulated clouds experience increased irregularity, but little changes in aspect ratio. Irregularity-driven cloud perimeter growth strongly indicates cloud splitting, with a more pronounced effect than aspect ratio-driven perimeter growth. Smaller cloud-shell gradients of properties across cloud boundaries are associated with more irregular clouds, suggesting enhanced mixing of clouds with the cloud-free environment. These findings indicate that the shallow cumulus evolution is partly driven by the irregularity of the cloud edge through lateral mixing and cloud splitting.