The role of cloud-cloud interactions and entrainment-mixing in the lifecycle of shallow cumulus clouds

Limited understanding of the key factors that govern the lifecycle of cumulus clouds, including the interactions among clouds and with surrounding environments, contributes to climate prediction uncertainty. To investigate these processes, we tracked the lifecycle of thousands of individual shallow cumulus clouds within a large-eddy simulation during the Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HISCALE) field campaign in the U.S. Southern Great Plains.

Our examination of these clouds followed two paths. First, we compared two distinct groups of clouds—those with growing cloud neighbors and those with decaying cloud neighbors. Clouds with growing neighbors were found to form over areas with larger surface heterogeneity than clouds with decaying neighbors. Clouds with growing neighbors also had less instability, less moisture and warmer air below cloud base than decaying neighbor clouds. This suggests that evaporation below the cloud base likely occurs before the formation of these clouds with decaying neighbor clouds due to the colder and moister air below cloud base. Larger instability leads to higher vertical velocity and convergence within the cloud, which causes stronger downdrafts and water vapor removal in the surrounding area. The latter appears to be the reason for the decaying neighboring clouds.

Second, we introduced two new metrics to assess the relationships between cloud shape and these processes: one reflecting the irregularity of cloud edges and another emphasizing the cloud horizontal aspect ratio. During the lifecycle of simulated cumulus clouds, cloud edge irregularity increased with minimal changes in aspect ratio. Irregularity-driven growth of the cloud perimeter was a strong indicator of cloud splitting, more so than growth driven by aspect ratio changes. Additionally, clouds with more irregular edges exhibited smaller gradients of properties at their boundaries, suggesting more intense mixing with the surrounding cloud-free environment.

These results advance insights into the interactions between cumulus clouds and their nearby environment entrainment that influence the evolution of cloud populations. Such knowledge can support the development of more accurate shallow cumulus parameterizations in the new generation of climate models.