PNNL

Abstract

Some of the fundamental puzzles of climate research are related to a limited understanding of the critical factors governing the organization and evolution of cumulus cloud fields. These factors include the forcings of shallow convection and subsequent entrainment-mixing. Here we tracked the life cycle of thousands of individual shallow cumulus clouds in a large-eddy simulation for a period during the Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HI-SCALE) field campaign in the U.S. Southern Great Plains. Concurrent evolution of the tracked clouds and their neighboring clouds are examined. Results show that, from Lagrangian perspective, the clouds in the growing stage generally grow faster than their neighboring clouds, and in the dissipation stage they dissipate faster than their neighboring clouds. Two groups of the tracked clouds with growing neighboring clouds and decaying neighboring clouds, respectively, show distinct characteristics in their life cycle. Clouds with growing neighboring clouds form above regions with surface heterogeneity, while clouds with decaying neighboring clouds are not associated with heterogeneous surfaces. Both subgroups have similar values of surface potential temperature. However, those with decaying neighboring clouds experience larger instability and a more humid boundary layer, indicating evaporation below the cloud-base is likely occurring before those clouds are formed. Larger instability leads to higher vertical velocity and convergence within the cloud, which causes stronger surrounding downdrafts and water vapor removal in the surrounding area. The latter is thought to be the reason for the decaying neighboring clouds. Understanding those processes provide insights into how cloud-cloud interactions could be represented in future cumulus parameterizations that predict the statistics of convective cloud populations.