Turbulence and Liquid Water Patterns in Simulated Small Cumulus Clouds

Scientists have observed for over 50 years that rain forms in warm cumulus clouds far more quickly than simple calculations would suggest. A number of hypotheses have been put forth that might explain why simple calculations fail to explain the observations, and one of these is that these calculations fail to include the enhancement of water droplet collisions by turbulence.

Several recent studies have shown that under moderate and extreme values of turbulent kinetic energy, drop collisions are greatly increased (by a factor of 10 or more) in cumulus clouds. However, these studies must often hold the values of turbulent kinetic energy and cloud liquid water content constant because of computational constraints. It is clearly evident that these variables are not constant within a growing cumulus cloud.

This study will identify patterns of variability in turbulent kinetic energy and liquid water content in cumulus clouds, on larger scales within a cloud through which a given droplet might travel. Using numerical simulations of small, warm cumulus clouds and newly developed visualization tools, trends in turbulent kinetic energy and liquid water content are investigated as the cloud grows. Droplet trajectories through the simulated cloud are used to understand variations in turbulent kinetic energy and liquid water content along different paths through the cloud. These results can be used to help constrain direct numerical simulations (DNS) that investigate the importance of turbulence to droplet collisions in warm cumulus clouds.