Predator-prey dynamics in warm cloud precipitation

How much water will remain in a cloud after it rains? The answer to this simple question is key to determining the energetic balance of the atmosphere and is therefore important for weather and climate prediction. We develop a coarse-grained approach to answer this question, verifying it using detailed simulations of cloud dynamics. Namely, we consider the stage when there are two species, large (rain) and small (cloud) drops in the cloud. We demonstrate that the dynamics of these two populations can be described using a predator-prey model containing two processes: 1) Raindrops preying on cloud drops through collision-coalescence, thereby increasing their mass. 2) Raindrops being large enough to fall at a significant rate, removing predators from the system. We further show that there are two distinct temporal regimes: first only accretion of cloud drops by rain drops occurs, and only then does fall-out of rain drops become important. Using the leading order analytical solution for the two regimes, we are able to determine the final mass in order-of-magnitude and explain the observed trends with the effective predator-prey rates, which we extract directly from simulations. Finally, I will address the processes which determine these effective predator-prey rates.  

Our approach could be useful for parametrizations of clouds in large-scale models of the atmosphere, where incorporating the full dynamics within clouds remains a challenge.