Mean Cold Pool Size of Quasi-Equilibrium Convection. Part I: Why Do Cold Pools Collide?

Abstract Precipitation-driven cold pools play an important role in organizing tropical convection. Previous studies of tropical convection in the radiative–convective equilibrium (RCE) setup found that cold pools tend to collide with each other and trigger new convection. It remains unclear why most cold pools do not have enough space to dissipate without collision. We explain it as the smaller mean cold pool radius R eq compared to its maximum potential radius R max . The latter denotes the radius needed for a cold pool’s buoyancy deficit to be dissipated by surface heating. Applying an energy balance constraint leads to an analytical solution for their ratio R max / R eq , which depends on the Bowen ratio, surface precipitation–evaporation ratio, and rain sedimentation efficiency. The theory predicts that in the regime of marine tropical convection where the Bowen ratio is much smaller than one, R eq cannot reach R max , and cold pools must collide frequently. This prediction is supported by large-eddy simulations using varying rain evaporation rates. In Part II, we combine the energy balance constraint with a convective life cycle model to obtain a theory of the mean cold pool radius R eq .