Mean Cold Pool Size of Quasi-Equilibrium Convection. Part II: The Survival Competition Hypothesis

Abstract The cold pool is a crucial component of tropical convection. However, what controls the mean cold pool size remains unclear. This two-paper series presents a theory of the mean cold pool radius in idealized quasi-equilibrium convection ( R eq ). Part I derives an energy balance constraint between R eq and the maximum potential radius of a cold pool ( R max ), showing that R eq cannot reach R max . Cold pools must be densely packed and collide frequently. This Part II derives another constraint between R eq and R max based on a cold pool survival competition hypothesis. A convective life cycle model with various candidate cold pool sizes is built. The type of cold pool producing the most intense next-generation cold pool is hypothesized to survive and set the spacing between convective towers. The size of the dominant cold pool type is determined by the trade-off between the mechanical lifting effect that favors a smaller cold pool, the thermodynamic forcing effect that favors a bigger cold pool, and the cloud radius feedback that also favors a bigger cold pool. Combining the energy balance and survival competition constraints, we obtain a solution for R eq , which has an analytically tractable upper bound. The upper bound is set by the cold pool’s fractional entrainment rate and the free-tropospheric relative humidity: a lower fractional entrainment rate or a drier free troposphere raises the upper bound of R eq . The R eq predicted by the theory agrees with a set of large-eddy simulations with different rainwater evaporation rates.