Large-Scale Responses to Induced Organization Gradients in a Cloud Model

Abstract To illuminate the nature of convective organization, a competition for uniformly supplied destabilization is staged in a nonrotating cyclic cloud model. Scattered convection in a control-run equilibrium is driven to aggregate in a belt covering half the domain by prescribing vertically sheared wind or warm-rain process denial. The equilibrium response is a belt-scale ascent which transports moisture toward the perturbed area. Larger and longer-lived rainstorms occur, whose competitive success earns the title of “organized.” Such success is revealed by the resulting domainwise circulations and suppressed rainfall over the unperturbed areas. Although shear is adverse to columnar updrafts, it has beneficial effects later, fostering wider (less entraining) updrafts and perhaps another kinetic energy source besides buoyancy. Warm-rain process denial (more purely adverse to columnar precipitation processes) reveals similar responses in equilibrant circulations and precipitation as the shear experiments, overall suggesting a “hardship breeds hardiness” interpretation. Sudden denial in a belt illustrates the multi-time-scale succession: cloud water builds up (few hours), a pulse of rain (hours 5–10) initiates the belt-scale circulation, and some continuing refinement (up to 20 h) warms the upper levels everywhere, reducing all updrafts’ buoyancy. Parameterization of organization in terms of such controlling factors is discussed. Significance Statement Deep convective “organization,” in the important sense of process efficiency, is shown to be related to cloud-pattern aggregation induced by vertical wind shear or microphysical process changes. Understanding the equilibrium relationship may require ecology-like logic about population succession: An adverse condition weakens existing entities within their life cycle (about an hour) but fosters development of more efficient entities, which end up dominating after a natural selection process an order of magnitude slower.