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要旨

Self-aggregation in numerical simulations of tropical convection is described by an upscale growth and intensification of dry and moist regions. Previous work has focused on determining the relevant mechanism that induces moist regions to get moister and dry regions to get drier. Though different mechanisms have been identified, the spatial evolution of self-aggregation is remarkably universal. The first part of this study shows that different mechanisms can lead to a similar evolution of self-aggregation, if self-aggregation is described by a phase separation of moist and dry regions, through a process called coarsening. Though it was previously introduced based on a convection-humidity feedback, coarsening, importantly, is not tied to a specific feedback process but only requires an intensification of local humidity perturbations. Based on different feedback loops, three simple models of the evolution of the humidity field are introduced, all of which lead to coarsening. In each model, diffusive transport of humidity is assumed, which approximates a humidity increase due to convection, within a finite region around convective cores. In the second part, predictions made by coarsening are compared with atmospheric model simulations. Analyzing a set of radiative-convective equilibrium simulations shows that coarsening correctly predicts the upscale growth of the moist and dry regions in the early stages of self-aggregation. In addition, coarsening can explain why self-aggregation is not observed for small domains and why the shape of the final moist region changes with the shape of the domain.