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Convective Self-Aggregation and Tropical Cyclogenesis under the Hypohydrostatic Rescaling

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The behavior of rotating and nonrotating aggregated convection is examined at various horizontal resolutions using the hypohydrostatic, or reduced acceleration in the vertical (RAVE), rescaling. This modification of the equations of motion reduces the scale separation between convective- and larger-scale motions, enabling the simultaneous and explicit representation of both types of flow in a single model without convective parameterization. Without the RAVE rescaling, a dry bias develops when simulations of nonrotating radiative-convective equilibrium are integrated at coarse resolution in domains large enough to permit convective self-aggregation. The rescaling reduces this dry bias, and here it is suggested that the rescaling moistens the troposphere by weakening the amplitude and slowing the group velocity of gravity waves, thus reducing the subsidence drying around aggregated convection. Separate simulations of rotating radiative-convective equilibrium exhibit tropical cyclogenesis; as horizontal resolution is coarsened without the rescaling, the resulting storms intensify more slowly and achieve lower peak intensities. At a given horizontal resolution, using RAVE increases peak storm intensity and reduces the time needed for tropical cyclogenesis-effects here suggested to be caused at least in part by the environmental moistening produced by RAVE. Consequently, the RAVE rescaling has the potential to improve simulations of tropical cyclones and other aggregated convection in models with horizontal resolutions of order 10-100 km.

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