Tropical cyclogenesis: effects of model resolution and hypohydrostatic rescaling

Tropical cyclogenesis is examined using the hypohydrostatic, or Reduced Acceleration in the VErtical (RAVE), rescaling in a model that explicitly represents moist convection. 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 non-rotating 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 slowing the group velocity of gravity waves and thus reducing 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, consistent with the idea that a dry bias inhibits cyclogenesis. 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 in models with horizontal resolutions of O(10-100 km).