Tropical cyclogenesis under the hypohydrostatic rescaling in climates with different equator-to-pole temperature gradients.

Abstract:

Simulations of strong tropical cyclones, category 3 to 5, in models without convective parameterization typically require a high horizontal resolution on the order of several kilometers, which is currently not feasible for global climate models. In this study we apply the hypohydrostatic rescaling, also referred to as RAVE (Reduced Acceleration in the Vertical, e.g. Kuang et al. 2005), as a means to improve the representation of tropical cyclones in a cloud-resolving model that explicitly represents moist convection (SAM). In effect, RAVE 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. At a given horizontal resolution, using RAVE increases peak storm intensity and reduces the time needed for tropical cyclogenesis, effects apparently caused by the environmental moistening produced by RAVE. In fact, using RAVE in this model within a global aqua-planet beta-plane configuration improves representation of both the mean tropical climate (eliminating the double-ITCZ problem) and of tropical cyclones (generating storms up to category 5).

Further, we present results of these simulations in which we vary the equator-to-pole temperature gradient and focus on the key characteristics of tropical cyclones, including the total number of cyclones, their genesis, strength and latitudinal distribution, and interaction with extra-tropical storms. We find that the tropical cyclone activity changes non-monotonically with reduction in the oceanic meridional temperature gradient – for small changes the cyclone activity decreases, but for larger changes it strongly increases, mainly because of the strong reduction in wind shear in mid-latitudes for weak meridional temperature gradients. The increase in tropical cyclone activity in warm climates is accompanied by the merging between the characteristics of extra-tropical storms and tropical cyclones, as seen in the paths of the storms as well as their property distributions. We discuss these results in the context of the past and future climate change.