Tropical-cyclone flow asymmetries induced by a uniform flow revisited

The dynamics of tropical cyclone intensification in a uniform flow is revisited as a first step towards an improved understanding of the asymmetric and symmetric spin-up dynamics within an environmental flow with deep vertical-shear. Complementing an earlier study by Thomsen et al., a comparison of the simulated updraft structure with that predicted by Shapiro’s asymmetric boundary layer theory for a translating vortex suggests strongly that, while the surface moisture fluxes act to maintain deep convection in the near-core region of the moving vortex, the asymmetric pattern of this convection is primarily frictionally controlled rather than by surface enthalpy fluxes. An analysis of three idealized numerical simulations with and without a uniform flow has revealed a number of interesting and important differences in the evolution of the simulated vortex. Several analysis techniques are presented and interpreted including a comparison of ventilation diagnostics for the simulations, the role of eddy processes, and a novel sector partitioning method that serves to extend the classical axisymmetric spin-up ideas to an intrinsically asymmetric flow. The latter analysis highlights asymmetric features of the spin-up process in which the storm-relative tangential velocity is amplified in the main updraught sector by the strong inward flux of absolute vorticity in the lower troposphere there. This vorticity is subsequently transported into the remaining sectors by the tangential circulation, where there is a weak outward vorticity flux that alone would lead to spin down. This geometrical feature broadly explains why there is a weaker spin up rate in the presence of storm translation.