Retrograde waves in tropical cyclone inner-core

This study presents an extension of Kelvin’s vortex wave model for the inner-core region of tropical cyclone-like vortices. By considering a more suitable approximation for tropical cyclones (TCs) for which the horizontal scale of the TC inner-core is significantly larger than the TC vertical depth and taking into account the TC inherent baroclinicity, it is shown that there exists a retrograde wave mode in which the wave propagation is opposite to the mean tangential flow for the azimuthal wavenumber 1. While this result appears to be similar to that obtained in Kelvin’s wave model, the retrograde mode in the TC-like vortices depends critically on the TC baroclinicity that Kelvin’s model does not contain. Idealised simulations of a TC-like vortex using the full-physics Hurricane Weather Research and Forecasting (HWRF) model indeed capture existence of the retrograde waves inside the vortex inner-core at the high intensity limit. Despite the brief existence of this retrograde mode, the emergence of retrograde waves in the HWRF simulations is of significance, because they may be related to subsequent wave growth and formation of mesovortices that are often observed inside the core region of intense TCs.