Impacts of initial vortex size and planetary vorticity on tropical cyclone size

Abstract

This is a numerical modelling study to understand how the initial vortex size, which is defined as the azimuthally-averaged radius from the tropical cyclone (TC) centre of the 10-m 17 m s-1 wind, and planetary vorticity (f) influence TC size change. Results from 16 f-plane experiments in a quiescent environment suggest that both of them are important in determining TC size change. With a given initial intensity and on the same f plane, an initially larger TC generally has a larger size at a later stage because it has a larger horizontal wind extent and higher winds outside the inner core. The larger vortex therefore possesses higher angular momentum (AM) in the lower troposphere to increase its size in the outer-core region through AM transport. However, an initially small TC may not be "destined" to be small during its lifetime, which agrees with the observation that TC size has a positive relationship with TC lifetime. In addition, a vortex can apparently grow by itself in a resting environment through fluxes of AM. A vortex at a higher latitude is also found to be not necessarily larger. Furthermore, size change is controlled to some extent by the lower-tropospheric inertial stability associated with the vortex. Consistent with observations, TC size appears to have a maximum at some optimum latitudinal region (~25°N in general). All the results agree well with the AM transport concept such that the outer-core area-integrated symmetric relative AM flux and Coriolis torque in the lower troposphere (especially those at the boundary layer) are important factors that govern size change.