The Effects of Initial Vortex Size on Tropical Cyclogenesis and Intensification

Three high-resolution numerical simulations are carried out to investigate the effects of initial vortex size on tropical cyclogenesis and intensification, starting with a relatively weak initial vortex with a maximum tangential wind speed of 5 m/s, but located at different radii (50, 100 and 150 km). In all simulations, there is a progressive organization of convectively induced, cyclonic relative vorticity into a monopole structure. As the initial vortex size is increased, the organization occurs later. In addition, the size of the vorticity monopole, the sizes of the inner and outer core tangential wind circulations and the lifetime intensity of the vortex all become larger as the initial vortex size is increased.

A simple slab boundary-layer model shows that, the smaller the initial vortex, the upflow at the top of the boundary layer is stronger and located closer to the circulation centre. Even though the upflow is relatively weak (of the order of a few mm/s), it is sufficient to provide a location with low convective inhibition, where deep convection can focus and amplify the vertical vorticity locally. The results serve to underpin a frequently cited observation that, when environmental conditions are favourable, small storms tend to spin up much faster than larger ones.