Eye formation by dynamical adjustment of the tropical cyclone's inner core

Tropical cyclones exhibit eye formation in the later stages of their development. It is postulated that the existence of condensational heating within an eyewall at a finite radius is a prerequisite for eye formation. A consequence of the resulting pressure fall under the eyewall is a simultaneous drop of surface pressure at the cyclone center due to gradient wind imbalance. The imbalance generally arises in high pressure systems with a small radius like that of the eye and it induces radial outflow at the surface together with downwelling at the vortex axis. However, warming and drying in the eye cannot result from this mechanism. Further downwelling must be forced by radial inflow at the top of the troposphere. The additional inflow can be explained by geostrophic adjustment of the inner core at a scale much smaller than the Rossby radius of deformation. With these mechanisms the eye nearly stagnates, becomes dry and possesses a temperature similar to that of the eyewall. Eye rotation with further warming must be induced by radial momentum fluxes from the eyewall into the eye. These implications are supported by a simple one-dimensional column model for the eye and the axisymmetric tropical cyclone model HURMOD.