The dynamics of the tropical cyclone core

The core of a tropical cyclone occupies the inner 100-200 km of the vortex. It is dominated by a cyclonic primary circulation in balance with a nearly axisymmetric, warm core low pressure anomaly. Superimposed on the primary circulation are weaker asymmetric motions and an axisymmetric secondary circulation. The asymmetries, which may be either internal gravity waves or Rossby waves, modulate precipitation and cloud into trailing spirals. The axisymmetric secondary circulation, driven by latent heat release and surface friction, comprises the following parts: surface inflow that extracts latent heat from the sea and replaces the frictional loss of angular momentum (M) to the sea; diabatically forced deep inflow that supplies an excess of M above frictional loss; the eyewall, an outward sloping locus of convective ascent; diabatically forced descent inside the eye; and upper tropospheric outflow. The eyewall usually moves inward as a result of differential adiabatic heating across the wind maximum. Eyewall succession occurs in intense cyclones when two concentric eyewalls are present and the outer replaces the inner. Because of their semibalanced dynamics, the primary and secondary circulations are relatively simple and well understood. These dynamics are not valid in the upper troposphere where the outflow is comparable to the swirling flow, nor do they apply to the asymmetric motions. Since the synoptic-scale environment appears to interact with the vortex core in the upper troposphere by means of the asymmetric motions, future research should emphasise this aspect of the tropical-cyclone dynamics.