Mechanisms of Tropical Cyclone Intensity Change

H. E. Willoughby, HRD/AOML/NOAA, Miami, FL

Tropical Cyclones' intensity, structure, and motion reflect a complicated interplay between internal dynamics and forcing by oceanic and atmospheric surroundings. One school of thought holds that on any day in late summer, the energy source in the tropical ocean can support intensities comparable with the most intense tropical cyclones observed. Another school believes that variations in the thermal structure of the surrounding atmosphere, departures of oceanic conditions from the climatological mean, and cooling due to the storms' action on the sea combine to make thermodynamics a limitation in many cases. Regardless of which hypothesis is true, meteorological factors must often inhibit intensification.

The most important factor is vertical shear of the environmental wind. Historically, shear was thought to "ventilate" the core of the cyclone by advecting the warm anomaly away. Recent analysis suggests, with substantial observational support, that the effect of shear is to force the convection into an asymmetric pattern such that the convective latent heat release forces flow asymmetry and irregular motion rather than intensification of the symmetric vortex.

In relatively weak shear, the convection tends to be organized into rings --for example, the eyewall-- coincident with local maxima of the swirling wind. Convective updrafts with roots in the energetic boundary layer entrain mass from midlevels and lift it to the tropopause. Horizontal convergence to replace this mass concentrates angular momentum at the ring causing the maximum wind to increase and the radius of maximum wind to contract. When two concentric rings are present, the outermost intercepts the energetic boundary-layer inflow, contracts and strangles the inner, often causing a dramatic weakening of the cyclone through "eyewall replacement".

The most intense tropical cyclones reach extreme wind speeds and low pressures not by long lasting, gradual intensification, but by the process of "rapid deepening" which may take the cyclone from 50 m s^-1 maximum winds to 80 m s^-1 and a minimum sea-level pressure near the theoretical limit for the prevailing sea-surface temperature in a day or two. Often rapid deepening follows interaction between the cyclone and a mid latitude trough or an upper-level low. There are two, not necessarily exclusive, plausible mechanisms for the interaction. The first is that superposition of the potential vorticity (PV) due to the trough and that due to the cyclone augment each other or that part of the trough's PV becomes incorporated into the cyclone. The second is that the deformation field associated with the trough induces waves on the cyclone that import angular momentum which spins up the upper part of the vortex, causes axisymmetric outflow, and destabilizes the convection in the cyclone core. Sometimes, rapid deepening occurs when the cyclone moves over warm oceanic features with mixed layers so deep that the oceanic heat source is relatively immune to cyclone-induced cooling. Often, episodes of rapid deepening end with an eyewall replacement.

This, then, is the present understanding of tropical cyclone intensity change. With the advent of new observational tools--most notably airborne Doppler radar, GPS-based dropsondes with improved thermodynamic sensors, and routine access to the upper troposphere aboard NOAA's new Gulfstream IV jet--it should be possible to refine this understanding into a quantitative prediction capability.