Revisiting the Relationship Between Eyewall Contraction and Intensification

The convective ring model of tropical cyclone intensification (Shapiro and Willoughby 1982; Willoughby 1990) posits that intensification and contraction of the radius of maximum winds (RMW) occur simultaneously. In this paradigm, the secondary circulation induced by eyewall heating leads to both contraction and “spin-up”, through radial advection of absolute angular momentum (M). From a suite of idealized simulations with differing initial RMW, we find that in general, the large majority of contraction occurs prior to the large majority of intensification. This phenomenon was presented briefly in Stern and Nolan (2011), but is explored in greater detail here.

We find that the quasi-steady size of the RMW increases with its initial size, in agreement with other studies. But the more substantial effect is that the time it takes for the vortex to achieve a quasi-steady size increases with the initial size. On the other hand, there is no systematic variation with initial size of the time it takes for the vortex to achieve a quasi-steady intensity. As a result, the time lag between the occurrence of a quasi-steady size and a quasi-steady intensity decreases systematically with increasing initial size of the RMW.

In our simulations, we find that the convective ring model does not well describe the manner of intensification, as propagation of the RMW across gradients of M first occurs, followed by the intensification “in place” at a nearly fixed RMW. Contrary to what is sometimes implied, the RMW cannot be treated as a material surface, as the value of M at the RMW changes greatly with time. We compare the results of our simulations to flight-level observations, and to calculations of the vortex response to heating in a linear model. Finally, comparisons to some real-data simulations are also made.