A Dynamical Perspective on Inner Eyewall Dissipation in Hurricane Wilma (2005)

In recent decades, with the aid of satellite and radar observations of tropical cyclone (TC), it is found that concentric eyewalls and the corresponding eyewall replacement cycle (ERC) are not uncommon. Generally, ERC leads to significant changes of TC structure, intensity and track. These changes may eventually affect the accuracy of the forecast and make the prior disaster preparedness less useful. Hence, advancing our understanding of ERC can minimise the influence of weather disaster by shedding light on what we should expect.

The dissipation of inner eyewall is an interesting stage of ERC which is not well-understood. After the outer eyewall formation, the inner eyewall gradually weakens and dissipates. Hence, it has been proposed in the literature that running out of fuel of the inner eyewall, due to the cut-off of momentum and diabatic heating fluxes by the outer eyewall, is the explanation. However, this proposed mechanism implies that an inner eyewall should die out within a certain time limit but it fails to explain the wide variety of the dissipation timing observed. The observation of Typhoon Usagi (2013) even shows that the coexistence of triple eyewalls was maintained for a long time, suggesting that the thermodynamics is not the only cause for the dissipation. Here, it is proposed that dynamical interaction between two eyewalls also plays a role. The results from a series of simple shallow water model experiments indicate that sufficient dynamical interaction between an inner vorticity monopole and an outer ring leads to the stretching and fission of the inner monopole. Analyses of the results from a hierarchy of models on the underlying physical processes will be discussed.