Western north Pacific typhoons with concentric eyewalls

In the past decade, the availability of satellite passive microwave data made it possible to reveal the concentric structure in typhoons that had previously been difficult with only the visible and infrared images. The eyewall replacement cycles in the concentric eyewall typhoons are very important to the accurate forecasts of rapid intensity change. This study examines the characteristics of typhoons with concentric eyewalls in the western North Pacific between 1997 and 2006. Approximately 24% of the typhoon cases examined possessed the concentric structure. A majority (83%) of these typhoons have strong intensity (categories 4 and 5 in the Saffir-Simpson Scale). In particular, there are 56% of category 4 and 71% of category 5 possessed the concentric structure among all strong typhoons examined.

The formation of the moat - the space between the inner and outer eyewalls - is a key feature for the concentric eyewalls. It is generally recognized that its mechanism is heavily influenced by the subsidence forced by the two eyewalls. On the other hand, the strain flow and the horizontal wind shear outside of the core vortex can also contribute to the concentric eyewall formation. Therefore a comparison of moat characteristics between the theory and observations can be used to assess the importance of the advective vorticity dynamics. Here we devise a theoretical parameter that is the moat width implied in the rapid filamentation dynamics process discussed by Rozoff et al, and, for each concentric eyewall typhoon, compute its value from the best track data maximum wind speed and the satellite estimated inner eyewall radius. The resultant “filamentation moat width”, regardless of the vortex outskirt wind structures, explains 40% (19%) of the variance of the satellite observed moat width for category 5 (4) typhoon. No significance variance is explained for the weaker typhoons. Thus, the mechanism of moat formation through rapid filamentation dynamics may be important in strong typhoons.

The evolution of intensity leading to and after the concentric eyewall formation is studied by comparing the composites of the formation cases and the non-formation cases. The intensity time series in both the concentric and non-concentric composites appear as an inverted “V” with the peak occurring at the time of concentric eyewalls formation and that of maximum intensity, respectively. The time series of intensity in the non-concentric composites agree fairly well with the composite results of for all tropical cyclones in the western North Pacific, and also in the North Atlantics, as reported by Emanuel. On the other hand, the intensity tendencies of the concentric typhoon composites are markedly different during the intensification phase. The concentric composites have a much slower intensification 12 h before the peak intensity (time of formation of concentric eyewalls) than that of the non-concentric composites. For Categories 4 and 5 the peak intensity of the concentric composite is comparable to that of the non concentric composite. However, 60 h before reaching the peak the concentric composites are 25% more intense than the non-concentric composites. So the key feature of the concentric formation appears to be the maintenance of a relatively high intensity for a longer duration, rather than a rapid intensification process that can reach a higher intensity. The intensity tendencies of both composites are similar in the weakening phase.