Western North Pacific Typhoons with Concentric Eyewalls

Abstract

This study examines the intensity change and moat dynamics of typhoons with concentric eyewalls using passive microwave data and best-track data in the western North Pacific between 1997 and 2006. Of the 225 typhoons examined, 55 typhoons and 62 cases with concentric eyewalls have been identified. The data indicate that approximately 57% of category 4 and 72% of category 5 typhoons possessed concentric eyewalls at some point during their lifetime. While major typhoons are most likely to form concentric eyewalls, the formation of the concentric structure may not be necessarily at the lifetime maximum intensity. Approximately one-third of concentric eyewall cases are formed at the time of maximum intensity. The moat is known to be heavily influenced by the subsidence forced by the two eyewalls. Rozoff et al. proposed that the rapid filamentation dynamics may also contribute to the organization of the moat. This paper examines the possibility of rapid filamentation dynamics by devising a filamentation moat width parameter. This parameter can be computed from the best-track typhoon intensity and the passive microwave satellite-estimated inner eyewall radius for each typhoon with concentric eyewalls. The filamentation moat width explains 40% of the variance of the satellite-observed moat width in the group with concentric eyewall formation intensity greater than 130 kt. The typhoon intensity time series in both the concentric and nonconcentric composites are studied. The time series of intensity is classified according to the 24-h intensity change before and after the concentric eyewalls formation. The averaged concentric eyewall formation latitudes in the groups with negative intensity change before concentric eyewall formation are at higher latitudes than that of the positive intensity change groups. Intensity of the concentric typhoons tends to peak at the time of secondary eyewall formation, but the standard model of intensification followed by weakening is valid for only half of the cases. Approximately 74% of the cases intensify 24 h before secondary eyewall formation and approximately 72% of the cases weaken 24 h after formation. The concentric composites have a much slower intensification rate 12 h before the peak intensity (time of concentric formation) than that of the nonconcentric composites. For categories 4 and 5, the peak intensity of the concentric typhoons is comparable to that of the nonconcentric typhoons. However, 60 h before reaching the peak the concentric composites are 25% more intense than the nonconcentric composites. So a key feature of concentric eyewall 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.