Tertiary eyewalls: observations and boundary layer response

While secondary eyewall research has received substantial attention in recent years, the related topic of tertiary eyewalls has not yet been thoroughly addressed by the scientific community. There are comprehensive studies about the frequency of occurrence of secondary eyewalls in nature as well as a number of scientific hypotheses that aim to explain their life cycle. In contrast, tertiary eyewalls, are known to exist but, to the knowledge of the authors, there are no studies of how frequently these hurricane configurations occur in nature, nor are there thorough attempts to unveil the physical processes that govern tertiary eyewall formation and evolution.

In this study we focus on tertiary eyewalls. First, we present an initial survey of passive microwave satellite imagery to identify tertiary eyewalls. The study spans 15 years (from 1997 to 2011) and focuses in the western North Pacific. The methodology of tertiary eyewall identification is objective, based on that described in Yang et al. (2013). Preliminary results suggest that roughly 5% of typhoon with secondary eyewalls also exhibit tertiary eyewalls. We present also detailed radar (courtesy of Satoki Tsujino of Nagoya University) and flight level data of individual tertiary eyewall cases that were exceptionally well observed.

Finally, the observational work is complemented by a simple modeling analysis to investigate whether boundary-layer dynamics can generate tertiary eyewalls. For this purpose we integrate a slab boundary layer model based on that described in Shapiro (1983). The model is initialized and forced with flight level data, from the U.S. Air Force Reserve Command C-130 (at about 3 km height), taken before the existence of the tertiary eyewall of the (Eastern Pacific) Hurricane Juliette (2001). The resulting model integrations are qualitatively and quantitatively compared with flight level data during the tertiary eyewall of Hurricane Juliette (2001). To the knowledge of the authors, the observational and modeling results presented here are the first attempt to explain tertiary eyewalls in tropical cyclones.