An Observational Analysis of Rapidly Intensifying Tropical Cyclones Undergoing Eyewall Replacement Cycles

Previous studies have shown that rapid intensification (RI) is a very important and notoriously complex aspect of the tropical cyclone (TC) intensity forecasting process. Trabing and Bell (2020) found the largest forecast errors tend to be in TCs with RI events. Another phenomenon that impacts TC intensity forecasting are the eyewall replacement cycles (ERCs). Sitkowski et al. (2011) detailed the traditional ERC process, providing great detail on each of the three phases: intensification, weakening, and reintensification. Notably, the classical ERC process takes on average 36 hours from the start of secondary eyewall formation (SEF) through the completion of the reintensification phase. In contrast, Fischer et al. (2020) conducted a case study analysis on ERCs that occurred during a period of RI in Hurricane Irma (2017). In this study, it was noted that ERCs were completed on the order of 12 hours, and that no temporary weakening of the maximum sustained winds in the inner eyewall was observed, which contrasts the classical ERC model described in Sitkowski et al. (2011).

The purpose of this study is to improve our understanding of TC intensity change during ERCs, and to see if Irma was a unique case, or if other storms have followed similar behavior. Hurricanes Harvey (2017) and Ian (2022) were both selected as cases of interest for this study, because both of these impactful major hurricanes showed signs of having undergone simultaneous RI and an ERC. These case studies will be compared to null cases, or instances of ERC occurrences that did not happen during a period of RI, such as those detailed in Sitkowski et al. (2011). After determining whether or not these storms (and other potential cases) did have simultaneous RI and ERC occurrences, the scientists will use airborne Doppler radar observations from the Tropical Cyclone Radar Archive of Doppler Analyses with Recentering (TC-RADAR) database (Fischer et al., 2022) to look at the environmental, vortex, and precipitation structures to identify any potential similarities that may lead to these simultaneous RI and ERC events. Ultimately, this research may provide a foundation for future work to design forecasting tools that can be used to improve intensity forecasting, as well as improved accuracy in the detection of RI events in impactful TCs.