An Investigation of Posttransition Intensity, Structural, and Timing Extremes for Extratropically Transitioning Tropical Cyclones

Extratropical transition (ET) is the process by which a tropical cyclone transforms into a non-tropical, or extratropical, cyclone after it encounters frontal systems and reduced sea-surface temperature at higher latitudes. Furthermore, ET is a poorly studied subject with large impacts on multiple developed countries.

In this study, we focus on improving understanding of the atmospheric conditions associated with different ET outcomes, specifically cyclone intensity post-ET, cyclone thermal structure (i.e., cold-core vs. warm seclusion) post-ET, and the time to complete ET. The only previous study that focused exclusively on this topic (Hart et al. 2006, Mon. Wea. Rev.) examined only a limited number of cyclones, with six to fifteen cyclones per composite, using coarse atmospheric analysis data that included a bogussed tropical cyclone vortex, yet the results are cited extensively. The primary goal of this research is to increase the number of analyzed cyclones using state-of-the-art high-resolution atmospheric reanalysis output.

We focus on the Atlantic basin and analyze all NHC-classified tropical cyclones that complete ET since 1995. Atmospheric fields are derived from ERA5 reanalysis data. The cyclone phase space of Hart (2003, Mon. Wea. Rev.) is used to objectively identify ET timing and post-transition cyclone structure. Cyclones are divided into three categories: (1) cyclones that become stronger, weaker, or did not change intensity after ET, (2) cyclones that have a cold-core or warm-seclusion structure post-ET, and (3) cyclones with fast versus slow time to complete ET. ERA5 reanalysis data are used to generate composites of atmospheric fields at selected milestones relative to a normalized ET timeline (e.g., times relative to the start and end of ET, as identified using the cyclone phase space), which allows for direct comparisons between individual cyclones.