An Investigation of Post-Transition Extremes for Extratropically Transitioning Tropical Cyclones
Mentor 1
Clark Evans
Start Date
1-5-2020 12:00 AM
Description
Extratropical transition (ET) is the process by which a tropical cyclone (i.e., hurricane) transforms into a non-tropical, or extratropical, cyclone after it encounters frontal systems and reduced sea-surface temperature at higher latitudes. In this study, we focus on improving understanding of the atmospheric conditions associated with different ET outcomes, looking at the cyclone intensity post-ET, cyclone thermal structure post-ET, and the time to complete ET. The only previous study that focused on this topic (Hart et al. 2006, Mon. Wea. Rev.) examined a limited number of cyclones using coarse atmospheric analysis data that included a fake tropical cyclone vortex. The primary goal of this research is to quantify the reliability of their results over larger samples using state-of-the-art high-resolution atmospheric reanalysis data. Herein, we focus on the North Atlantic basin and analyze all National Hurricane Center-classified tropical cyclones that complete ET since 1995. Atmospheric fields are derived from ERA5 reanalysis data. A cyclone phase space (CPS) is used to objectively identify ET timing and diagnose post-transition cyclone structure. Cyclones are divided into three categories: (1) cyclones that become stronger, weaker, or do not change intensity after ET, (2) cyclones that have a cold-core versus warm-seclusion structure post-ET, and (3) cyclones with fast versus slow time to complete ET. ERA5 reanalysis is used to generate composites of atmospheric fields at selected milestones along an ET-timing-relative timeline, which allows for direct comparisons between individual cyclones. Moreover, limitations of the CPS are highlighted when transitioning tropical cyclones instantly become warm-core extratropical cyclones.
An Investigation of Post-Transition Extremes for Extratropically Transitioning Tropical Cyclones
Extratropical transition (ET) is the process by which a tropical cyclone (i.e., hurricane) transforms into a non-tropical, or extratropical, cyclone after it encounters frontal systems and reduced sea-surface temperature at higher latitudes. In this study, we focus on improving understanding of the atmospheric conditions associated with different ET outcomes, looking at the cyclone intensity post-ET, cyclone thermal structure post-ET, and the time to complete ET. The only previous study that focused on this topic (Hart et al. 2006, Mon. Wea. Rev.) examined a limited number of cyclones using coarse atmospheric analysis data that included a fake tropical cyclone vortex. The primary goal of this research is to quantify the reliability of their results over larger samples using state-of-the-art high-resolution atmospheric reanalysis data. Herein, we focus on the North Atlantic basin and analyze all National Hurricane Center-classified tropical cyclones that complete ET since 1995. Atmospheric fields are derived from ERA5 reanalysis data. A cyclone phase space (CPS) is used to objectively identify ET timing and diagnose post-transition cyclone structure. Cyclones are divided into three categories: (1) cyclones that become stronger, weaker, or do not change intensity after ET, (2) cyclones that have a cold-core versus warm-seclusion structure post-ET, and (3) cyclones with fast versus slow time to complete ET. ERA5 reanalysis is used to generate composites of atmospheric fields at selected milestones along an ET-timing-relative timeline, which allows for direct comparisons between individual cyclones. Moreover, limitations of the CPS are highlighted when transitioning tropical cyclones instantly become warm-core extratropical cyclones.
