Determination of a consistent time for the extratropical transition of tropical cyclones. Part I: Examination of existing methods for finding "ET Time"

As a tropical cyclone moves poleward and interacts with the midlatitude circulation, the question of whether it will undergo extratropical transition (ET) and, if it does, whether it will reintensify or dissipate, is a complex problem. Several quantities have been proposed in previous studies to describe extratropical transition including frontogenesis, 500-hPa geopotential heights, and cyclone phase-space parameters. In this study, these parameters are explored for their utility in defining an ET time using the Navy's Operational Global Assimilation and Prediction System gridded analyses. The 500-hPa geopotential heights and frontogenesis currently do not have objective numerical definitions. Therefore, this study attempts to establish and examine threshold values that may be used to objectively define the ET time. Cyclone phase space already has numerical threshold values that can be examined. Results show that the 500-hPa geopotential height open wave distinguishes 81 of 82 cases, but it fails to discriminate between transitioning ET and recurving non-ET cases and cannot be determined automatically. The 2D scalar frontogenesis distinguishes 77 of 82 cases but does not discriminate between transitioning ET and recurving non-ET cases. Finally, phase space successfully distinguishes 81 of 82 cases for the "ET time" defined by the asymmetry parameter but is only successful at capturing transitioning ET and recurving nonET cases properly for 60 of 82 cases. All of the definitions are found to have disadvantages that preclude them from providing consistent guidance for when extratropical transition of a poleward-recurving tropical cyclone is occurring.