New Metric for Defining the Time of Extratropical Transition of Tropical Cyclones

Almost half of all tropical cyclones (TCs) in the Atlantic basin undergo extratropical transition (ET). During an ET event, wind fields often expand dramatically, resulting in more widely-felt impacts. Moreover, the heaviest precipitation typically shifts to the left-of-center (LOC), which can result in inland flash flooding hundreds of kilometers from the cyclone center. While several objective metrics to track and predict ET have been developed, they rely at least partially on internal tropical cyclone structure, for which numerical models show less skill. Further, these metrics fail to account for static stability, which plays a vital role in determining precipitation amounts. In this study, a coupled dynamic and thermodynamic metric using the eady moist baroclinic growth rate (EMBGR) is proposed to define the time of ET. The EMBGR parameter relies on well forecasted environmental flow characteristics and static stability. The time of ET deduced from the EMBGR is then compared using different methods i.e. HURDAT, storm precipitation distribution (left or right of center), interaction between the mid-latitude trough and tropical system from a vorticity perspective, and the Cyclone Phase Space