A New Dynamic-based Metric to Explore the Time and Intensity of Extratropical Transition of Tropical Cyclones

Almost half of all tropical cyclones (TCs) in the Atlantic basin undergo extratropical transition (ET). Re-intensifying ET (RIET) events often result in storm wind fields expanding dramatically and the heaviest precipitation shifting to the left-of-center (LOC). RIET events often result in more widespread tropical-storm force and greater winds and 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. A coupled dynamic and thermodynamic metric using the Eady moist baroclinic growth rate (EMBGR) is proposed to define the time and intensity of ET. The EMBGR parameter relies on relatively well forecasted environmental flow characteristics and static stability. The time and intensity of ET deduced from the EMBGR is then compared to several existing metrics: NHC Best Track, precipitation distribution (LOC/right-of-center), interaction between the TC and mid-latitude trough from a potential vorticity (PV) perspective, and the Cyclone Phase Space.