A New Dynamically-Based Metric to Discriminate between Re-intensifying and Decaying Extratropical Transition Cases

Almost half of all tropical cyclones (TCs) in the Atlantic basin undergo extratropical transition (ET).  ET cases that have the largest impacts on life and property are those that re-intensify as an extratropical cyclone.  Recent cases such as Irene (2011) and Sandy (2012) have exemplified the widespread wind and flood risks that such storms pose.  While several objective metrics to detect ET have been developed, they rely at least partially on internal TC structure, for which numerical weather prediction (NWP) models show less forecast skill.  Furthermore, these metrics 1) fail to account for static stability, which plays a vital role in determining precipitation amounts, and/or 2) do not necessarily attempt to discriminate between re-intensifying cases and decaying ET cases.  Here, a coupled dynamic and thermodynamic metric based on the Eady moist baroclinic growth rate (EMBGR) is developed to discriminate re-intensifying cases from decaying cases.  ET intensity skill deduced from the EMBGR metric is compared to previous metrics such as vorticity tendency and precipitation distributions.  Results from 36 years (1979 – 2014) of landfalling TCs using reanalysis data suggest that the EMBGR metric can provide a larger probability-of-detection (POD) and lower false-alarm-rate (FAR) for re-intensifying ETs than other metrics.  Future work should include testing the EMBGR metric in current global operational deterministic and ensemble NWP models.