Objective indicators of the life cycle evolution of extratropical transition for Atlantic tropical cyclones

Forty-six percent of Atlantic tropical storms undergo a process of extratropical transition (ET) in which the storm evolves from a tropical cyclone to a baroclinic system. In this paper, the structural evolution of a base set of 61 Atlantic tropical cyclones that underwent extratropical transition between 1979 and 1993 is examined. Objective indicators for the onset and completion of transition are empirically determined using National Hurricane Center (NHC) best-track data, ECMWF 1.125° × 1.125° reanalyses, and operational NCEP Aviation Model (AVN) and U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) numerical analyses. An independent set of storms from 1998 to 2001 are used to provide a preliminary evaluation of the proposed onset and completion diagnostics. Extratropical transition onset is declared when the storm becomes consistently asymmetric, as measured by the 900-600-hPa thickness asymmetry centered on the storm track. Completion of the ET process is identified using a measure of the thermal wind over the same layer. These diagnostics are consistent with the definitions of tropical and baroclinic cyclones and are readily calculable using operational analyses. Comparisons of these objective measures of ET timing with more detailed three-dimensional analyses and NHC classifications show good agreement.