9B.6
**Defining the extratropical transition lifecycle using objective indicators of transition
Robert E. Hart, Penn State Univ., University Park, PA
High resolution ECMWF reanalyses (1.125 deg resolution) are examined for their ability to diagnose the transitioning and post-transition phase of 60 Atlantic tropical cyclones from 1979-1993. The full lifecycle of each tropical cyclone is examined using both thermodynamic parameters (core temperature anomaly magnitude, Eliassen-Palm Flux) and dynamic parameters (magnitude and sign of vertical gradient of horizontal wind, vortex tilt, frontogenesis). Using each of these parameters, an objective description of the lifecycle of transition is produced, including an objective point of transition. A detailed comparison of these lifecycle analyses to NHC subjective diagnoses of transition is performed. Through this comparison, the subjective approach NHC uses for diagnosing extratropical transition will be evaluated.
A similar analysis of an independent dataset of high resolution forecasts will be presented. This dataset, comprised of AVN, GFDL, and NOGAPS forecast data (1998-1999), incorporates the use of bogussed (synthetic) cyclones at initialization. Using the analysis of this independent dataset, we can estimate the impact of bogussed cyclones on the evolution of modelled extratropical transition. Preliminary results indicate that bogussed cyclones delay the extratropical transition process by 36-60hrs. Such an impact will greatly influence the forecasts for intensity, rainfall distribution and radius of maximum wind, and suggests that bogussing of tropical cyclones just prior to and during extratropical transition may decrease forecast accuracy.
Session 9B, Extratropical Transition II (Parallel with Sessions 9A and J6)
Thursday, 25 May 2000, 10:15 AM-11:45 AM
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