The transition of a hurricane to an extratropical cyclone occurs multiple times in the Atlantic Ocean basin during the course of a hurricane season. Operationally, the current method of diagnosing the transition of a hurricane into an extratropical cyclone is mostly subjective and involves the assessment of the appearance of the storm in satellite imagery, the distortion of winds around the storm, the radius of maximum winds, and the underlying sea surface temperatures. By this current method, a single moment in time defines the difference between a tropical and extratropical cyclone. In reality, however, a transitioning cyclone will exhibit both tropical and extratropical characteristics for a period of time. The transformation from a warm-core, vertically stacked, and equivalently barotropic cyclone, to a cold-core, tilted, baroclinic cyclone, can take from twelve to seventy-two hours, and a subset of these storms may reintensify significantly as extratropical cyclones.

The goal of this study is to explicitly measure the extratropical transition of a tropical cyclone with the use of readily available synoptic tools. Preliminary results show that there are several dynamical measures that capture the essence of extratopical transition. Two such measures are thermal vorticity and the advection of vorticity by the thermal wind based on ideas developed by Sutcliffe over a half century ago. The use of deep tropospheric thermal vorticity is a valuable tool in diagnosing the thermal structure (warm or cold-core) of a cyclone. The gradient of deep tropospheric thermal vorticity with respect to storm location also allows insight into the interaction of the cyclone with baroclinic zones of varying intensity based upon the value of the gradient. Many memorable East Coast mid-latitude cyclones lie in similar thermal vorticity gradients to that of tropical cyclones undergoing extratropical transition, in that the cyclone resides under the slightly negative (warm-core) thermal vorticity.

The forcing for large-scale ascent can be measured by the advection of mid-level (700 hPa to 400 hPa) absolute vorticity by the tropospheric deep thermal wind. As tropical cyclones move into mid-latitudes and develop characteristics of extratropical cyclones, a positive/negative couplet of advection of absolute vorticity by the thermal wind becomes readily apparent. The development of this advection couplet can also be understood from the perspective of “potential vorticity thinking.” It will be shown that maps of thermal vorticity, advection of vorticity by the thermal wind, and potential vorticity can be used collectively to explain the process of extratropi-cal transition. This task will be accomplished by examining Hurricane Floyd of 1999, as well as strong transition composite cases versus non-transition composite cases utilizing the NCEP/ NCAR Reanalysis dataset from 1948-2001.