Determination of a consistent time for the extratropical transition of tropical cyclones. Part II: Potential vorticity metrics

As a tropical cyclone moves poleward and interacts with the midlatitude circulation, the question of whether it will undergo extratropical transition (ET) and, if it does, whether it will reintensify or dissipate, is a complex problem. Uncertainties include the tropical cyclone, the midlatitude circulation, the subtropical anticyclone, and the nonlinear interactions among these systems. A large part of the uncertainty is due to a lack of an understanding of when extratropical transition begins and how it progresses. In this study, absolute potential vorticity and isentropic, or Ertel's, potential vorticity is examined for its ability to more consistently determine significant times (i.e., beginning or end) of the ET life cycle using the Navy Operational Global Assimilation and Prediction System gridded analyses. It is found that isentropic potential vorticity on the 330-K potential temperature isentropic level is a good discriminator for examining the extratropical transition of tropical cyclones. At this level, a consistent "ET time" is defined as when the TC-centered circular average of isentropic potential vorticity reaches a minimum value. All 82 tropical cyclones moving into the midlatitudes meet this criterion. The completion of extratropical transition for the reintensifying cases is defined as when the storm exceeds an isentropic potential vorticity threshold value of 1.6 PVU at the 330-K potential temperature isentropic level. The success rate of this threshold value for the completion of extratropical transition for the reintensification cases is found to be 94.3% with a 27.6% false-alarm rate.