A description of tropical cyclone recurvature in terms of isentropic potential vorticity
Boerlage, Andrew P.
Elsberry, Russell L.
Nuss, Wendell A.
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Isentropic potential vorticity (IPV) fields calculated from the Na\7 Operational Regional Atmospheric Prediction System are analyzed to determine their usefulness as an aid to tropical cyclone recurvature forecasts. The IPV fields associated with Typhoon Nelson are calculated on a 80 km grid for the period 0000 UTC 4 October to 0000 UTC 7 October 19S8. In this preliminar>- study, IPV advection fields and the horizontal and vertical structure of the bogus vortex are examined to determine their contributions to the model forecast. The extremely broad bogus vortex is found to distort the IPV fields and cause regions of negative IPV, which is believed to result in inertial instability. Approximately 36 h are required for the model to adjust to the initial imbalance in the mass and wind fields. It appears that the unrealistic bogus vortex representation may have degraded the early portions of the model forecast. Analysis of IPV advection fields each 12 h during the NORAPS model integration showed that storm movement is primarily due to the influence of self-advection and the large-scale steering fiow. During later periods other advective features associated with adjacent synoptic systems begin to influence the storm movement. Forecasters with access to such IPV fields may be able to evaluate the likely validity of the model forecast of recurvature. The usefulness of the IPV representation from the NORAPS prediction in this single case study suggest that additional cases of tropical cxclone recurvature be examined in terms of IPV concepts.
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