Intensity changes or recurving typhoons from a potential vorticity perspective

Abstract

Potential vorticity (PV) interactions are evaluated in fourteen recurving typhoons to assess its usefulness as a forecast tool for intensity change. PV fields are derived from the large scale data in the ECMWF Global Advanced Operational Analysis data set. A deformation of the 1 potential vorticity unit contour around the typhoon on the 330 K isentropic surface coincides with the weakening of three typhoons, and may prove to be a useful forecast guideline for all recurving typhoons. Five typhoons have a trough with high PV on the 345 K isentropic surface approach within 10° radial distance. For these five typhoons, PV is not useful for forecasting intensity in isolation from other variables. The PV fields are too subtle by themselves to show a significant difference among typhoons which intensify, weaken or remain steady. Because of this, vertical shear of the horizontal wind and maximum potential intensity (MPI) are included with PV to examine intensity changes. Vertical shear of the horizontal wind calculated between 850-200 hPa in an intensifying typhoon is ≤ 15 mis, while shear in a weakening typhoon is ≥ 25 m/s. Shear increases as a PV anomaly propagates closer to the typhoon center. Weakening is associated with the upper PV anomaly passing within 50 from the center of the typhoon. When over fairly constant sea surface temperature (change of ≤ 1 °C ), two typhoons close to their MPI weakened. The typhoon which was far from its MPI intensified. Guidelines for forecasting intensity change using these parameters are presented.