Extended Abstract (1.9M)P2.12
Numerical study on the extratropical transition of Typhoon Bart (1991) over the western North Pacific Ocean
Jun Yoshino, Kyoto University, Kyoto, Japan; and H. Ishikawa and H. Ueda
Typhoon Bart (1999) has undergone a rapid weakening (a central pressure change of +50hPa/day) around Japan Islands and, after that, an unexpected re-intensification (-15hPa/day) over the Sea of Okhotsk. The purpose of this study is to investigate the detailed structure, dynamics and transitional process of mid-latitudinal Typhoon Bart using the mesoscale modeling system PSU/NCAR MM5. Furthermore, employing the gridded datasets from the successful simulation, a piecewise potential vorticity (PV) inversion is performed to determine the relative contributions of discrete pieces of the PV to the Bart's decaying and intensification.
Bart simulated by MM5 shows good agreements with the JMA best track for Bart and GMS-5 images. The cloud field of Bart has changed from axisymmetric pattern to asymmetry in its transitional process. Although Bart in the cyclolysis phase was far from an upstream synoptic trough composed of a high PV anomaly at upper levels, it began to come under the influence of the lower-level dry and cold air mass and upper-level negative PV anomaly. The cold air mass intrusion in Bart led to contribute to the large positive value of frontogenesis along the cold front. According to the results of the piecewise PV inversion, the upper-level negative PV anomaly, which would be generated by the diabatic heating in cloud, caused a rapid surface cyclolysis. The cloud activity associated with Bart resulted in the self-decaying with the generation of the negative PV anomaly aloft. However, a low-level positive PV anomaly associated with the Bart's warm core remnant was the most significant contribution to the deepening rate, despite of the development of the negative PV anomaly.
In the re-cyclogenesis stage, the simulated Bart wrapped typhoon cloud bands spirally and became an axisymmetric pattern again like its mature stage. The simulated PV field indicates that the upper-level high PV anomaly originating in high-latitudinal stratosphere, was vertically coupling with the low-level positive PV anomaly formed in the remnant of Bart. The spiral cloud bands in Bart produced the strong ascending motion resulting from the lifting of the tropopause, and accomplished the vortex stretching of Bart, and developed the divergent flow near the tropopause. The static piecewise PV inversion analyses showed that the upper-level dry positive PV anomaly with the synoptic perturbation was a minor contribution to the surface cyclogenesis, but it was essential factor to act on a positive feedback of cyclogenesis. The negative PV anomaly aloft, to the north of Bart, that had decreased its surface cyclonic activity in the weakening stage, played in an important role in evolving the outflowing jet streak and accelerating the positive feedback of cyclogenesis, resulting from the steeper PV gradient near the tropopause.
The re-intensifying and weakening processes of tropical cyclone could be explained by the dynamic outflowing jet streak process (negative PV anomaly) at upper levels and thermodynamic latent heating process (positive PV anomaly) at lower levels, which could interact with one another.
Poster Session 2, Poster Session P2 with Coffee Break
Wednesday, 25 June 2003, 3:30 PM-5:00 PM
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