Extratropical Transformation of Typhoon Vicki (9807): Structural Change and the Role of Upper-tropospheric Disturbances.

Abstract

Extratropical transformation of Typhoon Vicki (9807) is diagnosed in relation to its environment, particularly with respect to interactions with upper tropospheric disturbances, using the gridded dataset of global analysis (GANAL) produced by the Japan Meteorological Agency. TY Vicki made landfall on Japan with its maximum intensity on 22 September 1998, causing damage with strong winds. Vicki lost the diabatically generated potential vorticity (PV) anomaly at 00UTC 23 September 1998, when it was supposed to complete the extratropical transformation. It still had a considerable diabatic PV anomaly during passage over Japan, which is consistent with the cyclone intensity. At 06UTC 22 September, Vicki was located in the lower-baroclinic zone, and strong horizontal divergence directly above the cyclone center was dissipated in the upper troposphere, which suggest that was already beginning extratropical transformation. Regarding the upper tropospheric environment, Vicki was moving northward to the right entrance of the jet streak, and a trough, i.e., mesoscale PV anomaly, approached to the west of Vicki. To the north of the cyclone, mid-tropospheric warm frontogenesis and the associated northward out-flow were induced. To the west of the cyclone, the upper-tropospheric frontogenesis associated with the jet entrance and the trough contributed to upper-tropospheric outflow from the cyclone and midtropospheric dry-air inflow. These contributed to organizing a wide rainfall region in the northern semicircle of the cyclone and to limiting the rainfall region in the southwest of the cyclone center. It is also suggested that interactions between upper- and mid-tropospheric outflow, and the midlatitude jet streak to the north of the cyclone contributed to a deepened layer of horizontal divergence that delayed the weakening of the cyclone during the start of the extratropical transformation.