Extratropical transition of Typhoon Vicki (9807)

Synoptic-scale environment around typhoon Vicki was examined using 1.25-degree latitude/longitude GANAL analysis. Vicki made landfall on Japan at about 04 UTC September 22, 1998 and transformed into an extratropical cyclone on the next day. Although it still had characteristics of a tropical cyclone, e.g. upper- and mid-tropospheric warm-core structure and a distinct eye on the satellite imagery when it made landfall, it also had some characteristics of a baroclinic disturbance, e.g. lower-tropospheric horizontal temperature gradient and temperature advection.

By 18 UTC September 21, Vicki was a typical tropical cyclone that had strong horizontal convergence in the boundary layer and horizontal divergence near 150 hPa level, moving northward. At 00 UTC September 22, horizontal divergence spread to the north of Vicki in the mid-troposphere, although the dynamic tropopause was not lowered above it. Since there was an upper-level trough (PV maxima) west of Japan, southwesterly flow downstream of it and the upper- and middle-level warm core of the typhoon yielded horizontal warm advection and mid-tropospheric frontogenesis to the north of Vicki. In the same area, horizontal convergence in the lower layer was also increased in magnitude. It may have contributed to acceleration of movement of Vicki and decay of axisymmetrical structure. The upper-level trough was associated with dry air in the mid-troposphere. It also led to sudden dissipation of the warm core and extratropical transition.