Mesoscale features of diverging strong winds associated with a typhoon in extratropical transition

Typhoon 9807 (Vicki) passed through central Japan on September 22, 1998 and caused heavy damage on structures by strong winds. Maximum instantaneous wind speed was about 60 m/sec. The mesoscale features and mechanisms of the strong winds were studied by synoptic-scale and mesoscale analyses by using operational data including those of the Doppler radar at the Kansai International Airport which observed the core region of the typhoon.

The typhoon made landfall at about 04 UTC with typical structure, e.g. well-defined eye and warm core in the middle and upper troposphere. After that the structure changed rapidly. At 06 UTC low-level winds around the typhoon center already showed some characteristics of baroclinic disturbance. In addition, rather dry air approaching the typhoon center from the west was found at middle levels.

Strongest winds near the typhoon track and just east of it were southerly to westerly. This means that the strong winds were observed after the passage of the typhoon center. There were heavy rain areas on the backside of the center and a cold air pool was associated with the heavy rain areas

The strongest westerly or southwesterly winds at the surface were associated with a certain rainband. A composite dual Doppler radar analysis revealed that at the middle levels the circulation around the typhoon center and the westerly wind west of the typhoon converged in the rainband. Vertical cross sections normal to the rainband show that the air that entered the rainband from the west descended rapidly, diverged at the ground and flowed further inward. The strongest winds at the surface coincided with the diverging cold air. The air was probably cooled by the evaporation of precipitation.