Formation mechanism of a low-level jet during the passage of Typhoon Ma-on (2004) over the southern Kanto District

On 9 October 2004, Typhoon Ma-on hit the southern Kanto district in eastern Japan. Strong winds were observed in the left-rear quadrant of Ma-on near the typhoon center during its passage over the southern Kanto district in spite of the rapid translation speed of about 70 km h(-1). Numerical simulations of Ma-on were performed using a nonhydrostatic model with a horizontal grid spacing of 2 km. The simulation results showed that the strong winds on the left-rear quadrant of Ma-on were low-level phenomena after landfall. The low-level jet (LLJ) associated with the observed strong winds formed over the Sagami bay just after the typhoon center had passed. The typhoon moved over the pre-existing low-level cold air in the Kanto region. When the typhoon center reached the Sagami bay, the northerly flow of the low-level cold air formed a narrowed channel between the typhoon center and the Kanto Mountains in the west of the Kanto plain. The LLJ corresponded to the outflow response from the narrowed channel to the Sagami bay. Trajectory analysis illustrated that the parcels which passed through the LLJ traveled southward parallel to the Kanto Mountains. When they passed near the Tanzawa Mountains at the southern tip of the Kanto Mountains, significant subsidence, spreading and acceleration occurred toward the Sagami bay. The horizontal momentum budget analysis and diagnostic evaluation of pressure gradient force (PGF) revealed that the LLJ was mainly supported by the large-scale southward PGF due to Ma-on. However, locally generated mesoscale forcing due to the decreasing depth of cold layer worked around the exit of the narrowed channel of the cold air. Thus, we conclude that the dynamics and structure of the LIJ were close to those of "gap wind". The sensitivity experiments showed that the low-level cold air as well as the relative position between the high mountainous topography and the typhoon was essential for the LLJ formation.