Direction of Tornado Motions and Its Relationship with the Large-scale Wind Field

抄録

According to Niino et al. (1997), more than half of tornadoes in Japan from 1961 to 1993 moved toward the northeast quadrant. However, since this data was based on visual observations, the reported directions of tornado motions were biased toward 8 directions out of 16. Therefore, using tornado records from the database of gusty winds of the Japan Meteorological Agency, our study collects directional data of tornado motions in an objective way, and investigates comprehensively the relationship between directions of tornado motions and the large-scale wind field. First, the direction of tornado movement is calculated from the latitudes and longitudes of the starting and ending points of the damage path. These calculations show that approximately $70\%$ of tornadoes moved toward the northeast quadrant. The preference for the northeastward movement remains similar for the periods of 1961-1993 and 1994-2022, although the Japan Meteorological Agency changed several times their operation to collect tornado report data during the periods. This northeast preference is caused by the superposition of eastward and northward peaks. The eastward preference suggests that the cumulonimbus clouds are transported by the westerly wind at the middle troposphere to first order. In all seasons, a similar eastward preference is observed. In contrast, the distribution of the direction of movement of JJA and SON tornadoes exhibit a northward predominance, due to tornadoes associated with typhoons. The high correlations between the direction of tornado motions and the large-scale wind directions are also consistent with a notion that tornadoes are overall transported by winds along with cumulonimbus clouds. Nevertheless, predicting the direction of supercell movements using the method of Bunkers et al. (2000), the directions of tornado movements are found to deviate approximately $60^\circ$ counterclockwise from the supercell movements on average. This result quantitatively suggests that many tornadoes in Japan are not necessarily supercell-type.