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 through 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, by collecting directional data of tornado motions in an objective way, this study investigates 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 locations of occurrence and extinction. These calculations show that approximately 70\% of tornadoes moved toward the northeast quadrant regardless of the day of occurrences, regions, or weather conditions. The preference for the northeastward direction is presumably not a sampling bias because the qualitative results are the same for two independent time spans. This northeast quadrant 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. When classified by seasons, a similar eastward preference is observed in DJF and MAM. By 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 correlation between the direction of tornado motions and the large-scale wind direction is also consistent with a notion that tornadoes are transported by winds along with cumulonimbus clouds.