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[AAS10-03] Directional bias in tornado motions caused by the large-scale wind fields
Keywords:Tornado
A tornado is a violent updraft vortex that occurs in association with a cumulonimbus cloud. Tornadoes are generated when atmospheric conditions are extremely unstable, and can cause extensive damage over a narrow band-like area in a short period of time. Tornado genesis is difficult to predict, and its theoretical background is largely unknown. In Japan, in particular, the public pays little attention to tornadoes because they cause less damage than typhoons and other meteorological disasters. In addition, the number of statistical studies on tornadoes is limited because of the reliance on visual observation reports. Forecasting genesis and track of tornadoes is essential to minimize damage to people and structures.
According to Niino et al. (1997), more than half of tornadoes moved into 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, assuming that cumulonimbus clouds accompanied by tornadoes are carried by large-scale winds.
First, the direction of tornado movement is calculated from the latitude and longitude of the point of genesis and extinction, and is divided into 16 directions. These calculations show that approximately 70% of tornadoes moved into the northeast quadrant. The northeastward bias is presumably not a sampling bias because the qualitative results are the same for two independent time spans. Regardless of the year of occurrence, tornado size, or region, 60 to 80% of tornadoes moved toward the northeast quadrant. In particularly, the eastward bias suggests that the cumulonimbus clouds are being carried by the westerly jet stream overhead. When classified by season, there is a similar eastward bias for December-January-February (DJF) and March-April-May (MAM), but the distribution of the direction of movement of June-July-August (JJA) and September-October-November (SON) tornadoes also exhibits a northward bias, which is due to tornadoes associated with typhoons. The correlation between the direction of tornado motions and the large-scale wind direction is consistent with a hypothesis that tornadoes are transported by winds along with cumulonimbus clouds.
According to Niino et al. (1997), more than half of tornadoes moved into 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, assuming that cumulonimbus clouds accompanied by tornadoes are carried by large-scale winds.
First, the direction of tornado movement is calculated from the latitude and longitude of the point of genesis and extinction, and is divided into 16 directions. These calculations show that approximately 70% of tornadoes moved into the northeast quadrant. The northeastward bias is presumably not a sampling bias because the qualitative results are the same for two independent time spans. Regardless of the year of occurrence, tornado size, or region, 60 to 80% of tornadoes moved toward the northeast quadrant. In particularly, the eastward bias suggests that the cumulonimbus clouds are being carried by the westerly jet stream overhead. When classified by season, there is a similar eastward bias for December-January-February (DJF) and March-April-May (MAM), but the distribution of the direction of movement of June-July-August (JJA) and September-October-November (SON) tornadoes also exhibits a northward bias, which is due to tornadoes associated with typhoons. The correlation between the direction of tornado motions and the large-scale wind direction is consistent with a hypothesis that tornadoes are transported by winds along with cumulonimbus clouds.