Regionalization of Tornado Intensities Using Tornado Parameters and a Long-term High-resolution Reanalysis Data

This study examines regionalization of tornado wind speeds in Japan. A 50-year reanalysis dataset (the horizontal resolution of 5 km and hourly interval) is produced by downscaling using the WRF model and ECMWF reanalysis datasets. Tornado parameters such as 3-km storm relative helicity (SRH) and convective available potential energy (CAPE) are estimated from this long-term high-resolution data. Since CAPE could be likely reduced by convection, the largest CAPE is considered assuming an inflow sector for each proximity grid. The maximum 1-h precipitation is also calculated assuming the proximity inflow sectors. Thresholds are determined for the largest CAPE, SRH, and the maximum 1-h precipitation as a criterion that the most intense class (F3) of tornado in Japan could occur, and tornado possible frequency is considered by counting events in which all parameters exceed their threshold.

The results of the analysis indicate that the Pacific side of Japan along the south coast is the most favorable region for the occurrence of the most intense class of tornado. In Japan, most of all F3 tornadoes occurred in plain areas open to the Pacific Ocean. The frequency pattern is consistent with this, because the paths of past F3 tornadoes are included in the high frequency region. Topographic effect is also found, which results in two regions divided. It is suggested that the inflow of humid and unstable air in the high wind shear to plain areas is essentially important for supercell tornadoes in Japan. CAPE climatology shows the latitude- or the season-dependent pattern, which suggests that the threshold of CAPE is needed to set according to summer and winter seasons. The approach of this study is encouraging to consider climatology in terms of intense tornadoes and its possible change due to global warming.