A tornado struck Inabe City, central Japan, on the evening of 18 September 2012. This event is referred to as ‘Inabe tornado’ in this study. The parent storm of the Inabe tornado developed in the humid environment in which tornadoes occur occasionally in Japan. The purpose of this study is to reveal some of characteristics of a tornado and its parent storm in the humid environment in Japan, by a case study of the Inabe tornado. Especially, this study focused on the vortex structure and the development process of the parent storm.

To accomplish this purpose, three X-band polarimetric radars that are situated around Inabe city were used in this study. In addition, to understand the generation and development environment of the parent storm, the Japan Meteorological Agency mesoscale objective analysis (MANAL) data and upper-air observation data were used.

By using MANAL data and upper-air observation data, the environment of the parent storm of the Inabe tornado was characterized by humid environment without inflow of dry air, large low-level vertical wind shear, large storm-relative helicity (> 150 m² s^-2) calculated from surface to 3 km and medium CAPE (> 1000 J kg-1), which are favorite for the generation and development of supercell type storm.

A relatively long lived mesocyclone was identified within the parent storm of the Inabe tornado by using Doppler velocity data of PPI scans. The mesocyclone passed close to the damage track of the Inabe tornado several minutes after 15 JST, which corresponds to the occurrence time of the Inabe tornado. The morphology of radar echo of the parent storm was characterized by a hook echo, weak-echo region under an overhang echo, which were similar to typical supercells in the Great Plains. Three dimensional wind data of dual Doppler radar analysis was showed that the spatial collocation between strong updraft region and large vertical vorticity region persisted for about 1 hour: consistent lifetime with the mesocyclone. Consequently, it is revealed that the Inabe tornado originated from a supercell storm containing a mesocyclone. However, depths of the mesocyclone and the vault structure were shallow compared with typical supercells. Dual Doppler radar analysis revealed that the maximum vertical velocity and the height of it were much smaller and lower than those of typical supercells.

As other features of the parent storm of the Inabe tornado, a pair of an anticyclonic vortex and a cyclonic vortex was detected in the rear flank of the parent storm about 30 minute before the occurrence time of the Inabe tornado. Dual Doppler radar analysis suggests that this vortex pair was produced by the tilting of the horizontal vorticity associated with a rear-flank downdraft. Just before the occurrence time of the Inabe tornado, another smaller scale cyclonic vortex, which was referred to as “misocyclone”, was detected within adjacent to the mesocyclone of the parent storm of the Inabe tornado. Corresponding to the development of the misocyclone, a Descending Reflectivity Core (DRC) was observed. The DRC was located between the pair of anticyclonic vortex and cyclonic vortex.

This study compared characteristics of the environment and the structure of the parent storm of the Inabe tornado with typical supercells and other severe storms in Japan. The parent storm of the Inabe tornado had some similar features to typical supercells in the Great Plains, such as a hook echo, weak-echo region and the spatial collocation between strong updraft region and large vertical vorticity region. On the other hand, the parent storm had different features from typical supercells, such as a weak downdraft (weak outflow), a weak inflow, and the shallow depth of the meso- and miso-cyclone, which are similar to supercell-like storms (e.g. miniature supercell) that developed in the humid environment in Japan.

From results of the radar data analysis, as well as detailed analysis of environment, this study could show the formation mechanism of a weak tornado in the humid environment in Japan. In other words, it was revealed that the supercell which developed in the humid environment tends to provide a weak tornado (F0-1).

This study about the parent storm of the Inabe tornado showed a unique example of the detail structure of a supercell and the formation mechanism of a weak tornado in the humid environment in Japan by using three X-band polarimetric radars. Then, the result of this study would lead us to accumulate more case studies by using high resolution radar networks, such as phased array radars, to understand general features of a supercell and a tornado which generate and develop in the humid environment in Japan. In addition, the result of this study would make an impact for high resolution numerical experiments of tornadic storms in Japan.