Tornado Climatology focused on the shape of parent convective system in Japan

Tornadoes frequently occur in Japan. Their information is published in the tornado database by Japan Meteorological Agency. The database provides on the JEF scale, damaged area, synoptic environment, and more. Unfortunately, there is no information about the convective systems causing tornadoes. We expect that the tornado warnings could be significantly improved if we understand the variety of convective systems and their environmental indices. The present study aims to examine tornado climatology in Japan with respect to parent convective systems causing tornadoes. We picked the tornado events that occur in the observation range of JMA radars and XRAIN (Ministry of Land, Infrastructure, Transport and Tourism radar network) radars for 2013 to 2024, and then classify the convective systems based on the shape of >40 dBZ echo region in reflectivity, the size of vortices in Doppler velocity distributions, and moving velocity of the systems.

We analyzed 178 tornado events for 11 years. At this time, we classified six types of convective systems: Isolated cumulonimbus, Supercells, Cloud clusters, Quasi-linear convective system, Local front, and Inner rainband of typhoon. Cloud clusters were the most common system, followed by Isolated cumulonimbus. Furthermore, these two systems accounted for the majority of all systems. Therefore, we focused on these systems, looking for trends of occurrence with respect to location, time, season and so on.

As a result, Cloud clusters tend to occur on south coast of Japan facing the Pacific Ocean, Tohoku district faced on the Sea of Japan and inland areas, throughout year and in the morning and early afternoon. Isolated cumulonimbi tend to occur on the sea and coastal area in the late afternoon from summer to autumn. Most of tornado vortices tend to move east or landfall from the sea. The velocity difference of vortices tends to decrease, and their diameter tends to shrink after landfall. The lower limit of the maximum wind velocity of tornadoes causing damages is found to be 22 m/s at about 1 km AGL.

We found that there are various types in cloud cluster. For example, some supercells in the Outer rainbands of typhoon are just classified as Supercell. The other systems without mesocyclone in the Outer rainbands may be classified as Cloud cluster. Then, we will classify such systems in detail with respect to various observation scales, temporal change of shape and so on. In this presentation, we will report the results of the detailed investigation.