Volume 85 (2007) Issue 3 Pages 321-334
On January 18, 2001, a tornado formed over the Japan Sea near the Mikuni Town on the Hokuriku Coast of Japan. The “Mikuni Tornado” formed within a winter thundercloud, during a period of a cold air-outbreak, and heavy snowfall. The detailed structure of the tornado funnel cloud, and misocyclone with in the cloud are revealed from photographs, videotape, Doppler radar data, and the GPS sonde data. The tornado was generated 3 km offshore, close to the radar site. The tornado formed at the edge of a developing cell, had a single funnel, and was accompanied by a misocyclone near the cloud base; the position of the misocyclone corresponded well with a funnel cloud. The misocyclone had a diameter of 400-800 m and vorticity of 10−1 s−1, and coincided with the tornado funnel vortex in the cloud base. The funnel cloud had diameters of 150 m at the cloud base and 30 m near the surface, and a vorticity of 100 s−1. Although the funnel diameter at the cloud base changed remarkably during the life cycle of the tornado, the diameter near the surface remained largely unchanged. The funnel cloud began to dissipate after landing. The maximum wind speed of the tornado vortex is estimated to have been approximately 30 ms−1 (F0-scale), and the lifetime of the funnel cloud was 7 minutes. No damage or strong winds were recorded at the coastal radar site, which was located about 1 km from the tornado. The environmental feature that generated the Mikuni Tornado was vertical wind shear of the sub-cloud layer (3.5 × 10−2 s−1). The tornado was a non-supercell type tornado that formed under the uniform winter monsoon. This result indicates that small-scale snowclouds can produce tornadoes during periods of cold air outbreaks.