Observation of Tornadic Storms in Tokyo Urban Area By X-band Phased Array Weather Radar

Urban areas in Japan suffer from more and more disasters by severe weather such as heavy rain fall or tornadoes recently. Particularly around Tokyo in summer season, rapidly developing cumulonimbi produce heavy rain; occasionally causing floods.

To observe developing stage of cumulonimbi for analyzing and predicting severe weather, Japan Radio Co, ltd. have developed X-band phased array weather radar. With this radar, it takes only 30 seconds to finish volume scan of the cylindrical space with a radius of 80 km and height of 15 km. And more, this radar outputs radar parameter such as reflectivity with high spatial resolution of 50 m. The radar was installed in Chiba city and the experimental operation was started from the summer of 2015.

In this presentation, we show the observation results of tornadic storms, the 3D structures of which were revealed with high spatial and temporal resolution by the phased array weather radar.

The first case is an isolated cumulonimbus system along with a tornado on September 4th2015. The system was formed in the mountain region about 60 km west away from central Tokyo around 11:30, then the system moved toward east and produced heavy rain in urban area of Tokyo from 13:30 to 15:00. Along with the system, a tornado was sighted around 14:00 where was 60 km away from the radar site.

The phased array radar observation shows that the 3D shape of the system almost never changed and moving speed was almost constant of 24 km/h when the system across the Tokyo urban area. The south edge of the system had strong over hang echo (Figure 1). In this area, horizontal shear which grows taller had about 0.01/s of vorticity (Figure 2). The tornado sighted point is the southernmost point of the system. These analysis results reaches conclusion that the storm was supercell and the mesocyclone located at the south edge produced the heavy rain and sighted tornado.

The second case is a gust front that emerged from an isolated cumulonimbus system on August 4th 2016. The system was suddenly formed in the bay area of Chiba prefecture, just 25 km away west from the radar site.

The phased array radar detected a bow-like gust front emerging from the early developing stage of the cumulonimbus (Figure 3). The reflectivity of the gust front was in the range between 0 dBZ to 15dBZ. The echo height of the gust front was around 500 m. The gust front moved at the speed of 4 m/s toward to radar site and passed above the radar. Inside of the system, there was the strong echo which grew suddenly at 3.4 km height and fell down at the speed of 14 m/s. It takes only 4 minute for the echo to touch ground (Figure 4). Therefore it would be difficult to detect the falling of the echo by conventional parabolic radars. After the ground touch of the echo, strong Doppler velocity toward the radar was detected around ground level just before the gust front emerged from the cumulonimbus system. These analysis results reaches conclusion that the sudden strong rain fall generated strong downdraft and the gust front.