Rainband Array Formed on the Southern Part of Shikoku Island, Japan

Kochi prefecture located in the south part of Shikoku island in Japan is one of the heaviest rainfall areas in Japan and annual rainfall amount is approximately 2,500 mm even in plain area and more than 4,000 mm in mountainous area. Such heavy rain is mainly caused by the orographic effect on the slope of the Shikoku Mountains. Two types of orographic precipitation appear, depending on the wind field; stationary echo pattern and an alley of rain bands parallel to the mountain slope. We call the latter precipitation system the slope streaks type (Makigusa and Sassa 2014). Such slope streak system is formed when the center of a typhoon locates in the west of Shikoku island and the rotating wind around the typhoon approaches Shikoku island as moist southerly wind.

The heavy rain event of July 2018 from 28 June to 8 July yielded torrential rain in the wide area of western Japan and killed more than 200 people. During this event, maximum accumulated rainfall amount was 1850 mm at Yanase observatory in the mountainous area of Shikoku island. The slope streaks system was maintained for long time from 22 JST on 1 July to 21 JST on 4 July 2018. In the first half of this period, the typhoon, ‘Prapiroon’, located not the west of Shikoku but far southwest area near Okinawa islands. It might be caused by that southerly wind was enhanced between the cyclonic rotational wind around the typhoon and the edge flow of the Pacific High. The present observational study aims to clarify the characteristics of the slope streak system and its environment.

The radar data showed that maximum accumulated rainfall during this period was about 500 mm and the most intense rain band was observed in the east portion of the Shikoku island. The axis of each rain band was almost parallel to the wind direction at the altitude of 3 - 4 km and gradually rotated clockwise as the wind direction changed from south to southwest. The echo-top height of the rain band is mostly less than 8 km. The mean length and width of rain bands were 55 km and 9 km, respectively. Some rain bands were longer than 100 km. The special interval of rain bands was about 15 km.