The Debris Flow Disaster Caused by Back Building Squall Lines in Hiroshima, Japan

On 20 August 2014, a series of back building squall line brought the record-setting rainfall, exceeding 200 mm per 3 hours, in the north part of urban area in Hiroshima. Such heavy rainfall caused the debris flow and landslide at the steep slopes and hundreds of residence built on the bottom of the slopes were destructed. As a result, 74 people lost their lives inside of 15km x 5km area and thousands of people were forced to have refuge life.

According to the quick analysis of rain gauge data, radar echo data, and grid point value data, several characteristics related to the squall lines were found as followings, i) The squall lines were evolved near the southeast edge of the moist air band along the stationary front. The warm moist air continuously flow into the region along the edge of the northwestern Pacific high. ii) Mass of the warm moist air near sealevel was transported into the coastal area of Hiroshima through the Bungo channel, which is wedged between Kyushu Island and Shikoku Island. Both of them has mountain ranges with the height of 1500 m (880-870 hPa). The lowlevel moist air also came from North Kyushu, and the strong moisture convergence was occurred in Hiroshima. iii) The wind direction of the layer between 900 hPa and 500 hPa was nearly uniform (south west) around Hiroshima area. iv) The existence of the dry air mass both side of the squall line were able to enhance the instability and convection motion in the region.