Abstract
Line-shaped rainbands (LRBs), including Mesoscale Convective Systems (MCSs), stagnate on river basins and cause floods and landslides. Moreover, the frequency of LRBs is increasing around Hokkaido a part of northern Japan. Due to the complexity of the three-dimensional structures found in LRBs, some research dealt with two-dimensional observation data to identify them for the sake of simplicity. In previous studies, several methods to widely identify LRBs from radar rainfall data in terms of rainfall intensity, shape of rainfall area, and duration have been proposed. However, using the nationally consistent definition, the number of LRBs occurring in this region would be evaluated as very few due to the little rainfall climatology of this area. In this study, based on the previous studies, we adjust many combinations of threshold values, such as rainfall intensity, aspect ratio, and time duration in order to verify of validity the LRBs that occurred in northern Japan. The authors visually identified the six LRBs that caused the disaster and showed thresholds at which they were mechanically extractable by a computer program. The combination of thresholds that included at least six of those disaster-scale cases was "a rainfall area defined by a closed curve with a previous 3-hour rainfall of 40 mm or more, stagnating for 2 hours or more (covering 40% or more of its area), and with a maximum aspect ratio of 2.5 or more."
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