The occurrence of extreme precipitation events is now a serious concern in recent years in Japan. This study explores the atmospheric driving mechanisms of two extreme precipitation events occurred during 5–6 July 2017 and 5–8 July 2018 over western Japan. We identified that the atmospheric transport of large amounts of moisture and wind streams with wind speed of minimum 15 m s⁻¹ from south of Japan towards north on the days before these torrential precipitation events are mainly responsible for the July 2017 and July 2018 floods over western Japan. However, the contributions from the moisture advections (both vertical and horizontal) to the atmospheric water budget plays key roles to intensify the precipitations during the said torrential events. We also find that the prominent moisture flux convergence and well-developed moist conditions mainly maintain these heavy precipitation events over the downpour affected area. Our overall analysis suggests that the atmospheric factors driving to all these two heavy precipitation events are qualitatively robust and vital to explain the mechanism of extreme precipitations.

近年来，在日本，极端降水事件的发生现在是一个严重的问题。本研究探讨了日本西部2017年7月5日至6日和2018年7月5日至8日发生的两次极端降水事件的大气驱动机制。我们确定，大气中大量水分和风流的风速至少为15 ms ^-1这些洪灾事件发生前几天，从日本南部到北部，主要是造成日本西部2017年7月和2018年7月洪水的原因。然而，水分对流（垂直和水平方向）对大气水收支的贡献在增加上述洪流事件中的降水方面起着关键作用。我们还发现，显着的水汽通量收敛和发达的湿润条件主要保持了倾盆大雨影响地区的这些强降水事件。我们的整体分析表明，导致这两次强降水事件的所有大气因素在质量上都十分强大，对于解释极端降水的机理至关重要。