The Role of Typhoon Kilo (T1517) in the Kanto-Tohoku Heavy Rainfall Event in Japan in September 2015

From 9 to 11 September 2015, the Kanto and Tohoku regions of Japan experienced an extremely heavy rainfall event. The synoptic-scale field was characterized by two typhoons, Etau (T1518) and Kilo (T1517). After Etau made landfall in the Tokai region and transformed into an extra-tropical cyclone over the Sea of Japan, meridionally-oriented rain bands persisted over the Kanto region for about 12 hours and caused heavy rainfall, particularly over the Tochigi prefecture. During this time, Kilo approached the eastern ocean of the Kanto region. In this study, we examine the role of Kilo in this event by conducting numerical experiments using a stretched version of the Nonhydrostatic Icosahedral Atmospheric Model, configured with a minimum grid interval of about 5.6 km. The control experiment reproduced intense rain bands around the same period and place as the observed event, although they were not reproduced in an experiment with a longer lead time. Sensitivity experiments were conducted, in which Kilo was weakened by removing moisture in its central region with a longer lead time. In contrast to the expectation that reduced moisture would lead to a weaker typhoon, and hence weaker rain, the sensitivity experiment reproduced the rain band with a realistic location, but 5 % less precipitation than the control experiment. Furthermore, this experiment indicated that precipitation over the outer band of Etau, which covers the Kanto region, increased by 10 % compared to the control experiment. We found that a southeasterly wind, induced by a high pressure ridge between Kilo and the Kanto region, played a greater role in supplying moisture to the Kanto region than the strong easterly wind produced by the pressure gradient between Kilo and the Okhotsk high. In this case, weaker Kilo resulted in an enhanced northwestward moisture flux associated with the ridge, thereby inducing heavier rainfall over the Kanto region.