Unusually rapid intensification of Typhoon Man-yi in 2013 under preexisting warm-water conditions near the Kuroshio front south of Japan

Abstract

In 2013, sea-level pressure within Typhoon Man-yi dropped by more than 15 hPa in 6 h. The storm underwent extremely rapid intensification just north of 30°N near the coast of Japan. This study evaluated the importance of preexisting oceanic conditions around the Kuroshio Current for the rapid intensification, by performing a set of sensitivity experiments using an atmosphere-wave-ocean–coupled model. The results of both the sensitivity experiments and various observations suggest that warm water conditions in the ocean played a decisive role in the intensification and steepening of the sea-level pressure gradient within the storm area, whereas storm-induced sea surface cooling was important in suppressing excessive intensification during the early intensification phase of Man-yi. The rapid intensification was caused by excitement of a mesovortex inside the radius of the maximum surface wind. This unusual excitement was related to barotropic-convective instability induced by relatively high sea surface temperature and steep horizontal gradients in both sea-level pressure and tangential wind on the downshear side of the environmental vertical windshear vector. The local Rossby penetration depth around the mesovortex increased because of reduced static stability, increased relative vorticity, and an increase of the Coriolis parameter on the downshear side of the environmental vertical windshear vector where warm water in the ocean was transported along the Kuroshio Current. Preexisting high sea surface temperature conditions and storm-induced sea surface cooling also affected the extraordinarily heavy rainfall, particularly in the northern Kinki districts, which was simulated reasonably well in the numerical experiments.