A triggering mechanism of quasi-stationary convective bands in the vicinity of southwestern Japan during the summer season as deduced from moisture origins

Abstract

To find a plausible initiation mechanism of quasi-stationary convective bands (QSCBs) that lead to disasters, we investigated an extreme heavy precipitation event in Kyushu, southwestern Japan, in the middle of August 2021, using a regional spectral model incorporated with isotopic tracers aided by a water-tagging method. In the synoptic environment, the horizontal pressure gradient at low levels between an eastward-migrating Baiu frontal depression (BFD) and the North Pacific subtropical high (NPSH) is intensified to the south of a synoptic-scale front. To the south of the front, QSCBs also lie from the East China Sea to inland Kyushu. We confirm that the merging of the Asian monsoon moisture and the NPSH moisture creates a deep moist layer reaching middle levels from the surface in the vicinity of the QSCBs, providing an extremely unstable condition. Although such an unstable condition is indispensable for the maintenance of the QSCBs, an initiation mechanism is required that accounts for why the QSCBs emerge around the oceanic region where the two primary moisture merges; that appears to be the Ekman pumping mechanism. The strong boundary layer inflow of the NPSH moisture origin enhances the convective instability to the south of the QSCBs. Since the horizontal pressure gradient is attenuated in the vicinity of the BFD's center, on the contrary, the boundary layer flow north of the QSCBs decelerates rapidly, giving rise to upwelling due to the Ekman convergence. The persistent upwelling estimated in this event can lift the air of the Ekman layer up to a few hundred meters in a few hours, allowing it to readily reach the level of free convection. It is reasonable to conjecture that the persistent upwelling triggers the release of enhanced convective instability, leading to the formation of QSCBs. Continuous merging of the two remote moisture origins under the pronounced horizontal pressure gradient will further contribute to the reinforcement of the QSCBs.