Remote dynamic and thermodynamic effects of typhoons on Meiyu–Baiu precipitation in Japan assessed with bogus typhoon experiments

Abstract

To advance comprehensive knowledge of typhoon-induced remote precipitation and anomalous atmospheric circulations during the early summer rainy season (Meiyu-Baiu), we performed numerical simulations in which different bogus typhoons are imbedded in the July climatological mean field as ideal initial and lateral boundary conditions using a regional atmospheric model, and we compared experiments with and without the bogus typhoon. The dominance of an anomalous local high (ALH) remotely induced by a typhoon leads to a poleward shift of the primary Meiyu–Baiu frontal zone in the form of the enhancement of the North Pacific subtropical high (NPSH). Typhoon-induced negative potential vorticity advection plays a dominant role in forming the ALH around Japan. Despite the shift of the frontal zone, heavy precipitation in excess of 100 mm per day occurs along the Pacific coast of western Japan, which is triggered by two primary moisture inflows. One is the low-level moisture inflow along the moisture conveyor belt (MCB), which is an appreciable low-level moisture flux zone stretching from the Indian Ocean to a typhoon over the western North Pacific, due to the intensified horizontal pressure gradient between the typhoon and the NPSH. The MCB is more robust in the experiment in which the typhoon is close to the South China Sea where the monsoon westerlies prevail. The parcels along the MCB bring moisture into western Japan even though moisture losses due to condensation occur on the way, to some degree. The other is the moisture inflow through the atmospheric boundary layer along the southwestern periphery of the NPSH. The parcels along the NPSH periphery easily receive moisture from the underlying ocean as they approach western Japan. Both inflows are enhanced by the ALH formation over Japan. The combined effect of the typhoon and its induced ALH is responsible for the prominence of the two major moisture inflows into western Japan, leading to local heavy precipitation in that region.