Multiple Dynamics of Precipitation Concentrated on the North Side of Typhoon Hagibis (2019) during Extratropical Transition

Abstract

Torrential rain in Typhoon Hagibis caused a devastating disaster in Japan in October 2019. The precipitation was concentrated in the northern half of Hagibis during extratropical transition (ET). To elucidate the mechanisms of this asymmetric precipitation, synoptic-and mesoscale processes were mainly analyzed using the Japan Meteorological Agency Non-Hydrostatic Model. The present study demonstrates that the asymmetric processes were different depending on the ET stages. When Hagibis was close to the baroclinic zone at middle latitudes on around 12 October (the frontal stage), heavy precipitation in the northeastern part of Hagibis was attributed to warm frontogenesis and quasigeostrophic ascent, as reported in many previous studies. In contrast, when Hagibis was moderately distant from the baroclinic zone on around 11 October (the prefrontal stage), heavy precipitation in the northern part occurred in a slantwise northward ascending motion in the outer region. This slantwise motion developed in a region with strong westerly vertical shear, which was enhanced between Hagibis and a westerly jet stream. Based on the analyses of potential vorticity and absolute angular momentum, this region was characterized by reduced moist symmetric stability in the lower and middle troposphere accompanied by inertial instability in the upper troposphere and conditional instability in the lower troposphere. These results provide additional insights into the time evolution of asymmetric processes during ET in the absence of a distinct upper-tropospheric trough, particularly, the slantwise motion in the prefrontal stage.