Intense Rainfalls on August 17, 1968 over the Kiso-Hida and Nagara River Basin in Japan Associated with Intrusion of Middle Tropospheric Dry Airs over the Low-level Moist Belt

抄録

The synoptic-scale condition related to the intense rainfalls on August 17, 1968, over the Kiso?Hida and Nagara River Basin in the central part of Japan is studied by using European Center for Medium-Range Weather Forecasts 40-year reanalysis data and upper, synoptic-surface, local rain-gauge observation data and satellite cloud images.
The intense rainfalls occurred within a long cloud belt formed with a low-level moist belt (LMB), which had formed along the northwestern rim of the North Pacific subtropical anticyclone (NPSA). The LMB was sustained by large-scale moisture transport along the northern rim of the NPSA and mesoscale northward moisture transport along a small anticyclone embedded in the NPSA. In the lower and middle troposphere, dry air spread over the Japan Sea after the passage of a severe tropical storm (STS) over the Japan Sea owing to the prevailing westerlies to the southwest of the STS. The northern edge of the LMB was bounded by the dry air. The northern boundary of the LMB was signified as the dry front because of the strong moisture gradient and very weak thermal gradient.
Intense rainfalls occurred in the cloud belt, at a distance of ∼1200 km south of the STS, over the river basin where the large-scale and the mesoscale moist flows converged in association with the orographic convergence. Intense convective rainfalls were accompanied by the increase in the convective instability due to the southeastward intrusion of the middle tropospheric dry air over the LMB. Significant low-level jet stream and mesoscale depression were not found around the intense rainfall area.