A Lagrangian View of Moisture Transport Related to the Heavy Rainfall of July 2020 in Japan: Importance of the Moistening Over the Subtropical Regions

The transport and accumulation of moisture played an essential role in the extremely heavy rainfall of July 2020 in Japan. To better understand this event in terms of moisture sources and transport routes, backward particle trajectory analysis was conducted. We found two major moisture sources: transport from the tropics and uptake from the subtropics. A narrow moisture channel was found along the edge of the western Pacific Subtropical High (WPSH), transporting the moisture to the Baiu front. However, most moisture from the tropics was lost due to precipitation, and their contributions were reduced to about 15%. In contrast, the subtropical regions contributed over 80% moisture via evaporation and lower tropospheric convection. Among those regions, the western Pacific contributed the most (>33%). This study highlights the role of WPSH in moisture transport and demonstrated the importance of moisture uptake during transport.

Automated summary

The transport and accumulation of moisture played a crucial role in the heavy rainfall in Japan in July 2020. Backward particle trajectory analysis identified two major moisture sources: transport from the tropics and uptake from the subtropics. A narrow moisture channel along the edge of the western Pacific Subtropical High was found to transport moisture to the Baiu front. The study revealed that most moisture from the tropics was lost due to precipitation, while the subtropical regions contributed over 80% of the moisture through evaporation and lower tropospheric convection, with the western Pacific region being the largest contributor.