2:00 PM - 2:15 PM
[AAS08-02] Outflows of low-level cool air from the subarctic oceans to the Japan Sea during heavy rainfall events in summer
Keywords:Extreme weather event, heavy rainfall, cool air, cool air mass flux
Extreme weather events increasingly occur with climate change, and we need to clarify the mechanisms of the extreme weather events. In particular, record-breaking rainfall has occurred in Japan in recent years, accurate prediction of the heavy rainfall is a major challenge. Previous reports and studies have pointed out that one of the factors causing heavy rainfall in summer is the outflow of low-level cool air from the Okhotsk Sea to the Japan Sea, flowing into the frontal zone. However, few studies have focused on the detailed distribution of the low-level cool air around the Okhotsk Sea and its outflow pathway to the Japan Sea. This study uses atmospheric reanalysis data ERA5 to clarify the three-dimensional structure of the cool air and the pathways of the cool air outflow for the heavy rainfall events. Then, we consider the relation between low-level cool air and the occurrence of heavy rainfall. For the analysis of the low-level cool air, we use an isentropic analysis method, and in this study, we define low-level cool air as airmass below an isentropic surface of 296 K.
We mainly focus on the heavy rains in July 2018, July 2020, and August 2021. First, from latitude-time diagrams of the cool air mass flux from northern Japan to Sakhalin, we identified the location, time variation, and duration of the cool air outflow from the Okhotsk Sea. Regarding time variations, the cool air flows from the Sakhalin and the Soya Strait and then the amount of the cool air flowing from the Tsugaru Strait to the Japan Sea increases. Then, we found that the cool air exists along the mountain range along the Eurasian continent and flows southward along the mountain range. Finally, high cold air mass flux is followed by the heavy rainfall.
We mainly focus on the heavy rains in July 2018, July 2020, and August 2021. First, from latitude-time diagrams of the cool air mass flux from northern Japan to Sakhalin, we identified the location, time variation, and duration of the cool air outflow from the Okhotsk Sea. Regarding time variations, the cool air flows from the Sakhalin and the Soya Strait and then the amount of the cool air flowing from the Tsugaru Strait to the Japan Sea increases. Then, we found that the cool air exists along the mountain range along the Eurasian continent and flows southward along the mountain range. Finally, high cold air mass flux is followed by the heavy rainfall.