Tokyo, Japan – Researchers at Tokyo Metropolitan University have analyzed long-term precipitation radar data from satellites and found significant increases in precipitation over the past decade during the annual Meiyu-Baiu rainy season in East Asia. The data spans 23 years and gives unprecedented insight into changes in rainfall patterns. They showed that the increased precipitation was driven by the decadal increased moisture transport from the tropics and the frequent occurrence of the upper tropospheric valley over the front.

Every year from the second half of June to the first half of July, East Asia has a special rainy season known as the Meiyu (in China) or Baiu (in Japan) season, or “plum rain” from the ripening of plums along the Yangtze River. They are triggered by what is known as the Meiyu-Baiu Front, where the flow of moist air around the Asian monsoon region meets anti-cyclonic currents around the edge of the western North Pacific Sub-Tropical High (WNPSH). Although they are bringing much-needed water to the area, it appears that the floods they have triggered have taken a deadly turn recently, with widespread devastation; Floods in China and Japan in 2020 were particularly devastating. It is important for scientists and politicians to put this into a larger context: are these just anomalies or will they persist?

Although studied in depth, most studies use rain gauge readings and observations of cloud activity around the land. An overall picture of the precipitation in the entire region was lacking, in particular analyzes over long periods of time. Now a team led by assistant professor Hiroshi Takahashi has examined satellite data with radar measurements of precipitation. They combined two sets of data, the Tropical Rainfall Measurement Mission (TRMM) and the Global Precipitation Measurement Mission (GPM). The full data set spans 23 years and covers both the sea and the land with equal accuracy. By carefully analyzing the time series, they confirmed a significant increase in precipitation over the past decade. In particular, they showed that the number of extreme precipitation events that can trigger natural disasters has increased significantly.

The question is Why It has changed. The team focused on two aspects of precipitation development, moisture transport and changes in air flow in the upper troposphere. First, they showed that water vapor transport had increased along the WNPSH rim, mainly due to decreased tropical cyclone activity, a trend seen both in decade-by-decade comparisons and in the devastating 2020 season there were anomalous circulations in the upper Troposphere, which created a “trough” that drove the air up around the western edge of the Meiyu-Baiu Front, strongly correlated with increased precipitation.

By fully analyzing data spanning a far larger area and time period than before, the team’s findings put the recent changes in the Meiyu Baiu season in East Asia in the context of a globally changing climate. They hope that new standards for average rainfall will translate into new standards of disaster prevention.

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This work was supported by a KAKENHI Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) (No. 19H01375), the Environment Research and Technology Development Fund of the Environmental Restoration and Conservation Agency of Japan (Grant No. JPMEERF20192004), and the Japan Aerospace Exploration Agency / Earth Observation Research Center (PI No. ER2GPF012).

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