High-precision Observations of Precipitation System
Producing Torrential Rain in Baiu Front

First High-precision Observations in Lower Yangtze River Basin Using Doppler Radar Equipment to Elucidate Formation Process of Precipitation System at the Source

The FORSGC configured a global observation system to improve the precision of simulation models. This issue introduces the high-precision observations being conducted in the Lower Yangtze River basin using equipment such as Doppler radar.

Hiroshi Ueda, Biao Geng, Hiroyuki Yamada, Krishna K. Reddy
(Hydrological Cycle Observational Research Program)

T he FORSGC Cloud and Precipitation Processes Group Leader Hiroshi Ueda, Subleader Biao Geng, and Researchers Hiroyuki Yamada and Krishna K. Reddy have conducted the first high-precision observations in China's Lower Yangtze River Basin, which is located windward of Japan, of the processes forming the precipitation systems in the Baiu front that cause torrential rain in Japan.

By using a high-precision observation network, this research has enabled the three-dimensional structure and development processes of the precipitation systems occurring in low-pressure zones to be established. The group's research results form the topic of this press release.


Japan is subject to frequent heavy rainfall causing floods and landslides. Elucidating the mechanism that causes the Baiu front is crucial for preventing such disasters from occurring. Research utilizing meteorological satellites has revealed that the causes of the precipitation system that wreaks havoc in Japan during the rainy season have mostly been shown to originate on the windward side of Japan in China. Further, a close relationship is said to exist between the development of the Baiu front and the processes forming this precipitation system.

The present satellite data on its own, however, is inadequate for uncovering the mechanism causing the Baiu front precipitation system. A high-resolution observation of the spatial distribution of rain and wind within the precipitation system and their three-dimensional structure must be clarified. Towards this goal, the first high-precision observations targeting the Baiu front precipitation system were conducted in the Lower Yangtze River Basin, which lies on the windward side of Japan.

The purpose of these observations is to continuously monitor at high resolution the internal structure of cloud and precipitation towards understanding the three-dimensional structure of the precipitation system, and thereby revealing the process of its development. Conducting these observations will contribute to the improvement and verification of cloud and precipitation system models.

Fig. The Precipitation System Associated with a Low-Pressure Zone (Three-dimensional Structure)

Since June 2001, observations conducted jointly by Japan and China have focused on the Baiu season in the Lower Yangtze River Basin, and a high-precision observation network has been established for the first time using instruments such as Doppler radars. Between the evening of June 19 and 20 a low-pressure zone that developed quickly in the Baiu frontal zone above the continent traveled east, producing heavy rain from Western Japan to the Tokai district.

For the observations in this research, we were able to use a high-precision observation network to determine the three-dimensional structure of the precipitation system that occurs when a low-pressure zone is generated and the process by which it develops (see fig.). We succeeded in ascertaining the phenomenon forming the precipitation system whereby two humid air currents from the southwest and south collide to form rain clouds, which produce eddies and ascending air currents.

The observation data we obtained is extremely valuable as a basis from which to improve precision in climate variation and prediction modeling that is used to recreate the Baiu front and associated precipitation system. These observations have served to clarify the causes of torrential rainfall, and if they can be used to predict this phenomenon, will be tremendously useful in disaster prevention. The research group will continue observations this year.


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