Observational Research of the Baiu in the Lower Yangtze River Basin

The Frontier Observational Research System for Global Change (FORSGC) conducts research in close cooperation with the FRSGC in order to clarify the processes of global change. This time, Dr. Biao Geng, Subleader of the Hydrological Cycle Observational Research Program, reported the observational research being conducted in the Changjiang (Yangtze River) region and the future outlook for this research.

Biao Geng (Hydrological Cycle Observational Research Program)

T he Baiu front, which is active from Southeastern China to the Japanese Archipelago, is composed from multiscale cloud and precipitation systems. These systems play major role in the energy and water cycle in Asian monsoon regions. Elucidating the behaviors of the systems is important for understanding changes in the climate system.

During the Baiu season, the lower Yangtze River basin is susceptible to the enhanced vapor transport from the south. The cloud and precipitation processes in this region influence the abnormal weather conditions and climate variation in Japan.

The FORSGC Cloud and Precipitation Processes Group has configured a high-density observation network in the lower Yangtze River basin, including Doppler radars, bistatic receivers, wind profiler, automatic weather stations, and micro rain radar. The Group is conducting observational research of the Baiu front, mainly targeting the vapor transport to the precipitation systems that occur in the frontal zone and the three-dimensional structures and development processes of these precipitation systems.

Photograph : A photograph of a precipitation system accompanying the Baiu front that occurred within the observation zone on the evening of July 6, 2001. Relatively heavy rainfall (approximately 30 mm precipitation per hour) from this system was observed.

This observational research involves intensive field observation over two years from 2001 to 2002. The intensive filed observation implemented in 2001 coincided with the anomalous strength and position of the subtropical Pacific high. Many precipitation systems in the Baiu frontal zone have been observed and valuable data have been obtained for understanding the mechanisms of these systems.

Combined with the observation for next year that is predicted to show a different atmospheric pressure field compared with this year, we expect to reveal the processes of the precipitation systems in the lower Yangtze River basin.

The scientific results and high-resolution data on precipitation systems obtained from this observational research should definitely serve the further development and improvement of global climate models utilizing a 5-km mesh, which are being promoted by the FRSGC and explicitly handle mesoscale (several km to several 100km) precipitation systems. We also anticipate that this research will contribute significantly to the Earth Simulator, which is aimed towards predicting climate variation in monsoon regions and associated climate anomalies.

Horizontal distribution of wind and reflectivity fields of the precipitation system shown in the photograph at 16:06 (local time) and an altitude of 1.5 km. The data are obtained through a synthesis of data from three Doppler radars. The reflectivity levels show that the greater the reflectivity the stronger the rainfall. The circular symbol (.) indicates the location of the radar seen in the photograph.

The shaded area enclosed by broken lines indicates the area corresponding to the range visible in the photograph. The southernmost part of the precipitation system, which forms a band that stretches from SW to NE, corresponds to the black clouds in the photograph. Horizontal wind convergence is apparent in the areas with high reflectivity levels, showing that this system is accumulating together with the ascending current.

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