Abstract
Abstract. A special observation campaign (X-BAIU), using various instruments (wind profilers, C-band weather radars, X-band Doppler radars, rawinsondes, etc.), was carried out in Kyushu (western Japan) during the Baiu season, from 1998 to 2002. In the X-BAIU-99 and -02 observations, a line-shaped orographic rainband extending northeastward from the Koshikijima Islands appeared in the low-level strong wind with warm-moist airs. The weather radar observation indicated that the rainband was maintained for 11h. The maximum length and width of the rainband observed in 1999 was ~200km and ~20km, respectively. The rainband observed in 2002 was not so developed compared with the case in 1999. The Froude number averaged from sea level to the top of the Koshikijima Islands (~600m) was large (>1), and the lifting condensation level was below the tops of the Koshikijima Islands. Thus, it is suggested that the clouds organizing the rainband are formed by the triggering of the mountains on the airflow passing over them. The vertical profile of horizontal wind in/around the rainband was investigated in the wind profiler observations. In the downdraft region 60km from the Koshikijima Islands, strong wind and its clockwise rotation with increasing height was observed below 3km altitude. In addition, a strong wind component perpendicular to the rainband was observed when the rainband was well developed. These wind behaviors were related to the evolution of the rainband.
Highlights
The summer monsoon rainfalls are identified as Meiyu in China and Baiu in Japan
We calculated the Froude number (F r) as Fr=U/N h, where U is horizontal wind speed averaged below 600 m altitude, h is the height of the mountains of the Koshikijima Islands (600 m), and N is the BruntVaisalafrequency
Throughout periods A, B and C very moist air in which q was more than 15 g/kg existed below 600 m altitude and the layer below 3 km altitude was convectively unstable around the Koshikijima line
Summary
The summer monsoon rainfalls are identified as Meiyu in China and Baiu in Japan. A rainfall zone called the Baiu frontal zone stretches from the south of China to the Japan Islands during the Baiu season (June–July). Heavy rainfall frequently occurs over a region from the East China Sea to Kyushu, Japan, in the warm-moist airs flowing from the Pacific subtropical anticyclone to the Baiu frontal zone. Ogura et al (1985) analyzed a concentrated heavy rainfall occurring in the Nagasaki Peninsula, in the westernmost part of Kyushu Island They showed that a rainband migrated over the East China Sea, stopped its migration when its tip reached the Nagasaki area, and remained there for 5 h. In period A strong and stationary rainfall was observed and precipitating clouds were generated in the region between 0.0◦ and 0.4◦, which corresponds to the southwest of the Koshikijima Islands, and moved northeastward, one after another. We consider that the difference in the length of rainbands depends on whether the rainband is able to develop around NA or not
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