Combined wind profiler-weather radar observations of orographic rainband around Kyushu, Japan in the Baiu season

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 11 h. The maximum length and width of the rainband observed in 1999 was ∼200 km and ∼20 km, 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 (∼600 m) 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 60 km from the Koshikijima Islands, strong wind and its clockwise rotation with increasing height was observed below 3 km 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.

Statistical Study of Band-Shaped Rainfall Systems, the Koshikijima and Nagasaki Lines, Observed around Kyushu Island, Japan

“…Some previous studies indicated that the formation and maintenance mechanisms of these two lines were related to the topography of Koshikijima Islands and Nagasaki Peninsula (Morita et al 1998;Yoshizaki et al 2000;Tashiro 2004;Umemoto et al 2004). Meanwhile, Kato and Niino (1998) showed from numerical experiments that those mechanisms of the Koshikijima line hardly depended on the topography of Koshikijima Islands.…”

Multiscale Features of Line-Shaped Precipitation System Generation in Central Japan during Late Baiu Season

“…However, the development of line-shaped convective systems occurs in limited areas within a meso-a-scale system, because these systems are strongly influenced by orography. On the one hand, several authors pointed out the importance of orographic wave disturbances that cause line-shaped precipitation systems with rainfall exceeding 150 mm day À1 around Kyushu, called Koshikijima and Nagasaki lines, when an intense southerly or southwesterly from the tropics in the lower layer with a height of about 1 km is maintained for 12 h by meso-a-scale systems (Kato et al 2003;Tashiro 2004;Umemoto et al 2004;Kato 2005). On the other hand, several authors pointed out that such a line-shaped convective system develops around a meso-b-scale (20-200 km horizontal scale) low.…”