地形性降雨の発生過程に関する数値実験
Numerical experiments on the formative process of orographic rainfall

氏名: 鳥山 暁人
指導教官: 守田 治



 The Izumi heavy rainfall early in July 1997 was caused by narrow, severe linear echoes directed from Koshiki-jima to Izumi continually for a long time. It is supposed that these echoes are first formed by the triggering effect of Koshiki-jima, and then intensified by the interaction with larger scale disturbances. In the Baiu season, such linear echoes frequently occur in the East China Sea side of the Kyushu island.

 This study has two objects. The first is to reproduce orographic rainfall by numerical experiments, and then to investigate its formative process; the second is how it depends on shapes and scales of orography.

 The model used is ARPS, which is a non-hydrostatic, quasi- compressible numerical model developed by Oklahoma University. The x-direction is taken as the Koshiki-jima - Izumi (230°- 50°) line and the y-direction at the right angle. The wind at the lower layer blows along the x-direction. Twenty-two Rawinsonde observations at Fukuoka and Kagoshima during 7-9 July are used as initial data. The orography is a bell-shaped mountain lengthened to the x-direction (hereafter referred to as the x-mountain), mimicking Koshiki-jima. Orographic rainfall can be reproduced only with the data of Kagoshima at 3 JST on 8 July.

   From a series of experiments, it is found that orographic rainfall needs high humidity at the lower layer and high instability. Orographic rainfall is not formed by the forced updraft in front of the mountain or convergence by wind around the mountain at the near surface, but by convergence at the back of the mountain at about the 1400m level.

 Next, shapes and scales of the model mountain are changed. In the x-mountain case, narrow, severe linear echoes are formed from near the mountain. Echoes in the circular mountain case are slightly wide and formed away from the mountain, but as severe as the x-mountain case. In the case of the mountain lengthened to the y-direction, it is hard to form orographic rainfall. As the mountain becomes bigger, convection becomes strong and echoes develop easily, but features of orographic rainfall is not changed so much, compared with the x-mountain case.