1988年9月22日千歳竜巻のライフサイクル

Abstract

In the afternoon of September 22, 1988, a supercell storm developed over the Ishikari Plain, in Hokkaido, Japan. A single-funnel F1 tornado was spawned in Chitose City, near Sapporo. The characteristics of the structure and the life cycle of the Chitose tornado were revealed by radar observations, ground survey, photographs, videotape, and meteorological data analyses. The Chitose tornado had a lifetime of 20 minutes and funnel width of 150 m at cloud base. The life cycle of the tornado was divided into four stages according to the detailed structure of the funnel and damage characteristics : the formation stage was characterized by a descending funnel, the mature stage characterized by touchdown of a visible funnel and a formation of strong dust cloud, the shrinking stage characterized by tilting and meandering funnel, and decaying stage that lead to the dissipation of the funnel. The mesocyclone of 7 km in diameter developed at 2.5 km AGL in the parent cloud and was accompanied by a misocyclone of 1 km in diameter. The angular momentum of the tornado circulation at the height of 400 m AGL was nearly equal to that of the misocyclone at 2.5 km AGL. The time and space relationship between surface damage, the tornado funnel and the mesocyclone was revealed. The width of the surface damage area was nearly equal to the width of the dust cloud (200 m) and was 10 times larger than the surface funnel width. Although the surface funnel initially formed right below the misocyclone, the funnel top near the cloud base and the funnel bottom near the surface moved in quite different ways, since the parent cloud moved at a speed of 5 ms -1 and the surface funnel at a speed of 1.2 ms -1 .