In moist atomosphere, the severe precipitation phenomenon often occurrs by a forcing of topography over a mountain region. Especially, insummer, many cumulonimbus clouds in the mountain region develop greatly and cause severe precipitation. In this study, the structureof the cumulonimbus clouds which developed over the south slope of the Ibuki mountains in the afternoon of July 5, 2000, was investigated byDopplar radar observation. The Ibuki mountains are located in the southwest area of Gifu prefecture in Japan (136.6°E,35.6°N) and have the strike from southwest to northeast.
A cold air trough moved over Japan and the atmosphere was convectivity unstable. Many cumulonimbus clouds therefore developed in the mountain regions. The direction of vertical wind shear of the environment was from northwest to southeast whichwas almost parallel to the direction of the south slope of the Ibuki mountains. The cumulonimbus clouds were observed by Dopplar radars forabout 2 hours over the south slope of the Ibuki mountains. It hadabout 20 cellar echos with successive generation and disappearance. Itwas aimed to the behavior of the cellar echos. First cellar echo,named Primary cell, weakening with tilting to downshear sidemoved down the slope. Thereafter some new cellar echos, named Secondarycells, occurred and developed with tilting to downshear side or standing upright on the upstream side of Primary cell,around the occurence point of Primary cell. There were 6 systems in theconvective echo and they occurred with lining in parallel to the strike of the Ibukimountains. The systems of cellar echos, especiarily the development of Secondary cells, contributed to the development of the cumulonimbus clouds.
When Secondary cells were developing, the warm wind from the mountain side to theconvective echo was observed for about 1 hour at an AMeDAS point, Tarumi, in the Ibuki mountains. It is considered that the existence of the warm air over the top side of the mountains has influenced to the development of Secondary cells.