Numerical Simulations of an Orographically-Induced Mesoscale Cloud System and Subsequent Cyclogenesis off the South Coast of Japan

摘要

In cold seasons, a low-level mesoscale cloud system associated with an orographically-induced shear line often develops off the south coast of central Japan. Numerical experiments using high resolution limited-area models based on Japan Spectral Model (JSM) were performed for the 14 October 1991 case, where active development of the cloud system and associated cyclogenesis occurred. The model reproduced the evolution of the disturbance well, which is described as follows. In the initial stage of the cloud formation, a strong low-level northerly wind in the upwind side exhibited a splitting pattern around the Chubu Mountains, while the wind in the lee side was weak. High θe air that was formerly brought by a typhoon was trapped in the south coast area. Then an easterly wind gradually intensified along the south coast of Kanto, which caused the westward migration of the cloud system, and a NW-SE oriented shear line with strong positive vorticity in the lee of the Chubu Mts. A band-shaped precipitation area was simulated along the northeast side of the shear line, whereas on the southwest side, there was a dry air mass that descended the south slope. At the 18-h forecast, a shallow, mesoscale cyclone appeared on the shear line, which was nearly in geostrophic balance with the strong cyclonic circulation. The mesoscale cyclone then combined with the middle-level trough that progressed along the south coast and developed further as a synoptic-scale, extratropical cyclone. Simulated results show that the initial growth of the disturbance is triggered by the orographic effects, and that the later development is strongly influenced by the approach of the middle-level trough.