An observational study of cyclogenesis in the lee of the Japan central mountains

Summary An observational study is conducted of the lee cyclogenesis that occurred on 11 December 1990 in the southeast region of the Japan central mountains (JCM). The area of the region, roughly defined by the 1000m above sea level is 250×250 km. The principal results are summarized as follows:i)The parent low, which triggered the lee cyclogenesis, was initiated by an upper-level vortex and accompanied by a low-level jet and surface cold front on the upstream side of the JCM. The system moved eastward along the northern edge of the JCM and dissipated.ii)The surface cold front approached the JCM, became blocked in the middle of the JCM, and stagnant for about three hours. South of the JCM, however, the cold front advanced eastward with no blocking. This resulted in the formation of a strong horizontal wind shear zone along the southeastern edge of the JCM, between the warmer air on the downstream side of the JCM and the colder air to the south. A shallow cyclonic vortex, confined to within 3 km above ground level was subsequently generated in the horizontal wind shear zone.iii)The vortex moved eastward along a meso-scale coastal warm front, which previously existed for more than ten hours over the Kanto Plain, since the morning of 11 December. The vortex coupled with the upper-level vortex when it moved over the warm sea area east of the Japanese Islands, and then rapidly developed (12hPa/12h).