AN OBSERVATIONAL STUDY OF CYCLOGENESIS IN THE LEE OF THE JAPAN CENTRAL MOUNTAINS

An observational study is conducted of the lee cyclogenesis that occur red on II December 1990 in the southeast region of the Japan central m ountains (JCM). The area of the region, roughly defined by the 1000m a bove sea level is 250 x 250 km. The principal results are summarized a s follows: i) The parent low, which triggered the lee cyclogenesis, wa s initiated by an upper-level vortex and accompanied by a low-level je t and surface cold front on the upstream side of the JCM. The system m oved eastward along the northern edge of the JCM and dissipated. ii) T he surface cold front approached the JCM, became blocked in the middle of the JCM, and stagnant for about three hours. South of the JCM, how ever, the cold front advanced eastward with no blocking. This resulted in the formation of a strong horizontal wind shear zone along the sou theastern edge of the JCM, between the warmer air on the downstream si de of the JCM and the colder air to the south. A shallow cyclonic vort ex, confined to within 3 km above ground level was subsequently genera ted in the horizontal wind shear zone. iii) The vortex moved eastward along a meso-scale coastal warm front, which previously existed for mo re than ten hours over the Kanto Plain, since the morning of 11 Decemb er. The Vortex coupled with the upper-level Vortex when it moved over the warm sea area east of the Japanese Islands, and then rapidly devel oped (12hPa/12h). These results show that, although the area of the JC M is one-third that of the European Alps, the ways in which the JCM in fluences cyclogenesis are, in many respects, similar to those of the A lps.