Genesis of a Weak Polar Mesoscale Cyclone in the Lee-side Shear-Zone South of the Central Part of Japan on 7-8 March 1992-An Observational Case Study-

Genesis of a weak polar mesoscale cyclone (PMC) over the coastal Pacific south of Honshuu Island (central part of Japan) on 7-8 March 1992 is studied using data obtained at two research vessels and observation stations in Japan, satellite cloud images, and objective-analysis data. A cumulus-line that formed over the coastal Pacific south of Honshuu Island developed into a comma-shaped cloud system associated with a weak PMC within several hours. A weak cyclonic circulation with a pressure deficit of similar to 1 hPa and precipitation of similar to 2 mm h(-1) were observed in association with the PMC. Synoptic-and meso-scale analysis showed that the PMC formed in a shear-zone over the coastal Pacific south of Honshuu Island during a period of weak polar-air outbreak. The shear-zone formed between the northwesterly streams passing along the western side of the high-mountains area in Honshuu Island and northeasterly streams passing along the eastern side of the high-mountains area. This PMC generated under strong baroclinicity in the low-middle troposphere. However, the PMC did not develop into a significant depression and disappeared within similar to 2 days after generation. The synoptic-scale conditions for this non-developed PMC are compared with those for developed PMCs described in previous articles. The conditions for this non-developed PMC were characterized by absence of the associated upper cold trough, background low-level anticyclonic circulation, shallow moist layer, and PMC location outside a zone of maximum sea-surface temperature gradient.