Volume 95 (2017) Issue 2 Pages 127-145
Polar mesoscale cyclones (PMCs) frequently develop over the Japan Sea. Genesis of PMCs over the East China Sea is rare, but can occur under certain synoptic-scale conditions. In this observational case study, the feature of a PMC generated over the eastern East China Sea on 20 February 1975 is studied using observation data including those obtained during Air-mass Transformation Experiment, satellite cloud images, and objective-reanalysis data.
The PMC with a comma-cloud formed within cyclonic polar-air streams induced by an upper cold trough and a synoptic-scale parent cyclone that developed near Japan. Within 3-hour period after the generation of the PMC, its central pressure deepened from 1016 to 1012 hPa. Strong surface winds occurred in the trailing portion of the comma-cloud. The large-scale conditions for the PMC's genesis were characterized by the southward intruding of the cold core in the upper cold trough beyond 34°N to the East China Sea, positive vorticity advection at 500 hPa, and the moist-neutral layer formed over the warm Tsushima Current in the eastern East China Sea.
The PMC, after passing over Kyushu, developed as it moved eastward along the Pacific coast of Japan. The PMC developed further into the secondary cyclone comparable to the parent cyclone, as it moved in the low-level baroclinic zone over the northwestern Pacific. The large-scale conditions for the development were characterized by the upper cold trough and the low-level baroclinic zone formed over the zone of maximum sea-surface temperature gradient along north of the Kuroshio extension.