Polar/comma-cloud lows over the Japan Sea and the northwestern Pacific in winter

Abstract

The features of polar/comma-cloud lows in the vicinity of the Japan Islands in winter 1986/87 (December, January and February) are studied. In the present paper, the polar/comma-cloud low (abbreviated as p/c low) is defined as a mesa -scale low that forms in polar air stream north of the main polar frontal zone, and forms spiral or comma shaped cloud system of 200~700km extent in its formation and mature stages. This study shows the following features; The p/c lows develop most frequently in midwinter over the Japan Sea (45~35ºN) and the northwestern Pacific Ocean (55~40ºN), where strong low-level baroclinicity is maintained by surface fluxes in the polar airmass between the continent area and the relatively warm sea area. They rarely appear over the Asian Continent, the Yellow Sea and the East China Sea. The p/c lows form in the low-level cyclonic flow of polar air induced by developed synoptic-scale lows. The p/c lows develop in the vicinity of meso-α-scale cold vortices in the middle troposphere embedded in the synoptic-scale trough or cut-off low. The p/c lows develop in a zone 500~1000km north of the major polar frontal zone, where the baroclinicity in the lower~middle troposphere is strong. The characteristics of p/c lows in these regions are compared with characteristics of p/c lows in the northeastern Atlantic Ocean and the northeastern Pacific Ocean.