Structure and Environment of Polar Mesocyclones over the Northeastern Part of the Sea of Japan

Abstract

olar mesocyclones (PMCs) are mesoscale cyclonic vortices that develop poleward of the main polar front. The Sea of Japan is one region where PMCs frequently occur during winter. In this paper, the general characteristics of the structure and environment of PMCs that form over the northeastern part of the Sea of Japan and move southward are examined using composite analysis and numerical simulation. The composite analyses show that the synoptic-scale environment of the PMCs is characterized by a zonal temperature gradient at lower levels and a trough accompanied by cold air at upper levels. These elements of the environmental field form a reverse shear. The mesoscale structure of the PMCs exhibits characteristics of baroclinic development, while it is also accompanied by condensational heating. The numerical simulation in which the composite fields are used for the initial and boundary conditions successfully reproduces a realistic PMC. In this numerical simulation, the mesoscale structures are almost smoothed out in the initial and boundary fields, indicating that the PMCs spontaneously form and develop when the large-scale environment becomes favorable. Sensitivity experiments in which moisture is removed demonstrate that condensational heating is crucial for the genesis and development of the PMC. The sensitivity experiments also show that the warm sea surface temperature in the Strait of Tartary and the Sea of Japan to the west of Hokkaido Island, and the topography of the Sikhote-Alin mountain region provide favorable conditions for the development of the PMCs.