Structure and Environment of Polar Mesocyclones over the Sea of Japan: Composite Analyses and Numerical Experiments

Polar mesocyclones are meso-α- and meso-β cyclonic vortices that develop poleward of the main polar front during winter. The Sea of Japan is one of the regions where polar mesocyclones occur frequently. The general characteristics of the structure and environment of polar mesocyclones over the Sea of Japan are examined using composite analyses and numerical simulations. Polar mesocyclones are detected using an objective tracking method and classified according to their genesis location and direction of movement. A composite field using two kinds of objective analysis of the Japan Meteorological Agency (Mesoscale Analysis and Global Analysis) was made for each type of polar mesocyclones. The composite analysis shows that the synoptic-scale environment associated with these mesocyclones is characterized by a trough moving eastwards in the upper levels and a negative sea level pressure anomaly to the east that causes a cold air outbreak at low levels. However, the paths of the upper-level trough and the negative sea level pressure anomaly differ among the types of polar mesocyclones.　A numerical simulation using a non-hydrostatic model with a horizontal grid size of 5km, in which the initial and boundary fields are given by the composite field of the Global Analysis for each type of polar mesocyclone, does successfully reproduce a polar mesocyclone of the corresponding type. It is noted that mesoscale structures are almost smoothed out in the initial and boundary fields, demonstrating that the synoptic environment almost determines the genesis location and moving direction of the polar mesocyclones. Also performed are numerical experiments to examine sensitivities to condensational heating, distribution of sea surface temperature, and the topography of the Sikhote-Alin mountains at the east coast of the Eurasian continent. The results of the sensitivity experiments show that condensational heating is crucial for the genesis and development of all types of the polar mesocyclones. It is also shown that the high 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 polar mesocyclones that develop over the northeastern Sea of Japan. On the other hand, the blocking of the cold monsoon by the Changbai Mountains is found to be crucial for the polar mesocyclones that develop over the western Sea of Japan.