General Characteristics of Polar Mesocyclones over the Sea of Japan

Polar mesocyclones (PMCs) are meso-α- and meso-β cyclonic vortices that develop poleward of the main polar front. The Sea of Japan experiences frequent occurrence of PMCs during cold seasons. In order to reveal the general characteristics of PMCs over the Sea of Japan and the environment for their development, a method to track PMCs objectively and automatically is developed and is applied to 3-hourly Meso Analysis data of Japan Meteorological Agency during six cold seasons (November-March) from November 2007 to February 2014. Tracks of PMCs are found to be concentrated in the western part of the Sea of Japan and around Hokkaido Island, which is consistent with previous studies with a subjective detection of PMCs from satellite images. PMCs are found to be classified into several typical types according to locations of their genesis: Type-NW PMCs are initiated in the northwestern part of the Sea of Japan, Type-SW PMCs in southwestern part of the Sea of Japan, and Type-HI PMCs to the west of Hokkaido Island. Type-NW PMCs are further classified into three types according to the direction of their movement: Type-NWe moves eastward, Type NWse southeastward, and Type-NWs southward.

In order to reveal characteristics of environments for PMCs' development, a composite analysis for each type pf PMCs is performed. The results of the composite analysis are summarized as follows:

(1) Type-NW PMCs are formed in a convergence zone extending southeastward from the root of Korean Peninsula. The convergence zone corresponds to a surface trough extending westward from a preceding synoptic-scale cyclone which starts to develop as a trough at 500 hPa approaches the Sea of Japan from west. The direction of their movement depends on their distance to the preceding synoptic-scale cyclone.

(2) Type-NWe PMCs are located near or sometimes collocated with the synoptic-scale cyclone, and to the east of an advancing trough at 500 hPa. Type-NWs PMCs are located far from the synoptic-scale cyclone and to the south of a trough at 500 hPa. Type-NWse PMCs have intermediate characteristic between Type-NWe and NWs PMCs.

(3) Type-SW PMCs are formed in a convergence zone between northeasterly over the Sea of Japan and northwesterly from Korean Peninsula when a surface trough extends from south over the East China Sea. Type-SW PMCs reach their peak intensity when a trough at 500 hPa advances eastward and comes nearly right above the PMCs.

(4) Type-HI PMCs are formed at the western edge of a surface trough extending westward from a synoptic-scale cyclone over the Pacific Ocean. These PMCs move southward as the synoptic-scale cyclone moves northeastward and the surface trough rotates counterclockwise. A cold vortex exists above the PMCs

Numerical simulations are also performed in which the composite fields are used for the initial and boundary fields, where no incipient PMC is imposed in the initial field. It turned out that the numerical simulations successfully reproduce all the above types of the PMCs from their genesis to the mature stage, demonstrating the generality of the composite field as the environment of the PMCs.