Roles of an upper-level cold vortex and low-level baroclinicity in the development of polar lows over the Sea of Japan

Abstract

The relationship between a synoptic-scale, upper-level cold vortex and a meso-α scale polar low (PL) over the Sea of Japan, and the role played by low-level baroclinicity in the development of the PL were investigated. We examined three PLs from the perspective of potential vorticity (PV) and energy budgets, using mesoscale analysis data with a horizontal resolution of ~11 km. During the development stage of each PLs, a meso-α scale upper-level PV (UPV) anomaly, which intrudes to around the 700–600 hPa level and which is just a part of a synoptic-scale PV anomaly accompanying the cold vortex, moved to the west side of a low-level PL. The PL was located in a wide region of low static stability in the lower troposphere. These conditions suggested that the synoptic-scale cold vortex, accompanied by a cold dome and the different scales of UPV anomalies, played a role in creating a favourable environment for the PL development. One of the common characteristics of the PL cases studied here was that, in contrast to the low-level PL, the upper-level disturbance associated with the meso-α scale UPV anomaly showed no development at all. This is likely because of the mobile UPV anomaly cut off from its source. Low-level baroclinicity, which varied with time, was an important factor in differentiating the PL shape and the energy sources necessary for PL development. Furthermore, the strength of baroclinicity, through its enhancement of mesoscale fronts and convection, affected diabatic processes, as well as baroclinic development.

• mesoscale front
• PV anomaly
• energy budget analysis

7. Acknowledgements

This paper is a part of a master's thesis submitted to the Graduate School of Environment Science, Hokkaido University, in March 2009. U. Shimada is deeply grateful to Prof. Y. Fujiyoshi, Prof. A. Kubokawa and Dr. M. Kawashima of Hokkaido University for helpful discussions. The authors thank two anonymous reviewers for valuable comments that have greatly improved the manuscript. This work was supported by MEXT KAKENHI Grant Number 25106708 and partly supported by the Green Network of Excellence Program (GRENE Program) Arctic Climate Change Research Project.