Environmental Conditions in the Early Stage of Cold-Season Cyclones Formed in the West of Japanese Islands

抄録

The pre-existing environmental conditions favorable for the development of cold-season extratropical cyclones formed in the west of the Japanese Islands are investigated. We focus on the early stage of cyclones to isolate environments unaffected by developing cyclones. Using reanalysis data and a feature-tracking algorithm, we identify and categorize cyclones into “strong” and “weak” based on their maximum intensity. Strong cyclones are characterized by pre-existing anticyclonic southerly winds and consequent warm and moist environments over the formation area. A preceding upper-level trough appears to strengthen these low-level southerly winds by inducing divergence behind it. Further analysis of strong cyclones reveals that stronger cyclones are accompanied by a more extensive intrusion of low-level cold air from the Asian continent, which enhances temperature gradient over the formation area. Correspondingly, the subtropical jet stream intensifies and increases upper-level divergence in its left-exit region above the cyclones. A concurrent upstream trough in the polar-front jet and ridge in the subtropical jet likely contribute to this enhanced upper-level divergence. These findings suggest that differences in early-stage environmental conditions of extratropical cyclones may explain the difference in their maximum intensity.