An Investigation of Cold Vortices in the Upper Troposphere over the Western North Pacific during the Warm Season

Abstract

Sixty cold lows in the warm months (June?October) of 1982?87 (1984 missing) over the western North Pacific were studied by using rawinsonde data, airplane reports, and satellite cloud-tracked winds. The general characteristics of cold lows were analyzed. Various cloud types were derived from the cloud observations of the Geostationary Meteorological Satellite of Japan. Cloud distribution was investigated over different sectors of the vortex, over different regions of the jet streak, and at different life stages of the vortex. Structure of the vorticity and divergence fields was also studied for selected cases of cold lows. Cold lows generally moved westward with a mean speed of 4.1 m s?1 and a mean lifetime of 6.3 days. The frequency of occurrence exhibited significant interannual and monthly variations and reached a maximum in August and a minimum in October. One of the interesting findings of this study is the existence of a type NW (northwest) and/or a type S (south) jet streak for the majority of cold lows (87%). The type NW jet mainly formed in the northwest sector and then propagated downstream and dissipated in the southwest sector, while the type S jet usually formed in the southwest and southeast and then propagated downstream and dissipated in the northeast or northwest sector. Larger horizontal length scales and longer lifetimes were found for the cold lows accompanied by a jet as compared to those without a jet. A thermally direct circulation over the entrance region and a thermally indirect circulation over the exit region associated with both the type NW and the type S jets were derived from the thermal structure, the divergence pattern, and the cloud distribution over the vortex region. The cloud distribution over the vortex region was observed to be mainly modulated by the jet streaks. The evolution of a cold low was primarily related to the accompanied type S jet streak.