| dc.description.abstract | In winter, migration of cold air from the north causes persistent low temperature in Taiwan, causing damage. In previous research, there were cut-off low formations before cold surge, and accompanied with a high pressure in the north. Some researches show that cut-off low often causes flash flood. The purpose of this paper is to evaluate the impact of cut-off low on cold surge.
Daily Northern Hemisphere 500hPa weather chart from Japan Meteorological Agency Tokyo were need to find out cut-off low events. Results show that there were 902 events between 1996/04~2005/03. The maximum cut-off low frequency was observed over Europe and North Pacific. When cut-off low occurs, they are often accompanied with an abnormal ridge in the north, causing a high pressure formation. This kind of cut-off low usually lives longer than 9days.
The case spans the period 15-29 December 2004, and occurred at northern Caspian Sea. During the life of cut-off low, there are several jets accompanying with cut-off low. According to the characteristic of jet, the whole lifetime divided into three stage: (i) develop, (ii) intense, and (iii) decay. Before cut-off low formation, warm core was at 200hPa. After 60 hours, a cold core happened on 500hPa. Due to the change of temperature structure, stability increased in mid-tropophere and decreased in upper-troposphere. During the intense stage, cut-off low and the high pressure are stationary, at the same time, the surface high intenes. On the profile of the potential vorticity, it observed that potential vorticity transport from upper-troposphere to surface. When cut-off low decayed, the tropopause ascends. At the same time, cold surge occurred in Taiwan. | en_US |