关键词: 宁夏, 寒潮暴雪, 等熵位涡, 机制分析, 预报指标

Abstract: Using the conventional meteorological observation data, the hourly observation data from the automatic meteorological station and the ECWMF 6-hourly reanalysis data with the resolution of 0.125 °×0.125 °, based on comprehensive diagnosis analysis methods, a short lifespan process of a cold wave and record-breaking snowstorm in Ningxia Province, China from April 2nd to April 3rd, 2016 was investigated in terms of tempo-spatial distribution characteristics of disasters, atmospheric circulation change, weather systems evolution, the sources and paths of cold airs , outbreak mechanism of cold wave and forecast indicators by using potential vorticity on the isentropic surface of 315 K. The results indicate that under the stable circulation background, the small trough of westerly belt on 500 hPa was moved eastward from the north side of the Gansu Corridor, catching up with the larger trough on the southeast side and then merged and reinforced. The northwesterly strong cold air behind the trough invaded Ningxia along with the cold air from the south of Lake Baikal. The surface cold anticyclone was split and moved southward rapidly with cold front, leading to the outbreak of cold wave and heavy snowstorm behind the cold front. This process was classified as the type of upper air small trough moving eastward and merging, which was characterized by the suddenness, locality, short duration (about 12 hours) and strong disasters. It is difficult to predict accurately by using conventional observation data because of the delay of observational data obtained in forecasting, the difficulty in tracing the evolution of the trough in the height fields and the inadequate tempo-spatial resolution of existing air sounding data. However, The isentropic potential vorticity（IPV）can clearly trace the source and path of cold air. The analysis of IPV indicated that, the cold air on top of the troposphere from Aral Sea was moved southeasterly, then merged with the cold air in the mesosphere of the troposphere in Qinghai-Tibet Plateau and thus enhanced. The cold air in the bottom of the stratosphere from east of Novaya Zemlya was moved eastward slowly, then turned southward from Lake Baikal at a higher speed and supplementing the direct cold air mentioned above, and the interaction of the two caused the cold wave in Ningxia. The IPV also represented the upper atmospheric circulation during the cold wave. The areas of great abnormal values of IPV indicated the evolution of small trough quantitatively and clearly which had better forecast significance than the conventional meteorological data. The temporal evolution of IPV coincided with the stages of cold wave's initiation, break-out, recession and ending, and the cold air activity can be predicted 6 hours ahead. From the space perspective, the zones of the cold wave and the snowfall shared the identical distribution with the areas with values above 1.0 PVU, and the zones having severe temperature drop were those with the value being larger than 0.8 PVU. This study indicated that the 315K isentropic potential vorticity can be used as an effective tool for analyzing and predicting the sudden cold wave and extreme snowstorm process in spring，and also as a key forecast indicator for fine quantitative forecast in Ningxia. More researches on the catastrophic weather events should be conducted in order to predict the threshold of IPV more accurately.

Key words: Ningxia, cold wave and heavy snow, isentropic potential vorticity, mechanism analysis, forecast indicator