Abstract:

The area along the northern slope of the Tianshan Mountains has a high incidence of snowstorms in northern Xinjiang, China. As one of the main cities, Urumqi City has essential research value. Our research is based on the daily precipitation data of five national weather stations in Urumqi City during the snowfall period from 1961 to 2019 (November to March of the following year) and NCEP reanalysis data of 0.25°×0.25° and 1°×1° 4 times a day. Fifty-three times the characteristics of the snowstorm process and the large-scale circulation background are analyzed and summarized. Additionally, the Urumqi City snowstorm impact system is classified. Furthermore, the high and low altitude configurations of different types of snowstorms are synthesized and analyzed to obtain the structural characteristics of various systems. The results are as follows. (1) The frequency of snowstorms in Urumqi City increased by 0.3 times·(10a)^-1, with a quasi-20-year oscillation period and the highest frequency of occurrence in March (40%), followed by November (32%). (2) The snowstorm in Urumqi City is divided into southwest airflow in front of the trough, eastward movement of the upper-altitude trough, and strong front area. The strong front area has the highest proportion, but the snowfall is small. The southwest airflow in front of the trough lasts for a long time, and the snowfall is the largest. The east-moving type is the least. Meanwhile, the affected area is larger, and the snow intensity is stronger. (3) The main impact systems of the snowstorm in Urumqi City are 300 hPa polar front jet, 500 hPa west or southwest airflow, 700 hPa low altitude northerly jet, and 850 hPa northwest. (4) The mechanism of the formation of the Urumqi City snowstorm is that the low-level northerly airflow meets the mountain accumulation to force the warm and humid air to uplift to form a “cold cushion”, and form a strong vertical wind shear and a deep frontogenesis zone with the southwest airflow above 500 hPa. However, the causes of the snowstorm are different due to the difference in the duration of the strong frontogenesis and front slope and extension height of the three processes. (5) The water vapor transport of the snowstorm is mainly southwest, west and northwest, and the southwest airflow pattern and high altitude in front of the trough. The eastward trough type is guided by the southwest airflow and is directly transported to the sky over the snowstorm area. In contrast, the strong front type is transported by the relay of water vapor to form a water vapor confluence. We summarized the system structure characteristics of Urumqi City snowstorm weather caused by different influence systems. Consequently, we obtained that there are significant differences. Further research can improve the understanding of the evolution process of snowstorms along the Tianshan Mountains in northern Xinjiang. By combing forecast ideas, we can provide effective forecast services reference basis.

Key words: snowstorm, circulation classification, frontogenesis function, water vapor transport, Urumqi City