关键词: 强沙尘暴, 特强沙尘暴, 能见度, 特征分析, 天气形势

Abstract: In this paper the heavy sandstorms are that whose shortest visibility is less than 500 m, and the severe sandstorms less than 200 m. These data were picked out from all the sandstorm records from 1954 to 2003 in Shaanxi province, with each record regularly observed by 3 or 4 times. Consequently there were 41 heavy sandstorm processes and 6 severe sandstorm processes picked out, with each sandstorm process recorded by more 3 stations or more. The analysis results show that the least frequency period of heavy and severe sandstorm processes in Shaanxi province is 1990s. Heavy sandstorm processes in Shaanxi happened mainly in spring, and secondly in winter. Thereinto the April is the most frequently in spring and the February is the most one in winter. Besides spring and winter, they also happened in June and November. Based on the 500 hPa circulations at 08h the day before the 41 heavy sandstorm processes happened, the circulations and affecting systems causing heavy sandstorm processes in Shaanxi are divided into four types: cold (crossing) trough from Mongolia, downslide trough before ridge, eastward movement of short wave trough and northwest airflow. Cold (crossing) trough from Mongolia is the most important pattern that caused heavy sandstorm processes in Shaanxi, which accounted for 45%. The circulations and affecting systems causing the 6 severe sandstorm processes in Shaanxi are all cold (crossing) trough from Mongolia. According to path analysis upon the invading cold air on surface, the cold air paths causing heavy sand storm processes in Shaanxi are divided into the west, the northwest, and the north way.

Key words: heavy sandstorm, severe sandstorm, visibility, feature analysis, weather patterns