Climate Characteristics of Snowfall over the Qilian Mountain and Its Ambient Areas

Daily snowfall data of nighttime (20pm～08am),daytime(08am～20pm) and full day(20pm～20pm) during winter(from November to February) from 1960 to 2004 from 55 meteorological stations in the Qilian Mountain and ambient areas(90°~104° E,32°~42° N)are analyzed for the temporal and spatial distribution characteristics of various snowfall intensities,snowstorm weather impact systems and terrain influence.It is found that in the Qilian Mountain the number of middling snowfall days is most correlated to snowfall,with a correlation coefficient of 0.87 and the most snowfall days occurring in northeast.In various regions the least snowfall days appear in the west,the least intensity appears in the middle,but the most flurry days appear in the middle,middling snowfall is more and stronger in the middle and south,heavy snow and snowstorm are most in the north,and the most strong ones in the south.Nighttime snow days are more obviously,nighttime flurry and middling snowfall are stronger than those in daytime.As to annual variation,the annual snowfall is increasing continuously in the west,the least snow occurred in the 1970s in the middle areas and in the 1990s in the north,but the most occurred in the 21 century in the west,north and middle,a peak occurred in the 1970s in the south,and then was decreasing continuously.In the inter-annual variation,there are obvious periods of 3～4 years,5～7 years and 12～14 years for the snowfall days.The primary weather backgrounds of snowstorm are types of north horizontal trough pressing southwards and type of Xinjiang cold trough developing and moving eastwards,account for 38.1% and 52.4%,respectively.All snowstorms occurred in the mountain windward slopes of winter monsoon and gorges.