A Study On Winter Snow And Rain In Northern China

Based on the NCEP/NCAR reanalysis, the station precipitation data of China during 1960-2008, and the aerosol optical depth (AOD) measurements in northern China over 1980-2004, this paper studies the annual variability of winter snow/rain in northern China and associated atmospheric circulation and aerosol distribution characteristics by using the statistical cluster analysis and the composite method. The results show that since 1960s the winter precipitation in northern China has been in general increasing. Verified by test of significance, the anomaly analysis showed that among the winters of 1990-2008, the typical more rain/snow years (MRSYs) are 1998,2003 and 2006 while the typical less snow years (LRSYs) are 2005, 1997 and 2001. It is also found by the composite analysis that the main differences of atmospheric circulations between MRSYs and LRSYs include:(1) in MRSYs, over the northern Polar region and Taymyr Peninsula there exists the strong cold air in the low-level and it migrates southward to North China (Northwest, North, and Northeast China), establishing a channel favorable for the continuous transport of cold airs. In LRSYs, however, such a channel for conveying cold airs does not exist;(2) in MRSYs, the Northern Hemisphere frontal zone and westerlies are over North China, and the low-level geopotential height field from eastern China to West Pacific exhibits a "east high, west low" pattern, good for easterly and southerly airflows to move northward along 110°E, while in LRSYs the 500 hPa prevailing westerly winds stay far away from China, and the low-level southeasterly conveyor belt moves to the high latitude areas in Siberia to the east of Russia, not helpful for the development of precipitation in northern China;(3)in MRSYs, the westerly airflows at 850 hPa overlaps the near-surface easterly airflows around 110°E, where large-scale upward motions combined with local-scale updrafts develop into strong slanted climbing airflows, forming a vertical circulation beneficial for the generation of heavy snows in eastern China, but in LRSYs the low-level westerlies and easterlies near 110-115°E are not present, and the vertical circulation structure good for snowfalls moves eastward into the Pacific Ocean.(4) the correlation analysis on AOD and the total precipitation in winter over 1980-2004 in norhtern China reveals that the annual variation of winter rain/snow in northern China is positively correlated to the annual variation of AOD in this region with a correlation coefficient of 0.415 at the 0.001 significance level. The composite analysis of AOD anomaly further finds that between the MRSYs and LRSYs, the atmospheric AOD is significantly different and the largest difference lies in eastern Inner Mongolia, northern Hebei, Jilin and southern Heilongjiang provinces.By using principal component analysis, wavelet analysis and correlation analysis to explore the spatial and temporal characteristics of winter snowfall in Northeast China and the correlation with global sea surface temperatures(sst),the study finds that:(5)The high frequency oscillation periods(2-3a and 6-7a) and a quasi-decadal scale 15a of the low-frequency oscillations are always exist during the past 48 years.(6) Winter snowfall reveals increasing trend in the past 48 years as a whole in Northeast region.However, the winter snowfall in the area north of 45°N increased significantly, while the winter snowfall does not change significantly in south of 45°N.(7) The existence of significantly positive correlation between sst in the North Atlantic 10-40°W,30-50°N and the 1st/2nd time coefficient, and between the winter snowfall in the northern part of the study area and sst,but the correlation between winter snowfall in the south region and the global sst is not obvious. The warming of sst may lead to a significant increase in snowfall in the northern part of the study area.