Spatial-Temporal Characteristics of Observed Key Parameters for Snow Cover in China during 1957—2009

Using the observed snow cover data from meteorological stations,in this study,the temporal and spatial characteristics of snow depth,snow water equivalent(SWE) and snow density in China during 1957-2009 are analyzed. It is revealed that the annual mean snow depth,SWE,and snow density were 0.49 cm,0.7 mm and 0.14 g·cm-3 over China as a whole,respectively.On average,they were all smallest over the Tibetan Plateau(TP),and were greater over Northwest China(NW).The inter-annual variation was smaller for snow density in China,but the inter-monthly variation was obvious.The snow densities were greatest in March and April for Northeast China(NE) and NW,respectively,and were greatest in June for the TP.Spatially,the annual mean snow densities were below 0.14 g·cm-3 in most of China,and were higher in the TP,NE and NW,which were 0.14～0.18 g·cm-3 in the northern part of NE and the middle and southwest parts of the TP,0.18～0.26 g·cm-3 in South China and the mid-south part of the TP.The inter-monthly variations were consistent for snow depth and SWE.For China as a whole,the monthly mean snow depth was greatest in January,but the SWE in February was quite close to that in January due to larger snow density in February.In October and November when snow started accumulating,the snow depth and SWE in the NE were the maximum,and were the maximum in the NW since then until fully melting of snow cover.Spatially,the regions with greater annual mean snow depth and SWE were located in NE,northern Xinjiang Autonomous Region,and a small fraction of the southwestern TP. The trend analyses indicated that the annual mean snow depth and SWE both presented increasing trends over China as a whole,NE,NW,and the TP during 1957-2009.Although the trend in China was not significant,the amplitude of variation became greater and greater in the second half of the 20th century.It was generally determined by the variation of snow cover in NE,NW,and the TP.Seasonally,for China as a whole,all trends were negative except for ann insignificant positive trend in winter,but only the negative trend in spring was significant at 95% confidence level.For the regions with greater snow cover,only the positive trend over NW in winter and the negative trends over the TP in summer for snow depth were significant,and for SWE,only the negative trends over the TP in spring and summer were significant.Spatially,the statistically significant(95%-level) positive trend regions for annual mean snow depth lay in the eastern Inner Mongolia,the northern part of NE,the northwestern Xinjiang Region,and the northeastern TP,and the significant negative trend regions mainly lay in the southeast part of NE,most of North China,and the southern TP.The distribution of positive and negative trends for annual mean SWE were similar to that of snow depth in position,but not in range.The range showing positive trends was not as large as that of snow depth.SWE decreased significantly in most of China except for the most of NE and the northern Xinjiang Region in winter.