| Snow cover, as an important component of land cover, is one of the most active natural materials on the Earth’s surface. The Tibetan Plateau, with the highest elevation against the same altitude area of the world, its snow layer is affected directly and significantly under the background of global warming. The dynamics of snow cover area (SCA) exert a considerable influence on the surface albedo, climate change, hydrological and atmospheric circulation,energy balance. Meanwhile, The alternate process between accumulating and melting of snow directly affects hydrothermal environment of the soil and the growth of vegetation and further affect the activity rhythm of animals and microorganisms, thus, it can cause profound impact on plant community.Monitoring spatial and temporal variations of snow cover and analyzing the relationship with climatic factor on different scales have important social, economic, and ecological significance.In this study,10350 Moderate Resolution Imaging Spectroradiometer(MODIS) 8-day composite snow products, meteorological data from 128 ground stations, digital elevation model data and natural zone data was utilized and provided snow cover pictures from 2001 to 2015 over the Tibetan plateau after geometric correction, projection transformation, cloud removal processing, mosaic and shearing. Then, processing based on geographic information system and statistical analysis based on SPSS and R was realized to analyze annual and inter-annual variations of snow cover for 15 years.Next, using correlation and stepwise regression analysis, the variation regularity of SCA and snow cover days(SCD) of Tibetan Plateau and its relationship with climate factor were studied in different natural zone.It systematically revealed regional differences of climate factors impacts on snow cover changes, the results showed that:(1) The annual change of the snow cover can be described as "bimodal and one valley", the two peaks located in the middle of February and late October, and low was in late July, the timing of snow cover onset and melt was not stable and had the fluctuation.Among the four seasons, snow coverage of winter is the largest (21.45%), next was spring, summer (5.63%) was far lower than the other three seasons. The annual snow cover area of Tibetan Plateau is between 3.63×105 km2-4.91×105 km2, among, The SCA is large in 2002,2005 and 2008, and low in 2001,2003 and 2010. SCA decreased with slow fluctuation by the rate -1.82×104km2/10a, however, the degree of change is not obvious.Overall, SCA in last five years of study period was characterized by more obvious decline than the first two five years. Snow melting occurs mainly in the summer and results suggest a tendency for a decrease with slow fluctuation,and for the period during 2001 to 2014, the decrease was statistically significant(p<0.01) with the rate -3.15×104km2/10a. SCA of autumn made the biggest contribution to the all-year snow(r=0.778), followed by winter(r=0.667). There also was a slightly decreasing tendency from 2001 to 2015 for most of months.The results indicated that there were significant spatial differences of the Tibetan Plateau snow cover which showed uneven distribution. In the peripheral high mountains, snow cover distributed extensively and had a long duration, but in the vast interior, it was rare or even thin, and sub-frigid zone contains a few natural areas with rich snow cover. Certain patch shape distribution of snow in winter becomes scattered distribution in spring. However, snow cover in spring did not appear large ablation. Summer snow mainly located in high mountains, snow cover in the process of transition from summer to autumn accumulated and extended quickly, and also began in some zones with high altitude.(2) From SCD point of view, the area has following characteristic form majority to minority:unstable snow zone, short duration zone, middle duration zone, perennial snow-free zone, long duration zone, persistent snow zone. Overall, long duration zone and persistent snow zone were characterized by a significantly decreasing tendency (p<0.05 and p<0.01), there was a slightly decreasing tendency for perennial snow-free zone and unstable snow zone but not statistically significant.Area with longer snow days are mainly distributed in the northwestern and southern of the region and the plateau edge. it has certain correlation between the change of SCD and altitude, Kunlun mountains, karakoram, the qilian mountain, mount tunggula, the Himalayas, nyenchen tanglha were the main mountain ranges covered by long snow cover days. There was evident spatio-temporal difference for SCD change in 2001-2005, 2006-2010 and 2011-2015, but decreasing tendency is the major change in all three periods, especially in 2006-2010 and 2011-2015, decreasing area accounted for more than half of the total area. For 2001-2015, majority of the pixels (57.35%) were characterized by a decrease in SCD(S<0) and scatteredly located in the most parts of the plateau, mild decrease is the major. In contrast, only 0.13% of pixels were characterized by decreases in SCD(S>0), meanwhile,42.15% of pixels were characterized by invariability in SCD (S=0) and were mainly located in the central of the region.Most of the natural zones present decreasing tendency with fluctuation for SCD in 15 years, among them, decline in IIAB1 is significant.SCD in different temperature zone was characterized by:plateau sub-frigid zone>plateau temperate zone>Mountainous subtropical zone, these results suggest that SCD has a close relationship with overall pattern of climatic transition from warm-wet in southeast to cold and drought in northwest.(3) Further examination of the relationship between the SCA and climatic factors in different seasons, hold more significant correlation with temperature, humidity, precipitation>0.1 mm days (rainy days), precipitation, and vapor pressure.There was primary negative correlation between SCA and temperature, vapor pressure, and primary positive correlation between SCA and humidity, rainy days, precipitation. SCA in summer had very significant negative correlation with temperature (r=-0.708; p<0.01), and SCA in winter had very significant positive correlation with humidity(r=0.676;p<0.01), precipitation days(r=0.600; p<0.05), was significant negative correlation with temperature(r=0.554;p<0.01).The results of stepwise regression analysis showed that SCA in spring and winter were influenced by multiple climatic factors, and dominant factor is the humidity and vapor pressure. Snow cover change in summer is temperature driven type, The increase(0.3℃/10a) in mean temperature in summer, could also contribute to the summer melting of persistent snow cover(-1.29×104km2/10a). SCA in winter was most sensitive to humidity (p<0.01), wet and cold climate is conducive to the maintaining and extension of winter snow.In different natural zones, the correlation of SCD with climatic factors presented spatial variation. There was primary negative correlation between SCD and temperature, sunshine time, sunshine percentage, and primary positive correlation between SCD and humidity, rainy days. The influences of precipitation on SCD in different natural present the opposite result. Stepwise regression suggested that SCD in most of natural zones was most sensitive to the change of the temperature and humidity, SCD change in Ngari montane was a typical temperature-humidity influencing type. SCD in Southern Tibet montane and Eastern Qinghai-Qilian montane was negatively affected by temperature. Humidity played the leading role of SCD in Golog-Nagqu. Vapor pressure was also the important climate factor which affecting SCD in some natural zones, precipitation days was a remarkable positive factor driving the change of SCD in Southern Qinghai. In summary, the sensitivity of SCD of different zones to various important climatic factors varies, and climatic driving and factor contribution rate vary to some extent. |
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