积雪是地表覆盖的重要组成部分,也是气候变化的良好指示器,监测积雪变化对研究全球水循环、气候变化有着极为重要的意义.本文利用正-背景数据学习算法,即Presence and Background Learning Model(PBL模型)对中国积雪进行估测,得到Special Sensor Microwave Imager 5 days(SSM/I 5 d)中国积雪分布图,并利用Mann-Kendall趋势检验法对1992~2010年中国积雪进行了时空变化分析,具体探讨了年积雪日数变化、积雪年季分布与变化特征、稳定积雪区变化.结果表明, PBL模型在中国地区积雪估测精度表现良好,在与Interactive Multisensor Snow and Ice Mapping System(IMS), MODIS/Terra Snow Cover 8-Day L3 Global 500 m Grid(MOD10A2)等积雪产品的比较中取得了较高的一致性,年平均总体精度达到0.88. 1992~2010年间中国三大主要积雪区积雪日数都有显著下降趋势,但青藏高原西北部则有显著上升趋势.前者是由气温上升所引起,后者则是由降水增多所引起.在积雪的季节分布上,新疆与东北-内蒙古地区积雪面积都是冬季最大,春、秋季相对较小;青藏高原积雪面积则是春、秋、冬季都很大,积雪的季节变化规律远不如其他两大积雪区明显.研究期间,中国稳定积雪区面积无显著变化趋势,仅青藏高原年降雪日数波动较大.
英文摘要:
Snow cover is an important component of land cover on the Earth's surface and is also a good indicator of climate change. Hence, monitoring the spatial distribution and temporal variation of snow cover is of great significance to the study of the global water cycle and climate change. Traditional snow cover monitoring has been primarily based on in situ observations; however, the uneven and low-density distribution of meteorological stations made it difficult to reflect the overall picture of snow cover in some regions. To solve this problem, we used the Presence and Background Learning (PBL) algorithm to estimate the snow cover in China and obtained the 5 days (5 d) snow cover maps of the Special Sensor Microwave/Imager (SSM/I). The PBL model is a type of one-class classifier that needs no negative training sample in the training set. The cornerstone of this method is to combine SSM/I Brightness Temperature data and in situ observations to estimate the probability of the existence of snow cover based on the Artificial Neural Network (ANN). The estimation result indicates that the average annual overall accuracy of the PBL model in China is 0.88, which shows good agreement with the Interactive Multisensor Snow and Ice Mapping System (IMS) snow cover products. Compared with other snow cover products, such as MODIS/Terra Snow Cover 8-Day L3 Global 500 m Grid (MOD10A2) and AMSR-E/Aqua 5-Day L3 Global Snow Water Equivalent EASE-Grids(AE_5DSno), the performance of the PBL model is better at predicting the snow cover distribution in China. After obtaining the SSM/I 5d snow cover maps of China, we analysed the temporal and spatial variations of snow cover in China during 1992-2010 using the Mann-Kendall test, which included the variation of annual snow cover days, seasonal snow cover distribution and change characteristics, and the variation of the stable snow cover area. The results suggest that China's snow cover is primarily distributed in the Tibetan Plateau, Xinjiang, northeast China, and Inner Mongolia, which are all high altitude or high latitude regions. From 1992-2010, the following occurred. (1) The number of snow cover days decreased significantly in the three major snow cover regions of China due to rising temperatures, while there was an observably upward trend in the northwestern Tibetan Plateau with the increase of precipitation. (2) The snow cover area of Xinjiang and Northeast-Inner Mongolia reached its maximum in winter and was relatively small in the spring and autumn. The Tibetan Plateau's snow cover area is generally the largest in the spring, and it is also very large in the autumn and winter. Thus, the seasonal variation characteristics of snow cover there is not as obvious as that of the other two major snow regions. (3) In the four years of 1996, 2003, 2004 and 2006, the snow cover area in the spring in Xinjiang was much lower than that in the autumn, which indicates a warmer winter event caused by warmer than usual temperatures in the winter. (4) Although no significant change in the snow cover area was found in all regions during the study period, the change in snow cover on the Tibetan Plateau is particularly noteworthy due to its area having increased greatly since 2005, which might be the beginning of the increase of snow cover area in this region. (5) The stable snow cover area of China is 3.39 million km~2. In the three major snow cover regions, the stable snow cover area of the Tibetan Plateau is the largest (1.68 million km~2), the Northeast- Inner Mongolia region is the second largest (1.05 million km~2), and the Xinjiang region is the smallest (0.63 million km~2). The stable snow cover area had no significant variation trend in China during 1992-2010only the snow cover days of the Tibetan Plateau had a larger interannual fluctuation.