基于MODIS的青藏高原季节性积雪去云方法可行性比较研究

青藏高原地处中纬度地区，季节性积雪分布破碎，地面观测站点稀少，中分辨率成像光谱仪（MODIS）可为该地区提供每日积雪监测数据，然而云是光学遥感最大的影响因素，为研究MODIS每日积雪产品去云方法在青藏高原的适用性，根据原理将去云方法归纳总结为五大类，并对每种方法的“潜在假设”开展分析讨论。结果显示：基于时间连续性的方法适用性强，去云效果明显，上下午积雪连续的平均概率为72.5%，而2~5d的连续积雪的概率为5.6%~43%不等，可靠性差；临近像元法可去除零散分布的云，平均正确率达到95.5%，但去除云量较少；基于高程的去云算法在山区适用性好，而在高原腹地由于坡度较小而错判概率较大；采用被动微波遥感数据进行去云则依赖于微波对云的识别率，往往误差较大；采用数学方法拟合积雪边界在积雪破碎、降雪融雪较快的青藏高原地区，物理意义较弱。通过分析研究表明，青藏高原地区MODIS日积雪产品的去云，需综合多种算法的区域适用性，充分考虑青藏高原地形及积雪本身的特征，逐步完善每日积雪去云工作。

Comparative study of the feasibility of cloud removal methods based on MODIS seasonal snow cover data over the Tibetan Plateau

The Tibetan Plateau is located in a mid-latitude area,where seasonal snow cover scatters spatially and climate stations is not enough. Moderate Resolution Imaging Spectrometer(MODIS)provides daily products of snow cover for this region. However,cloud cover is the major limitation to optical remote sensing observation. In order to analyze the applicability of the cloud removal methods,the methods are generalized into five catego-ries according to their principles,and then comparative analysis is carried out to search their "hidden assump-tions". The result demonstrates that methods based on time continuity is well applied and has obvious cloud re-moval effect. The consistency rate of Terra and Aqua is 72.5%on average. While the consistency of 2-days to 5-days ranges from 5.6%to 43%,of which the reliability is poor. The adjacent pixels method combination is able to remove a little cloud,with a high correct rate of 95.5%,but the removed cloud amount is only a little. The method based on altitude is well applied in high mountain areas,while it is not working well in plateau hinter-land due to its small slopes. Passive microwave remote sensing data for cloud removal are dependent on the rec-ognition rate of microwave on cloud,which brings about larger error. Other mathematical method is of weak physical significance in the Tibetan Plateau,where snowfall and snowmelt is frequently and snow cover is scat-tered. By comparing and analyzing the methods above,it is acquired that regional advantages and characteristics of the Tibetan Plateau should be considered into cloud removal,in order to gradually improve the MODIS daily snow cover product.