祁连山区MODIS积雪反照率产品的精度验证及云下积雪反照率估算研究

积雪反照率在全球气候和能量收支平衡模型中起着重要的作用.利用祁连山地区大冬树垭口站点反照率实测数据对由TM/ETM+得到的反照率数据进行标定,然后将TM/ETM+反照率数据通过升尺度对MODIS逐日积雪反照率(SAD)产品在晴空条件下的精度进行了验证.同时,发展了一个基于MODIS SAD与AMSR-E SWE数据融合并结合Noah积雪反照率参数化方案估算MODIS SAD数据云下积雪反照率的算法,通过统计分析纠正了云对积雪反照率的影响,对云下积雪反照率进行了验证分析.结果表明:MODIS SAD产品在祁连山地区的精度要低于大面积积雪覆盖的平坦地区(如格陵兰岛),其平均绝对误差及均方根误差分别为0.0548和0.0727;云下积雪反照率估算方法可以有效地获取云覆盖下积雪像元的反照率值,纠正后的无云MODIS SAD数据与地面观测值有较好的一致性,其平均绝对误差为0.078.

Accuracy validation of the MODIS snow albedo products and estimate of the snow albedo under cloud over the Qilian Mountains

Snow surface albedo plays an important role in the radiation budget and global climate models. In this paper, the observed albedo data are used to calculate the surface albedo as the true value to validate the albedo derived from TM/ETM+ image at 30 m, then the TM/ETM+ albedo is aggregated to 500 m resolution and compared with the MOD10A1 snow albedo daily (SAD). The results show that MODIS SAD products do not reflect snow albedo very well over mountains. However, the overall absolute error, standard deviation for the MODIS SAD products is 0.0548 and 0.0727, respectively, so it still can be used to study the snow cover change. A method for estimating snow albedo under cloud is developed. By using the MOD10A1 SAD and AMSR-E snow water equivalent (SWE) products, the snow under cloud is recognized, then use the SWE and Nash LSM snow albedo model to estimate the snow albedo under cloud. We can use this method to produce MODIS daily cloudless snow albedo product. The observed albedo data are used to validate the accuracy of the new SAD product. There is a relatively consistency between the MODIS SAD products after cloud removal and the observed ones, with an overall absolute error of 0.078.