MODIS V006和V005全球反照率产品精度对比分析

摘要

反照率作为一种非常重要的地表能量平衡、全球变化研究的参数,在众多研究领域中得到了广泛的应用,到目前为止已经有多种全球范围的反照率产品进行业务化生产和发布,针对不同反照率产品质量评价的研究也变得愈加重要.MODLAND团队在MODIS V005反照率产品反演算法的基础上通过改进16天周期内观测数据加权的方法生产出新版本的反照率产品MODIS V006.本文针对MODIS两个版本V005及V006的反照率产品,利用FLUXNET地面站点数据,比较验证两个版本反照率的总体精度以及在不同地表类型条件下的精度差异,同时通过交叉验证的方法分析二者的差异及稳定性.验证结果表明,MODIS V006反照率产品虽然在全反演高质量的数据比例上较V005有所下降,但是在同等条件下V006在提高时间分辨率的同时其精度也有所提高,在不同的地表类型条件下精度也优于V005,且在时间序列分布上具有稳定比例的高质量数据,可以满足大多数应用的精度需求.%Land surface albedos represent an important parameter in the Earth's energy balance,and they play a crucial role in the global change and climate systems.Numerous global or regional land surface albedo products with various spatial resolutions and temporal frequencies have been released.Thus,the quality analysis of albedo products has become increasingly important.On the basis of the algorithm of a version 5 albedo product (MODIS V005),a new daily product version (MODIS V006) with a high temporal resolution is generated with MODLAND.The new albedo product requires urgent validation,and its accuracy should be known before extensive use.On the basis of sites with adequate spatial representativeness in the FLUXNET network,direct comparisons between different versions ofalbedo products and in situ albedos were conducted.To investigate the accuracy under various land cover type conditions of the two versions of albedo products,we divided the ground measurements into six classes and calculated several evaluation indicators.Two different versions of products were also compared using a cross-validation method under the prior knowledge that V005 has been extensively estimated and that the method achieves good performance in various validation works.At the same time,the Quality Control (QC) flag was investigated to analyze the stability of the new albedo products.The direct comparison results indicate that two versions of products show good consistency against the in situ albedos.However,V006 performs better than V005 during the same experimental time.For the accuracy under different land cover types,all the evaluation indicators briefly show that V006 products achieve a better match with in situ albedos than V005 products.As the V006 has a high temporal resolution,accurately capturing the rapid changes of land surface albedos is relatively easy,especially during snow-covered days.The comparison result obtained through the cross-validation method indicates that except for a few slightly high values,the V006 product effectively matches the V005 product.Although the percentage of QC flag indicating the good quality of the V006 product is lower than that of the V005 product,data show the stable performance of both products in the span of three years.The MODIS V006 product has a shorter temporal resolution and higher accuracy than the V005 product.The trends of the two albedo products over the time series highlight their good consistency.However,V006 performs better than MODIS V005 during snow-covered times.This finding is expected to play a significant role in many other research fields.