Reprocessed MODIS Version 6.1 Leaf Area Index dataset

IntroductionThe leaf area index (LAI) data sets were generated by reprocessing the MODIS version 6.1 LAI products.The raw data used include the MODIS LAI Version 6.1 products MCD15A2H (2002.7.4-2021), MOD15A2H (2000.2.18-2002.6.26) (Myneni et al., 2021) and MODIS Land Cover Type product MCD12Q1 (2001-2021) (Friedl and Sulla-Menashe, 2022).The algorithm is mainly based on the two-step integrated method developed by Yuan et al. (2011), and the method of background value calculation was updated.For each year being reprocessed, the nearest 9-year data include itself are used.We use the corresponding year's land cover data for calculating the multi-year average, local per class mean, per class mean and multi-year per class mean (ref. Fig. 4, Yuan et al., 2011).Data descriptionThese monthly LAI data were provided at 0.5-degree resolution covering the period 2000-2021. Data of each year is stored in one NetCDF file, namely lai_monthly_0.5_{YEAR}.nc.For LAI data with more spatial or temporal resolutions, see Land-Atmosphere Interaction Research Group at Sun Yat-sen University (bnu.edu.cn).CautionThe reprocessed data for downloading consists two MODIS version 6.1 products, i.e., MCD15A2H (2002.7.4-2021) and MOD15A2H (2000.2.18-2002.6.26). The prefix “MCD” stands for a combined product, whose algorithm chooses the best pixel available from all the acquisitions of both MODIS sensors located on NASA’s Terra and Aqua satellites, while “MOD” data are retrieved only from the Terra satellite. We have found that their temporal-mean values were different especially in the equatorial region, which may result in an unrealistic trend (see Lin et al., 2022 for detailed discussion). Therefore, attention should be paid when using the reprocessed products for long-term trend analysis and data starting from year 2003 (i.e., only MCD) was recommended for LAI trend study. <strong>Data citation</strong>Lin, W.; Yuan, H.; Dong, W.; Zhang, S.; Liu, S.; Wei, N.; Lu, X.; Wei, Z.; Hu, Y.; Dai, Y. Reprocessed MODIS Version 6.1 Leaf Area Index Dataset and Its Evaluation for Land Surface and Climate Modeling. <em style="color:rgb(34, 34, 34);">Remote Sens.</em> <strong style="color:rgb(34, 34, 34);">2023</strong>, <em style="color:rgb(34, 34, 34);">15</em>, 1780. https://doi.org/10.3390/rs15071780Yuan, H., Dai, Y., Xiao, Z., Ji, D., Shangguan, W., 2011. Reprocessing the MODIS Leaf Area Index Products for Land Surface and Climate Modelling. Remote Sensing of Environment, 115(5), 1171-1187. doi:10.1016/j.rse.2011.01.001

一、引言
本数据集通过再处理MODIS 6.1版本叶面积指数（Leaf Area Index，LAI）产品生成。所用原始数据包括MODIS LAI 6.1版本产品MCD15A2H（2002.7.4-2021）、MOD15A2H（2000.2.18-2002.6.26）（Myneni等，2021），以及MODIS土地覆盖类型产品MCD12Q1（2001-2021）（Friedl与Sulla-Menashe，2022）。
本研究采用的算法主要基于Yuan等人（2011）提出的两步集成方法，并对背景值计算流程进行了更新。针对每一个待再处理的年份，我们选取包含该年份在内的最近9年的数据进行处理。我们使用对应年份的土地覆盖数据，计算多年平均值、局地类均值、类均值以及多年类均值（参见Yuan等人2011年研究中的图4）。

二、数据说明
本数据集为2000-2021年的月尺度LAI数据，空间分辨率为0.5度。每年的数据存储为一个NetCDF格式文件，命名格式为`lai_monthly_0.5_{YEAR}.nc`。如需更高空间或时间分辨率的LAI数据，请访问中山大学陆气相互作用研究组官网（bnu.edu.cn）。

三、注意事项
本次提供的可下载再处理数据包含两款MODIS 6.1版本产品：MCD15A2H（2002.7.4-2021）与MOD15A2H（2000.2.18-2002.6.26）。前缀“MCD”代表联合产品，其算法会从NASA的Terra和Aqua两颗卫星搭载的MODIS传感器的所有观测数据中选取最优像元；而“MOD”前缀的数据仅从Terra卫星获取。我们发现两类数据的时间平均值存在差异，尤其在赤道区域，这可能导致不合理的趋势结果（详细讨论参见Lin等人2022年的研究）。因此，在使用该再处理产品进行长期趋势分析时需格外谨慎，建议使用2003年及之后的数据（仅MCD系列产品）开展LAI趋势研究。

四、数据引用
1. Lin, W.; Yuan, H.; Dong, W.; Zhang, S.; Liu, S.; Wei, N.; Lu, X.; Wei, Z.; Hu, Y.; Dai, Y. 再处理MODIS 6.1版本叶面积指数数据集及其在陆面与气候模拟中的评估. *遥感（Remote Sens.）*, 2023, 15, 1780. https://doi.org/10.3390/rs15071780
2. Yuan, H., Dai, Y., Xiao, Z., Ji, D., Shangguan, W., 2011. 用于陆面与气候模拟的MODIS叶面积指数产品再处理. *环境遥感（Remote Sensing of Environment）*, 115(5), 1171-1187. doi:10.1016/j.rse.2011.01.001