MODIS/Aqua Land Surface Temperature/3-Band Emissivity 8-Day L3 Global 0.05Deg CMG V061

A new suite of Moderate Resolution Imaging Spectroradiometer (MODIS) Land Surface Temperature and Emissivity (LST&E) products are available in Collection 6.1. The MYD21 Land Surface Temperature (LST) algorithm differs from the algorithm of the [MYD11](https://doi.org/10.5067/modis/myd11_l2.061) LST products, in that the MYD21 algorithm is based on the ASTER Temperature/Emissivity Separation (TES) technique, whereas the MYD11 uses the split-window technique. The MYD21 TES algorithm uses a physics-based algorithm to dynamically retrieve both the LST and spectral emissivity simultaneously from the MODIS thermal infrared bands 29, 31, and 32. The TES algorithm is combined with an improved Water Vapor Scaling (WVS) atmospheric correction scheme to stabilize the retrieval during very warm and humid conditions.

The MYD21C2 dataset is an 8-day composite LST product that uses an algorithm based on a simple averaging method. The algorithm calculates the average from all the cloud free [MYD21A1D](http://doi.org/10.5067/MODIS/MYD21A1D.061) and [MYD21A1N](http://doi.org/10.5067/MODIS/MYD21A1N.061) daily acquisitions from the 8-day period. Unlike the MOD21A1 data sets where the daytime and nighttime acquisitions are separate products, the MYD21A2 contains both daytime and nighttime acquisitions as separate Science Dataset (SDS) layers within a single Hierarchical Data Format (HDF) file. The LST, Quality Control (QC), view zenith angle, and viewing time have separate day and night SDS layers, while the values for the MODIS emissivity bands 29, 31, and 32 are the average of both the nighttime and daytime acquisitions. Additional details regarding the method used to create this Level 3 (L3) product are available in the Algorithm Theoretical Basis Document (ATBD).

Known Issues
* Users of MODIS LST products may notice an increase in occurrences of [extreme high temperature outliers](https://landweb.modaps.eosdis.nasa.gov/displayissue?id=117) in the unfiltered MxD21 Version 6 and 6.1 products compared to the heritage MxD11 LST products. This can occur especially over desert regions like the Sahara where undetected cloud and dust can negatively impact both the MxD21 and MxD11 retrieval algorithms.
* In the MxD11 LST products, these contaminated pixels are flagged in the algorithm and set to fill values in the output products based on differences in the band 32 and band 31 radiances used in the generalized split window algorithm. In the MxD21 LST products, values for the contaminated pixels are retained in the output products (and may result in overestimated temperatures), and users need to apply Quality Control (QC) filtering and other error analyses for filtering out bad values. High temperature outlier thresholds are not employed in MxD21 since it would potentially remove naturally occurring hot surface targets such as fires and lava flows.
* High atmospheric aerosol optical depth (AOD) caused by vast dust outbreaks in the Sahara and other deserts highlighted in the example documentation are the primary reason for high outlier surface temperature values (and corresponding low emissivity values) in the MxD21 LST products. Future versions of the MxD21 product will include a dust flag from the MODIS aerosol product and/or brightness temperature look up tables to filter out contaminated dust pixels. It should be noted that in the MxD11B day/night algorithm products, more advanced cloud filtering is employed in the multi-day products based on a temporal analysis of historical LST over cloudy areas. This may result in more stringent filtering of dust contaminated pixels in these products.
* In order to mitigate the impact of dust in the MxD21 V6 and 6.1 products, the science team recommends using a combination of the existing QC bits, emissivity values, and estimated product errors, to confidently remove bad pixels from analysis. For more details, refer to this dust and cloud contamination [example documentation](https://landweb.modaps.eosdis.nasa.gov/data/userguide/MOD21_dust_QC_examples.pdf).
* For complete information about known issues please refer to the [MODIS/VIIRS Land Quality Assessment website](https://landweb.modaps.eosdis.nasa.gov/knownissue?sensor=MODIS&sat=Aqua&as=61).

新一代中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）地表温度与发射率（Land Surface Temperature and Emissivity, LST&E）产品套件已在数据集合集6.1（Collection 6.1）中发布。MYD21地表温度（Land Surface Temperature, LST）算法与[MYD11](https://doi.org/10.5067/modis/myd11_l2.061)产品的算法存在差异：MYD21算法基于ASTER温度/发射率分离（Temperature/Emissivity Separation, TES）技术，而MYD11则采用分裂窗算法。MYD21 TES算法采用基于物理原理的方法，可同时从MODIS热红外波段29、31和32中动态反演地表温度与光谱发射率。该算法结合了改进的水汽缩放（Water Vapor Scaling, WVS）大气校正方案，以在极端高温高湿环境下稳定反演过程。

MYD21C2数据集为8天合成地表温度产品，其算法基于简单平均方法：从8天周期内所有无云的[MYD21A1D](http://doi.org/10.5067/MODIS/MYD21A1D.061)与[MYD21A1N](http://doi.org/10.5067/MODIS/MYD21A1N.061)每日观测数据中计算平均值。与昼、夜观测分别作为独立产品的MOD21A1数据集不同，MYD21A2将昼、夜观测数据整合为单个分层数据格式（Hierarchical Data Format, HDF）文件中的独立科学数据集（Science Dataset, SDS）图层。其中，地表温度、质量控制（Quality Control, QC）、观测天顶角与观测时间均设有昼、夜独立的SDS图层，而MODIS发射率波段29、31和32的数值则为昼、夜观测的平均值。有关该3级（Level 3, L3）产品的制作方法的更多细节，可参阅算法理论基础文档（Algorithm Theoretical Basis Document, ATBD）。

已知问题
* MODIS LST产品用户可能会发现，相较于传统MxD11 LST产品，未经过滤的MxD21 6版及6.1版产品中[极端高温异常值](https://landweb.modaps.eosdis.nasa.gov/displayissue?id=117)的出现频次有所增加。这种情况在撒哈拉等沙漠区域尤为突出，未被检测到的云与尘埃会对MxD21和MxD11的反演算法产生负面影响。
* 在MxD11 LST产品中，算法会基于广义分裂窗算法所用的32波段与31波段辐射亮度差异，对受污染的像素进行标记，并将其在输出产品中设为填充值。而在MxD21 LST产品中，受污染像素的数值会被保留在输出结果中（可能导致温度被高估），用户需通过质量控制过滤与其他误差分析手段剔除无效值。MxD21未采用高温异常值阈值，因为这可能会误移除火灾、熔岩流等自然高温地表目标。
* 撒哈拉及其他沙漠区域大规模沙尘爆发导致的大气气溶胶光学厚度（Aerosol Optical Depth, AOD）升高，是MxD21 LST产品中出现高异常地表温度值（以及对应低发射率值）的主要原因。未来版本的MxD21产品将加入来自MODIS气溶胶产品的沙尘标记和/或亮温度查找表，以过滤受沙尘污染的像素。需注意的是，在MxD11B昼夜算法产品中，基于多云区域历史地表温度的时间分析，多日产品已采用了更先进的云过滤手段，这可能会对受沙尘污染的像素进行更严格的过滤。
* 为缓解MxD21 V6及6.1版产品中的沙尘影响，科学团队建议结合现有质量控制比特位、发射率数值与产品估算误差，以可靠地从分析中移除无效像素。更多细节可参阅该沙尘与云污染[示例文档](https://landweb.modaps.eosdis.nasa.gov/data/userguide/MOD21_dust_QC_examples.pdf)。
* 如需了解已知问题的完整信息，请参阅[MODIS/VIIRS陆地质量评估网站](https://landweb.modaps.eosdis.nasa.gov/knownissue?sensor=MODIS&sat=Aqua&as=61)。