Sentinel-3B OLCI Level-3 Global Binned Earth-observation Reduced Resolution (ERR) Diffuse Attenuation Coefficient of Downwelling Irradiance (KD) Data, version 2022.0

The diffuse attenuation coefficient of downwelling irradiance at 490 nm, Kd_490, quantifies how quickly blue-green light diminishes with depth—an indicator of water clarity and the depth to which sunlight penetrates. Kd_490 is generated from ocean-color reflectance using the KD2 algorithm. Lower values indicate clearer water (deeper light penetration); users commonly apply Kd_490 to estimate euphotic depth, track coastal water quality and turbidity, interpret bloom/light environments, and supply inputs to biogeochemical and ecosystem models. As with all ocean-color products, uncertainties can be larger in optically complex coastal or inland waters; refer to the file metadata and algorithm documentation for quality flags and usage notes.

This activity was informed by the Satellite Needs Working Group (SNWG), an interagency effort of the U.S. Government dedicated to identifying and addressing Earth observation needs across U.S. civilian federal agencies.

Geophysical variables in this suite include:
- Kd_490 — Diffuse attenuation coefficient at 490 nm, KD2 algorithm (m⁻¹)

490纳米处下行辐照度的漫衰减系数Kd_490（diffuse attenuation coefficient of downwelling irradiance at 490 nm）可量化蓝绿光随深度的衰减速率，是反映水体透明度与太阳光穿透深度的指标。Kd_490通过KD2算法（KD2 algorithm）由海洋水色反射率（ocean-color reflectance）反演生成。较低的Kd_490值对应透明度更高的水体（太阳光穿透深度更深）；使用者通常可借助Kd_490估算真光层深度（euphotic depth）、监测近岸水体水质与浊度、阐释水华与光照环境，并为生物地球化学及生态系统模型提供输入参数。与所有海洋水色产品类似，该数据在光学特性复杂的近岸或内陆水体中不确定性更高；相关质量标识与使用说明请参阅数据集元数据及算法文档。

本数据集的研制工作由卫星需求工作组（Satellite Needs Working Group, SNWG）提供指导，该工作组为美国政府跨部门协作机构，致力于识别并满足美国民用联邦各机构的地球观测需求。

本数据集套件包含的地球物理变量（geophysical variables）如下：
- Kd_490：490纳米处漫衰减系数，采用KD2算法计算，单位为m⁻¹。