How quickly sunlight fades into the ocean (NOAA-21, near-real-time)

The Ocean Biology DAAC produces near real-time (NRT) products using the best-available combination of ancillary data from meteorological and ozone data. As such, the inputs and the calibration used are less than optimal. Near real-time products provide a snapshot of the data during a short time period within a single orbit. The KD data suite provides diffuse attenuation coefficient of downwelling irradiance at 490 nm, Kd_490, which 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. Geophysical variables in this suite include: - Kd_490 — Diffuse attenuation coefficient of downwelling irradiance at 490 nm, KD2 algorithm (m⁻¹))