How quickly ocean water dims sunlight (Terra)
What it measures. A global map of how quickly blue-green sunlight fades as it goes down through ocean water (the diffuse attenuation coefficient at 490 nm), which is a stand-in for water clarity and how deep light reaches. Lower values mean clearer water.
How it's made. Calculated from ocean-color reflectance measured by the MODIS instrument on the Terra satellite, run through an algorithm (KD2) and binned onto a global grid.
How & where you'd use it. Scientists use it to estimate how deep sunlight penetrates, track coastal water quality and murkiness, interpret algae blooms, and feed ocean ecosystem models; results are less certain in murky coastal or inland waters.
What's measured
Coverage & cadence
- Time span2000-02-24 → ongoing
- Measured byTerra (MODIS)
- Processing levelLevel 3
- Spatial extent-180, -90, 180, 90
- FormatsnetCDF-4
- StatusACTIVE
What you can do with it
- Watch sea-surface temperature and marine heatwaves
- Spot algal blooms and ocean-colour shifts
- Support fisheries and coastal monitoring
Official description
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)
Get the data
import earthaccess
earthaccess.login(strategy="netrc") # free Earthdata Login
results = earthaccess.search_data(
short_name="MODIST_L3b_KD",
version="2022.0",
bounding_box=(-122.5, 37.2, -121.8, 37.9), # your area (W,S,E,N)
temporal=("2024-01-01", "2024-12-31"), # your dates
)
files = earthaccess.open(results) # stream straight from OB_CLOUDBrowsing CMR needs no login. Downloading or streaming bytes needs a free Earthdata Login + the earthaccess package. Official links
- Earthdata Search allows users to search, discover, visualize, refine, and access NASA Earth Observation data. GET DATA
- View the version history of this product. VIEW RELATED INFORMATION
- NASA's Ocean Color Web - Data Distribution Site GET DATA
- Terra Homepage VIEW RELATED INFORMATION
- MODIS Homepage VIEW RELATED INFORMATION
- Satellite data procesing can be difficult. We're here to help! Visit the Oceandata Help Hub to navigate learning content for accessing visualizing, and analyzing data products. VIEW RELATED INFORMATION
- Algorithm Theoretical Basis Document for Diffuse Attenuation Coefficient of Downwelling Irradiance (KD) VIEW RELATED INFORMATION