Advancing Ocean and Land Surface Remote Sensing with EarthCARE’s lidar ATLID

The ESA-JAXA EarthCARE mission delivers cutting‑edge measurements of clouds, aerosols, and Earth’s radiation budget, quantifying the coupled interactions among the three. A key instrument on the mission is the high‑spectral‑resolution lidar (ATLID), which produces data useful beyond its primary role in atmospheric science. Although developed for atmospheric observations, ATLID’s ability to quantify the near-surface ocean backscatter also supports ocean‑optical applications, including examining how subsurface lidar signal attenuation is influenced by optical constituents such as phytoplankton and colored dissolved organic matter.

The co‑polar Mie surface return from ATLID provides estimates of aerosol and cloud optical depth, which are essential for calibrating the near-surface ocean Rayleigh signal. Once corrected for atmospheric attenuation, the isolated Rayleigh component can be used to infer chlorophyll concentrations using established bio-optical models. The surface depolarization ratio from ATLID also enables reliable discrimination between ocean and sea ice, ensuring chlorophyll retrievals are limited to open-water areas. The methodology’s validation includes using NASA HSRL2 data from the NightBLUE campaign to corroborate ocean subsurface retrievals.

The global performance of ATLID-derived chlorophyll retrievals is validated through comparisons with established satellite data from PACE-OCI, Aqua-MODIS and Sentinel-3 OLCI, as well as reanalysis products from the Copernicus Marine Environment Monitoring Service (CMEMS). Initial findings show strong agreement, with ATLID successfully capturing large-scale chlorophyll gradients, particularly in open-ocean areas. ATLID’s ability to operate in high latitudes and night-time conditions, where passive sensors face limitations, represents an important step forward. These capabilities show promise in extending the temporal and spatial coverage of ocean-color data. The retrieved chlorophyll concentrations may be used to help refine estimates of ocean albedo within EarthCARE’s Level 2 radiative‑closure studies.

Additionally, over land ATLID surface depolarization ratios correlates well with the Normalized Difference Vegetation Index (NDVI) and, over desert surfaces, also shows a relationship with the TROPOMI Lambertian Equivalent Reflectance (LER). This demonstrates ATLID’s ability to characterize surface-atmosphere interactions and reinforces its relevance across both ocean and land domains.

In summary, EarthCARE ATLID’s surface return, corrected for aerosol attenuation using the co-polar Mie surface returns, introduces a novel and unique method for global chlorophyll retrievals. This first demonstration showcases how atmospheric lidar can complement existing remote sensing products like MODIS, OLCI, and CMEMS, while offering valuable contributions to both ocean and land classification, such as desert albedo and NDVI analysis.