Evaluating Geometry-Dependent Surface Lambertian-Equivalent Reflectivity (GLER) over Water with OMI L1B measurements

Trace gas retrievals such as NO₂ from satellites rely on accurate modeling of the surface reflectivity. Traditional algorithms take advantage of reflectivity climatologies which do not properly account for the anisotropy of the Earth's surface. In order to account for the anisotropy of the Earth's surface a Geometry-dependent surface Lambertian-equivalent reflectivity (GLER) approach has been developed, which can be implemented within existing cloud, aerosol and trace gas operational retrievals. The first version of the GLER product at 466 nm was tailored to the OMI FOVs (13km by 24km at nadir) by averaging the high spatial resolution (1km) bidirectional-reflectance distribution function (BRDF) information from NASA's MODIS MCD43GF product over land and the Cox-Munk surface wave slope distribution over water with a contribution from water-leaving radiance. The vector discrete ordinate radiative transfer model (VLIDORT) is used to model the radiances for the exact viewing geometry of each OMI FOV. Lambertian equivalent reflectivities (LER) derived from OMI L1B radiances at 466 nm screened for aerosols and clouds are used to evaluate the GLER product. Over water GLER is a few percent lower than the OMI derived LER. The greatest difference occurs over sun glint regions possibly due to both the strong sensitivity to, and uncertainty in, the wind speed measurements used for GLER calculations. GLERs at shorter wavelengths such as 354 nm are also compared with OMI-derived LERs as they can be important in rotational-Raman scattering and O₂-O₂ cloud algorithms. In future work, GLER will be produced for other spectral regions and potentially other satellite instruments (e.g., GOME-2, OMPS, S5P/TROPOMI).