A geometry-dependent surface Lambertian-equivalent reflectivity product at 466 nm for UV/Vis retrievals: Part I. Evaluation over land surfaces using measurements from OMI

Abstract. The anisotropy of the Earth's surface reflection has implications for satellite-based retrieval algorithms that utilize climatological surface reflectivity databases that do not depend upon the observation geometry. This is the case for most of the current ultraviolet and visible (UV/Vis) cloud, aerosol, and trace-gas algorithms. The illumination-observation dependence of surface reflection is described by the bidirectional reflectance distribution function (BRDF). To account for the BRDF effect, we use the concept of geometry-dependent surface Lambertian-equivalent reflectivity (GLER), which is derived from the top-of-atmosphere (TOA) radiance computed with Rayleigh scattering and surface BRDF for the exact geometry of a satellite-based pixel. We present details on the implementation of land and water surface BRDF models. We evaluate our GLER product over land surfaces using observed and computed sun-normalized radiances at 466 nm. The input surface BRDF parameters for computing TOA radiance are derived from MODerate-resolution Imaging Spectroradiometer (MODIS) satellite observations. The observed TOA radiance for comparison is from the Ozone Monitoring Instrument (OMI). The comparison shows good agreement between observed and calculated OMI radiances in typical geographical regions, with correlation coefficients greater than 0.9 for a majority of the selected regions in the year of 2006. Seasonal variations of clear-sky OMI radiances (i.e., with minimum clouds and aerosols) closely follow those computed using MODIS-derived GLER over land. GLER also captures the cross-track dependence of OMI radiances, although the observations are slightly higher than those computed using GLER presumably owing to residual cloud and aerosol (non-absorbing) contamination, particularly over dark surfaces (heavily vegetated regions such as mixed forest, croplands and grasslands). The standard OMI climatological surface reflectivity database predicts higher radiances than GLER and OMI observations with different seasonal variations and cross-track dependence over most of the selected regions. Overall, our evaluation demonstrates that the GLER product adequately accounts for surface BRDF effects while at the same time simplifies the surface BRDF implementation within the existing OMI retrieval infrastructure; use of our GLER product requires changes only to the input surface reflectivity database.