Abstract
The lidar equation for ocean at optical wavelengths including subsurface signals is revisited using the recent work of the radiative transfer and ocean color community for passive measurements. The previous form of the specular and subsurface echo term are corrected from their heritage, which originated from passive remote sensing of whitecaps, and is improved for more accurate use in future lidar research. A corrected expression for specular and subsurface lidar return is presented. The previous formalism does not correctly address angular dependency of specular lidar return and overestimates the subsurface term by a factor ranging from 89% to 194% for a nadir pointing lidar. Suggestions for future improvements to the lidar equation are also presented.
Highlights
Ocean surface return analysis from spaceborne active remote sensing is a promising subject of study
We found that Eq (2) does not estimate the subsurface term correctly
The increase of lidar return at high wind speed present in CALIPSO observations [2] is consistent with treating the whitecaps as a lambertian surface and the fraction of the surface covered with whitecaps, W, as a power law of wind speed
Summary
Ocean surface return analysis from spaceborne active remote sensing is a promising subject of study. We can cite Bufton [4] who provided the first lidar equation including specular and subsurface terms and Menzies [5] who used a more complete formalism including whitecap reflectance We used those studies to analyze CALIPSO [6] specular returns and derive quantitative measurements of wind speed [1] and aerosol optical thickness [2]. With several space-based lidar missions being developed using UV lidars such as the Earth Cloud and Aerosol Radiation Explorer (Earthcare), the Aerosol Cloud Ecosystem (ACE mission), and one with large off-nadir angles (ADM aeolus), a correct formalism to estimate the surface and subsurface return is critical This further emphasizes the need for a precise determination of the calibration error arising from the use of the ocean as a reference target and for a better understanding of the oceanic subsurface processes. We have relied on what has been developed in the last decades for passive and active measurements
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