Polarization Calibration of Spaceborne Lidar Using Dense Cirrus–Scattered Solar Background with Molecular Scattering Correction

Abstract. Accurate calibration is essential for spaceborne polarization-sensitive lidars, as biases in depolarization ratio measurements can significantly affect the retrieval of cloud and aerosol properties. A polarization calibration technique based on solar background signals scattered by optically thick ice clouds (OTIC) provides a semi-continuous daytime calibration capability that complements onboard pseudo-depolarizer (PD) methods. This method was successfully applied to data from the Cloud-Aerosol Transport System (CATS) lidar at 1064 nm, where molecular scattering effects are negligible. However, at shorter wavelengths, molecular scattering of sunlight between the lidar and the OTIC layer polarizes the background signal and introduces systematic biases. We present a molecular scattering correction (MSC) scheme based on vector radiative transfer modeling (VRTM) to account for this effect and demonstrate its performance using observations from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The results show that molecular scattering introduces a daytime bias of approximately 1 % at 532 nm, which is effectively removed by the VRTM-based MSC, yielding close agreement with onboard PD calibrations. For the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) Atmospheric Lidar (ATLID) operating at 355 nm, model calculations indicate that molecular scattering contributions can be more than five times larger than at 532 nm, underscoring the necessity of applying an MSC when the OTIC calibration technique is employed. Together, these results establish the OTIC calibration technique, combined with MSC, as a robust approach for achieving accurate polarization calibration across current and future spaceborne lidar missions.