Retrieval of cloud fraction and optical thickness from multi-angle polarization observations

Abstract. We introduce an innovative method to retrieve cloud fraction and optical thickness based on polarimetry. The approach is well-suited for satellite observations providing multi-angle polarization measurements, such as the Hyper-Angular Rainbow Polarimeter (HARP2), the Spectro-Polarimeter for Planetary EXploration (SPEX), and the Multi-viewing Multi-channel Multi-polarisation Imager (3MI). The cloud fraction and the cloud optical thickness can be derived for each pixel from measurements at two viewing angles: one within the cloudbow at a scattering angle of approximately 140° and a second in the sun-glint region or at a scattering angle of approximately 90°. In the cloudbow, the degree of polarization depends mainly on the cloud optical thickness. Conversely, for a viewing direction in the sun-glint region or around 90° scattering angle, the degree of polarization depends on the clear fraction of the pixel, because at these scattering angles radiation scattered by cloud droplets is almost unpolarized whereas radiation reflected by the surface or scattered by molecules is highly polarized. Utilizing these dependencies, we developed a straightforward retrieval algorithm using a lookup-table approach. As a demonstration, we apply the methodology to airborne observations from polarization cameras of the Munich Aerosol Cloud Scanner (specMACS) instrument. The high spatial resolution data (10–20 m) has been averaged to a spatial resolution of approximately 2.5 km to mimic satellite observations. A comparison of the derived cloud fractions with the high spatial resolution images for specific cases, featuring low, medium and high cloud fractions, demonstrates the expected performance of the retrieval.