Polarization characteristics of scattered solar radiation in the presence of high-level clouds: an active-passive sensing approach

The report is dedicated to the analysis of polarization characteristics of zenith-scattered solar radiation in the presence and absence of high-level clouds (HLCs). The main objective is to evaluate the potential of passive polarimetry for detecting clouds that contain horizontally oriented ice crystals with specular reflective properties. The approach is based on a comparison of active (polarization lidar) and passive (polarimetric) measurements obtained using the High-Altitude Matrix Polarization Lidar (HAMPL) developed at the National Research Tomsk State University [1].

HLCs mainly consist of ice crystals whose shapes, sizes, and orientations vary. Under certain conditions, these particles align horizontally – such clouds are referred to as specular [2], as the flat facets of ice crystals reflect incident light in accordance with Fresnel's laws. The orientation of crystals in HLCs significantly affects both the transmission and scattering of shortwave solar radiation reaching the Earth’s surface, as well as the longwave radiation emitted back into space, making such cloud formations an important factor in climate regulation [3]. A general theory of electromagnetic wave scattering by ensembles of crystals has not yet been fully developed, nor is there a complete physical model describing the microstructure of crystalline HLCs. Therefore, identifying such clouds and determining and predicting the conditions under which they form remains a relevant challenge.

Active methods such as lidar measurements enable the identification of clouds with specular ice crystals based on a combination of features: a high polarization degree in the backscattered signal, characteristic values of the backscattering phase matrix elements, low optical thickness, and specific signal component ratios. In passive mode, HAMPL measures the Stokes vector and polarization degree of zenith-scattered sunlight. Comparing the behavior of these parameters under different conditions allows the effect of HLCs to be identified. The report analyzes data obtained on different days in 2025. In clear-sky conditions, the degree of polarization of scattered sunlight in the zenith showed a smooth diurnal variation, synchronized with changes in the solar zenith angle. In the presence of HLCs, deviations from this typical pattern were observed, especially in the afternoon. These anomalies coincided with the occurrence of cloud layers detected by the lidar at various altitudes, confirming their effect on the polarization characteristics of solar radiation.

The experimental data show that the polarization degree of scattered sunlight is a sensitive indicator of the optical properties of HLCs. This confirms the potential of polarimetric methods for diagnosing atmospheric conditions. The results highlight the importance of further research into the effect of atmospheric factors on polarization parameters and the need to improve passive polarimetry technologies, particularly when combined with active atmospheric sensing methods.

The work was performed with the financial support of the Russian Science Foundation, Grant No. 24-77-00097.

1. Bryukhanov I.D., et al. // Opt. Atm. Okeana. 2024. V. 37, No. 2. P. 105–113.

2. Platt C.M.R. // J. Appl. Meteorol. 1978. № 17. P. 1220–1224.

3. Kondratyev K.Ya. // Meteorolog. Gidrolog. 2004. No. 4. P. 93–119.