All-sky polarization imaging of cloud thermodynamic phase

Abstract

Knowing the cloud thermodynamic phase (if a cloud is composed of ice crystals or liquid droplets) is crucial for many cloud remote sensing measurements. Further, this knowledge can help in simulating and interpreting cloud radiation measurements to better understand the role of clouds in climate, weather, and optical propagation. Knobelspiesse et al. [Atmos. Meas. Tech. 8, 1537, (2015)] showed that, for simulated zenith observations, the algebraic sign of the S 1 Stokes parameter (related to the difference between perpendicular and parallel linear polarization in the scattering plane) can be used to detect cloud thermodynamic phase when observed with a ground-based passive polarimeter. In this paper, we describe the use of our all-sky imaging polarimeter to experimentally test this proposed method of detecting cloud thermodynamic phase in the entire sky dome. The zenith cloud phase was validated with a dual-polarization lidar instrument.

Aerosol-induced changes in sky polarization pattern: potential hint on applications in polarimetric remote sensing

“…Beside these applications, the sky polarization pattern is one unique application direction of polarimetric remote sensing (Eshelman, Tauc, and Shaw 2019), and is a useful tool in recognizing atmosphere state (Hasekamp, Litvinov, and Butz 2011), navigation application (Chahl and Mizutani 2012), etc. According to Rayleigh scattering theory, a concentric pattern of degree of polarization (DOP) exists in the sky determined by the position of sun.…”

Visualization of all-sky polarization images referenced in the instrument, scattering, and solar principal planes

“…Maximum error for the linear Stokes parameters was estimated to be AE1.2% with 100% linear input. This instrument has been used to characterize how skylight polarization changes with aerosols, 4,5 clouds, 6,7 and the underlying surface reflectance. 9 3 Sky Polarization Reference Planes The skylight DoLP has a distinct pattern that rotates through the sky in time, based on the position of the sun.…”

“…Although Rayleigh scattering theory predicts 100% polarization at a scattering angle of 90 deg, the actual maximum DoLP value is smaller because of multiple scattering from aerosols and the ground. [1][2][3][4][5][6][7][8][9][10] This pattern moves through the sky with the sun, with a magnitude that changes with environmental conditions and wavelength. 11,12 The DoLP pattern is, however, independent of the orientation of the polarizing elements used in the measurements.…”

“…The method presented by Li et al 45 was used to transform the Stokes S 1 and S 2 parameters from the instrument plane to the solar principal plane by using a rotation Mueller matrix [Eq. (6)]. The AoP in the solar principal plane was then derived from the realigned Stokes parameters.…”

“…In the atmosphere, the primary source of polarization is scattering by gas molecules; however, aerosols, [1][2][3][4][5] clouds, [6][7][8] and underlying surface reflectance 9,10 alter the observed sky polarization. To interpret polarization measurements or simulations correctly, it is important to understand how partially polarized skylight varies with environmental factors, [1][2][3][4][5][6][7][8][9][10] as well as with wavelength and solar position. [10][11][12] This is critical for military, environmental, and navigational applications.…”