Errors induced by the neglect of polarization in radiance calculations for three-dimensional cloudy atmospheres

Abstract

Remote sensing instruments observe radiation being scattered and absorbed by molecules, aerosol particles, cloud droplets and ice crystals. In order to interpret and accurately model such observations, the vector radiative transfer equation needs to be solved, because scattering polarizes the initially unpolarized incoming solar radiation. A widely used approximation in radiative transfer theory is the neglect of polarization which allows to greatly simplify the radiative transfer equation.It is well known that the error caused by multiple Rayleigh scattering can be larger than 10%, depending on wavelength and sun-observer geometry [1]. For homogeneous plane-parallel layers of liquid cloud droplets the error is comparatively small (below 1%) [2]. Therefore, in radiative transfer modelling for cloud remote sensing polarization is mostly neglected. However, in reality clouds are not plane-parallel layers of water droplets but complex threedimensional (3D) structures and observations of clouds usually include pixels consisting of clear and cloudy parts. In this study we revisit the question of the magnitude of error due to the neglect of polarization in radiative transfer theory for a realistic 3D cloudy atmosphere.We apply the Monte Carlo radiative transfer model MYSTIC with and without neglecting polarization and compare the results. At a phase angle of 90 • and 400 nm wavelength we find the maximum overestimation error of about 8% for complete clear-sky conditions. The error is reduced to about 6% in clear-sky regions surrounded by clouds due to scattering from clouds into the clear regions. Within the clouds the error is up to 4% with the highest values in cloud shadows. In backscattering direction the radiance is underestimated by about 5% in clear regions between clouds. For other sun-observer geometries, the error ranges between the two extremes. The error decreases with wavelength and in the absorption bands.

Submillimeter Wave Instrument radiometry of the Jovian icy moons

“…The role of polarization in the radiative transfer theory itself is now relatively well understood. The neglect of polarization in radiance calculations not only induces systematic errors (Adams & Kattawar 1970;Ilyushin 2012;Emde & Mayer 2018) but also prevents qualitative analysis of the physical mechanisms of the formation of the radiative field (Ilyushin 2019). However, the information value of the polarization in radiative transfer inverse problems is still questionable, and for any particular retrieval a special assessment should be made.…”