A new 3D Monte Carlo Radiative Transfer Model Including Polarization: Validations and Applications

English Abstract:

Non-spherical particles have a considerable impact on climate (Liou, 1986; Kaufman et al., 2002; IPCC, 2013). They absorb, scatter and change the polarization state of solar radiation depending on their shape, size, chemical composition and orientation. Even though these processes are complex, their knowledge is the prior condition for the interpretation of remote-sensing measurements. Additionally, the investigation of ground-based and airborne passive radiometric and polarization measurements require a vector radiative transfer model that accounts for both multiple scattering and polarization. Here we introduce a new three-dimensional (3D) vector radiative transfer model, SPARTA “Solver for Polarized Atmospheric Radiative Transfer Applications” and its validation through benchmark results. The model is based on the statistical Monte Carlo (MC) method (in the forward scheme) and calculates column-response pixel-based polarized radiative densities for 3D inhomogeneous cloudy atmospheres. Validations of the model output for different cases (Rayleigh scattering, aerosol particles, etc.) have been carried out against benchmark results, indicating an excellent agreement, considering the noise of the MC method in radiance calculations. Increasing the number of photons will reduce the noise. Two-dimensional (2D) runs for lidar-based Saharan dust cloud fields from SAMUM experiment will be discussed, performing 2D and one-dimensional (1D) reflectance calculations using SPARTA. In the areas with large spatial gradient in optical thickness, the radiance fields of the 2D scenario differ about ± 13% from the fields of the 1D scenario.