# IPRT polarized radiative transfer model intercomparison project - Three-dimensional test cases (phase B)

## TL;DR

This paper reports on the IPRT project's intercomparison of 3D polarized radiative transfer models using complex cloud and aerosol scenarios to establish benchmark results for remote sensing applications.

## Contribution

It extends previous 1D intercomparisons to 2D and 3D cases, providing benchmark results for polarized radiative transfer models in complex atmospheric conditions.

## Key findings

- Benchmark results for 3D polarized radiative transfer models are established.
- The intercomparison includes complex cloud fields generated by Large Eddy Simulation.
- Results are available on the IPRT website for use in remote sensing validation.

## Abstract

Initially unpolarized solar radiation becomes polarized by scattering in the Earth's atmosphere. In particular molecular scattering polarizes electromagnetic radiation, but also scattering of radiation at aerosols, cloud droplets and ice crystals polarizes. Each atmospheric constituent produces a characteristic polarization signal, thus spectro-polarimetric measurements are frequently employed for remote sensing of aerosol and cloud properties. Retrieval algorithms require efficient radiative transfer models. Usually, these apply the plane-parallel approximation, assuming that the atmosphere consists of horizontally homogeneous layers. For remote sensing applications, the radiance is considered constant over the instantaneous field-of-view of the instrument and each sensor element is treated independently in plane-parallel approximation, neglecting horizontal radiation transport between adjacent pixels. In order to estimate the errors due to the IPA approximation, three-dimensional (3D) vector radiative transfer models are required. So far, only a few such models exist. Therefore, the International Polarized Radiative Transfer (IPRT) working group of the International Radiation Commission (IRC) has initiated a model intercomparison project in order to provide benchmark results for polarized radiative transfer. The group has already performed an intercomparison for one-dimensional (1D) multi-layer test cases (Emde et al., 2015). This paper presents the continuation of the intercomparison project for 2D and 3D test cases: a step cloud, a cubic cloud, and a more realistic scenario including a 3D cloud field generated by a Large Eddy Simulation model and typical background aerosols. The commonly established benchmark results for 3D polarized radiative transfer are available at the IPRT website (http://www.meteo.physik.uni-muenchen.de/~iprt).