IPRT polarized radiative transfer model intercomparison project – Phase A
Introduction
An increasing number of remote sensing instruments measure the polarization state of electromagnetic radiation. Therefore, polarized radiative transfer codes are required to interpret and analyse the measurements and to develop retrieval algorithms. Sensors that measure polarization from space are, e.g., the Polarization and Directionality of the Earths Reflectances (POLDER) instrument onboard PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) [1] and the Thermal and Near Infrared Sensor for Carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) on the Greenhouse gas Observing SATellite GOSAT [2]. Future missions include e.g. the Climate Absolute Radiance and Refractivity Observatory (CLARREO) [3] and the Multi-Viewing Multi-Channel Multi-Polarization Imaging mission (3MI) on METOP-SG (Meteorological Operational Satellite – Second Generation). All-sky imaging systems are available to measure the polarized radiance distribution; such systems are described for instance by Liu and Voss [4], Kreuter et al. [5] and references therein. The Research Scanning Polarimeter (RSP) [6], [7] has been used for ground-based as well as airborne aerosol measurements. Other multi-channel polarimetric instruments are the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) [8] and the Observing System Including PolaRisation in the Solar Infrared Spectrum (OSIRIS) [9]. The commercially available ground-based polarimeter, CE318-DP, developed by CIMEL Electronic (Paris, France) is now available at several AERONET stations [10].
A large number of models for polarized radiative transfer have been developed in the last years for various specific applications. They mostly have been validated against existing benchmark data; e.g., Coulson et al. [11] and Nataraj et al. [12] for Rayleigh scattering; e.g., de Haan et al. [13]; Wauben et al. [14]; Garcia and Siewert [15] for layers including aerosols; and Kokhanovsky et al. [16] for cloud and aerosol scattering including realistic phase matrices. More references to published benchmark results are given on the IPRT website, section “benchmark results”. However, all existing benchmark results are limited to one or two plane-parallel layers with an underlying Lambertian surface. To simulate the measurements of the above-mentioned sensors, far more realistic settings are required. Reasonable height profiles of molecules, aerosols and clouds should be taken into account. For clear-sky atmospheres, a plane-parallel model geometry is a reasonable approximation. When clouds are analysed it is also important to look into effects resulting from the geometrical structure of clouds, commonly called 3D-effects, hence validated 3D vector radiative transfer codes are required. In order to simulate limb observations, fully spherical vector codes are needed. Polarization by the surface must be considered, in particular for aerosol remote sensing from space.
In order to support model developers and to set standards for polarized radiative transfer modelling the International Radiation Commission (IRC) has established the working group “International Polarized Radiative Transfer” (IPRT) which is charged by the task to provide benchmark data for polarized radiative transfer simulations for realistic atmospheric setups as needed to simulate the current and future satellite, airborne and ground-based polarimetric sensors. In order to establish this benchmark dataset a model intercomparison project has been launched. This paper summarizes the results from the first phase of the project. Six vector radiative transfer models from various international institutions have participated. The models use different approaches to solve the vector radiative transfer equation, among them are deterministic approaches based on discrete ordinates or spherical harmonics and also statistical approaches based on Monte Carlo methods. The test cases in the first phase include simple one-layer setups, cases with polarized surface reflection, and cases with realistic height profiles of molecules and aerosol particles.
The focus of this intercomparison project is the establishment of benchmark results, therefore all models were run in high accuracy mode. For realistic applications with limited computational time the models are usually run with lower accuracy. The first intercomparisons between models showed several larger differences, some of them due to model errors which have been fixed in the course of this project. The participants were allowed to provide corrected or more accurate data. Finally a very good agreement for all test cases has been found for most models. The commonly established benchmark results are available at the IPRT website (http://www.meteo.physik.uni-muenchen.de/iprt). The next phase of the intercomparison project will start soon with focus on 3D radiative transfer.
Section snippets
Radiative transfer models
Table 1 includes a summary of features of the participating radiative transfer models.
Model coordinate system and Stokes vector
For all test cases the Stokes parameters, which are defined as time averages of linear combinations of the electromagnetic field vector [60], [61], [62], [63], are calculated asHere, El and Er are the components of the electric field vector parallel and perpendicular to the reference plane respectively. The pre-factor on the right hand side contains the electric permittivity ϵ and the magnetic permeability μp.
The model coordinate
Model intercomparison
This section presents results of all models for an exemplary selection of viewing angles. For each test case, one or two plots show the simulated Stokes vector and the absolute differences between MYSTIC and IPOL, SPARTA, Pstar, SHDOM, and 3DMCPOL respectively. Out of range values are shown as arrows in the difference plots. Comparison plots for all viewing directions are provided on the IPRT website and as supplementary material.
In order to quantify the level of agreement between the models we
Conclusion and outlook
Overall, we found a very good agreement between all models and for the test cases of this intercomparison project. The achieved level of agreement is very high, for cases without clouds the relative root mean square difference is mostly below 0.05% for total intensity and linear polarization. However some significant deviations were found: for non-zero depolarization factors, for ocean reflection, and for simulations including cloud droplets. For these settings some of the models need to be
Acknowledgements
We thank Dr. Michael Mishchenko for providing the code to calculate the ocean reflectance matrix. Furthermore we thank Dr. Josef Gasteiger for providing optical properties of the aspherical aerosol particles.
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