Radiative Transfer Workshop III - Test Cases

The intercomparison will be carried out in two parts. The first part deals with a comparison of the absorption coefficient calculation (case 1 and 2), while the second part concerns the radiative transfer (case 3 and 4). Case 1 and case 3 are calculations with a full specified set of input parameters with the purpose of making a direct comparison of the model implementations. Case 2 and case 4 are calculations where only some input parameters are common and the remaining ones are to be set by the participants. These later cases are meant to test the absolute uncertainties that are present in the calculations (including uncertainties in input parameters).
The intercomparison of absorption coefficients will be carried out separately for line-by-line absorption and for continuum absorption.
Regarding the radiative transfer, different geometries will be considered: down, up, and limb looking.

A brief description of the cases and the corresponding input for each case is found below. We ask the participants to follow the instructions as close as possible.

Absorption Coefficient Intercomparison

The intercomparison of the absorption coefficients will be made in two steps, checking both, the correctness of the implementation (case 1), and the uncertainty in calculation when some of the input parameters are varying (case 2).

For the line-by-line absorption, a number of target lines were selected:

  • H2O@22.2GHz
  • O3@110.83GHz
  • O2@118.7GHz
  • H2O@183.3GHz
  • HCN@354GHz
  • ClO@501GHz
  • N2O@502GHz
  • HNO3@544GHz
  • CO@576GHz
  • O3@625.37GHz
  • HCl@625.9GHz
  • ClO@649.45GHz
A spectral range of 1 GHz, in steps of 10 MHz, around the center frequency of each selected line will be considered.

Beside these lines, the spectral range corresponding to same window regions were also includes.

Case 1: Check of the Absorption Coefficient Implementation.

The aim of this case is to check the implementation of the absorption coefficient calculation. A distinction will be made between the line absorption (case 1A) and continuum absorption (case 1B).

Case 2: Absorption Coefficient - Model Uncertainties

This case aims to investigate the uncertainties in different models when they use their usual standard input. Absorption coefficients should be computed for specified atmospheric conditions and frequencies but letting the remaining parameters (e.g. spectroscopic data, line shape) to be chosen by the participants.

Radiative Transfer Intercomparison

The intercomparison will be carried out for checking both, the implementation (case 3), and uncertainties in different models (case 4). For each case, the calculations will be performed for different geometries: down, up and limb looking. Corresponding to each case and geometry, specific input is provided.

Case 3: Check of the Radiative Transfer Algorithm Implementation

The aim of this case is to compare different radiate transfer algorithms. Pre-calculated absorption coefficients will be used as input and refraction will be neglected. A full description of needed input parameters, such as instrumental characteristics, is provided. The participants shall calculate both pencil beam monochromatic spectra and spectra as recorded by the instrument.

Case 4: Radiative Transfer - Full Model Uncertainties.

This case concerns the total deviation between the forward models. Deviations will be caused by the uncertainties both in absorption coefficient calculation and in radiative transfer calculation. Refraction, when having a significant influence, will also be considered. The calculations will be performed for given frequencies, atmospheric conditions, and instrumental characteristics but not fixed assumptions for the other parameters. The resulting spectra seen by the instrument will be compared.