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Background

Whether they are to be used for weather forecasting or climate projections, atmospheric radiation schemes require a fast yet accurate descrition of absorption and emission by atmospheric gases.  Gas absorption spectra can contain hundreds of thousands of absorption lines, far too many to represent explicitly. The state-of-the-art approach to approximating this complexity is the correlated k-distribution (CKD) method in which parts of the spectrum with a similar molar absorption coefficient, k, are grouped together and treated by a single pseudo-monochromatic radiation calculation. This works because absorption spectra are highly (although not perfectly) correlated in the vertical (see section 10.3 of A first course in atmospheric radiation by Grant Petty). Unfortunately, the tools and know-how to generate CKD gas optics models are historically unavailable to the vast majority of users and developers of radiation schemes. This means that practitionners must use one of the few off-the-shelf models available, whether or not it is optimized for their particular application.

What is ecCKD?

"ecCKD" is a free software tool for generating CKD gas optics models. It requires a limited amount of user input, primarily to specify the required band structure and an error tolerance, from which it can generate a CKD model in the form of look-up tables stored in a single self-describing CKD definition file in NetCDF format.  This can be read in by the free ecRad radiation scheme (currently only the ecckd branch), but the nature of the look-up table is simple enough that it could in principle be incorporated into any other radiation scheme. It is quite data intensive, requiring the 1 TB CKDMIP line-by-line gas absorption database to be available on disk.  A particular innovation is the ability to use the full-spectrum correlated-k method, first investigated for atmospheric applications by Pawlak et al. (2004) in the shortwave and Hogan (2010) in the longwave. The gas optics models generated tend to be both accurate and efficient. Their speed is characterized by the total number of "k-terms", i.e. the total number of pseudo-monochromatic radiative transfer calculations that need to be performed to compute broadband longwave or shortwave flux profiles.

Access ecCKD and documentation

  • ecCKD GitHub repository
  • Hogan and Matricardi (2022) article submitted to JAMES (coming soon)
  • User guide (PDF), generated from this source file
  • Abstract about ecCKD submitted to the International Radiation Symposium 2022

Related links

CKD definition files

Gas optics models evaluated by Hogan and Matricardi (2022), trained against the CKDMIP "Evaluation-1" dataset and evaluated against the independent "Evaluation-2" dataset:

Gas optics models trained against the combined "Evaluation-1" and "Evaluation-2" datasets:

Experimental gas optics models for radiance models used in remote sensing:


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