This page evaluates the ECMWF CKD tool "ecCKD", which builds on the methods described by Hogan (JAS 2010) and is under active development; the latest version is 0.5.
Longwave CKD models have been constructed with a range of k terms (g points) using not only the wide and narrow CKDMIP band structures, but also a single band for the entire longwave spectrum (the full-spectrum correlated-k method, FSCK). Note that the CKD models are optimized by minimizing the errors against the Evaluation-1 CKDMIP line-by-line dataset, so the evaluations here are not truly independent. Indepdent evaluation will be possible when the Evaluation-2 dataset is produced. The full set of plots are available in PDF files for each of the three CKDMIP "applications":
Six CKD models have been generated for each of the three applications and three band structures, leading to a total of 54 models. Some of the detailed information at these links may be summarized in terms of the relationship between accuracy (as quantified by six error metrics) of a CKD model and its efficiency (as characterized by the total number of k terms), which for the "climate" application is as follows:
Naturally the models tend to become more accurate with increasing numbers of k terms, although there appears to be a limit above which the number of k terms does not improve accuracy. The FSCK performance is typically better than either of the two band structures for the same number of k terms, and the performance is still reasonable even when the total number of k terms is only of order 20. The plots for the climate-fsck-27 model are presented and discussed below. The various plots show a few areas where ecCKD could be improved:
To illustrate the performance of ecCKD, the plots are shown for one of the better CKD models it produced, targeting the climate application with the FSCK band structure and 27 k terms.