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

Overview

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:

Detailed evaluation of the climate-fsck-27 model

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.