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":
- Evaluation of longwave ecCKD models targeting limited-area NWP
- Evaluation of ecCKD models targeting global NWP
- Evaluation of ecCKD models targeting climate modelling
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:
- All models show a negative bias in downwelling surface longwave that gets systematically worse for warmer and moister atmospheres. No such trend exists for OLR, although there is still a negative bias of typically 0.3 W m-2 that could be improved.
- The radiative forcing by methane tends to be too linear, not capturing the extent of the logarithmic dependence. This is believed to be because the automated procedure for allocating k terms allocates too few to the methane bands, far fewer than are allocated to carbon dioxide, for which the logarithmic dependence is well captured. This could be improved by reducing the error threshold for the allocation of methane k terms.
- ecCKD optimizes both broadband fluxes/heating rates, as well as those in individual bands. As a result, there is some compensation of errors between various bands which is larger than it should be in models with large numbers of k terms.
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. The parts of the spectrum contributing to each k term are illustrated below. The first term represents the most optically thin parts of the spectrum while terms 2-27 target the absorption by an individual gas. Most of the terms are for CO2 and H2O, except for terms 2, 17 and 21 for O3, term 10 for N2O and term 11 for CH4.
The following plots evaluate fluxes and heating rates for the four CKDMIP greenhouse-gas scenarios "Present", "Preindustrial", "Glacial Maximum" and "Future" (click on individual plots to expand). The evaluation has been performed using radiative transfer with four zenith angles in each hemisphere (8 streams). The shaded regions in the central three panels of each plot encompass 95% of the data. The upwelling TOA fluxes are underestimated by 0.3-0.4 W m-2.
