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- 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.
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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.
