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 longwave results are from version is 0.5 and the shortwave results from version 0.6. CKD models are generated from CKDMIP datasets using the following automated steps:
- The band structure is specified, and then within each band the high-resolution absorption spectrum of each individual gas is reordered using the median/present-day concentration case from the MMM dataset, with the ordering according to the height of the peak cooling rate in the longwave, and the height at which the optical depth from TOA reaches 0.25 in the shortwave. For the NWP applications, all gases except H2O and O3 are merged into a single "composite" gas. Note that the same ordering is used at all heights.
- An error tolerance is specified by the user, and the reordered spectra for each gas are divided into as many k terms (g points) as are needed so that the RMSE in heating rate for any individual k term is less than the specified tolerance. The partitioning of the spectrum is adjusted so that the error associated with each k term in a band (for single gas) is approximately equal.
- The k terms for the individual gases are combined to obtain a final set of k terms using the hypercube partition method of Hogan (2010).
- The Idealized dataset is used to compute a the look-up tables of absorption coefficient for each gas in each k term.
- A quasi-Newton scheme is used to optimize the coefficients of the look-up table to minimize the errors when computing heating rates and fluxes for the Evaluation-1 dataset.
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Shortwave CKD models have been constructed with a range of k terms using the wide and narrow CKDMIP band structures. The full-spectrum correlated-k method was tried but the results are not yet satisfactory. As in the longwave, the CKD models are optimized by minimizing the errors against the Evaluation-1 CKDMIP line-by-line dataset, so the evaluations here are not yet truy truly independent. The full set of plots are available in PDF files for each of the three CKDMIP "applications":
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Naturally the models tend to become more accurate with increasing numbers of k terms and in terms of RMSE in fluxes there appears to be scope for further increasing accuracy by futher increasing the number of k terms. When comparing models using the narrow and wide band structure for the same number of k terms, the narrow-band models tend to have a larger flux bias but a lower upper-atmosphere heating-rate RMSE. The plots for the climate-wide-38 model are presented and discussed next.
Detailed evaluation of the climate-
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wide-38 model
To illustrate the performance of ecCKD, the plots are shown for one of the better shortwave CKD models it produced, targeting the climate application with the wide band structure and 38 k terms. The parts of the spectrum contributing to each k term are illustrated below. ecCKD automatically selects k terms with the aim of each one having a roughly similar error in reproducing the fluxes and heating rates in the parts of the spectrum it represents. It can be seen here that in the first three wide bands the k terms have been selected similarly to the longwave according to the locations of the strongest and weakest absorption lines. In the fourth wide band where line absorption is very weak, five k terms are used stacked in order of increasing Rayleigh optical depth. In the final wide band the k terms are selected according to the strength of continuum absorption by ozone and molecular oxygen.
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