...

- We should list the steps involved in creating CKD models, and study in detail the impact of different methodologies in each of these steps. The How CKD Tools Work page has a brief summary of how our tools work, and could form a basis for this.
**It would be useful if models who have not yet submitted information for this page could do so**. - What is the best way to spectrally average absorption coefficients to k terms, and
**what is the impact of different averaging techniques for the various tools?**- A quick study of the impact on longwave ecCKD models is shown here, implying that the best method amongst those tried is to average the transmission across an equivalent atmospheric layer over which pressure changes by a factor of 3 (although ecCKD's subsequent optimization step was found to be crucial).
- A recent paper by Webb, Solovjov and André suggests that the optimal approach is the geometric mean of the absorption coefficients at the bounds of the spectral interval in g space, i.e.
*k*_{opt }= sqrt(*k*_{1}x*k*_{2}) = exp((ln*k*_{1}+ ln*k*_{2})/2). This is similar to logarithmic averaging, but only considers the bounds of the spectral interval.

- Is there a fundamental limit to the accuracy of correlated-k models even with large numbers of k terms? This is implied by Slide 11 or Robin's ecCKD talk, and in some other talks, and is likely due to the inability to represent the imperfect rank correlation of spectra at different heights.
**It would be useful if CKDMIP participants could submit models with quite large numbers of k terms**(ideally all using the same "narrow" band structure) so that we can see if this fundamental limit is reached in all models, and whether the limiting accuracy is similar between models. What can we do about it? SOCRATES has a method to treat imperfect correlation in height - can this be shared and applied by other tools? - Other crucial aspects are the way spectra are sorted (either separately per pressure/temperature/gas or with a single sorting per gas) and the treatment of gas overlap.
**Can we test different approaches using the same tool?** - It would be great to have some more submissions from different tools, ideally using at least the "narrow" band structure.

...