Hello @johnwilkin and @JonathanGregory, Thanks for these points.
I think that the use of the `positive` attribute to determine the direction is sufficient and elegant - that works for me. I am now quite troubled by the use `nsigma`, though! How it is applied is surely dependent on whether one assumes one-based or zero-base indices `k`, as @JonathanGregory pointed out. And the possibility of it becoming incorrect when the vertical dimension is subsetted is not good. These problems all disappear, I think, if we insist on missing values for the out-of-range portions of `zlev` and `sigma`... It's just occured to me that we dispense with `nsigma` and use missing values instead, surely we don't need to know the direction at all? and could omit the new text that says: _"If the layers are numbered top-down i.e. with `k = 1` nearest the surface and increasing numbers for greater depth, then `sigma(k)` is defined for `1 \<= k \<= nsigma`. If the layers are numbered bottom-up i.e. with `k = 1` nearest the sea floor, then `sigma(k)` is defined for `nlayer - nsigma \<= k \<= nlayer`, where `nlayer` is the number of layers."_ With missing values, doesn't "_for levels k where sigma(k) has a defined value and zlev(k) is not defined:_ _for levels k where zlev(k) has a defined value and sigma(k) is not defined:"_ give us all we need? David -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://urldefense.us/v3/__https://github.com/cf-convention/cf-conventions/issues/314*issuecomment-825037412__;Iw!!G2kpM7uM-TzIFchu!jiDl7OQBsa__Sw2ebQ7a1BKqVPV4-0G4oS_vkswjd6HwauBrq03uyEPs_N5RCs8kSYTsqc5l9t0$ This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
