> Ok, we can recommend but not require. Do you think "surface" is > reasonable to assume when z is missing?No, just because Z is missing you can't assume that it's at the surface.It could be something like "tropopause" and just that there is no additional data to set that reference.
In CF, data on particular surfaces that aren't defined by particular coord variables has this intended in the standard name e.g. "X_at_tropopause" or "X_at_sea_level". Cheers Jonathan