Hi again,

Great that we agree on the problem. OK, let's keep the present definition of VMR (that it refers to sum of all gases, not just "constant" ones).

We should then for sure introduce a rescaling method (or maybe several). I expressed myself poorly, I rather meant that introducing such a method is not a fully complete solution, if we consider the "fine print". What I had in mind is the Jacobian, the coupling between variable and constant gases should theoretically go into the expressions for the Jacobian. But that's just a "smart" comment. I don't say that it should be implemented, which would be a pain. Then Stuart's comment is more relevant, this could have consequences for the values given to absorption models.

To make the rescaling method easy to apply, I would suggest to make one specific for Earth, that automatically base the rescaling on H2O. There could be a generic one as well.

Yes, this puts some weight on the user. Hydrostatic equilibrium (HSE) is a similar case. Input profiles do not always fulfil HSE (this is the case for Fascod, if not a mater of geopotential vs geometric altitudes?). For HSE it is up to the user to apply this "fine tuning" or not. This including to include adding call of the HSE method in OEM iterations, to make sure that HSE is maintained after an iteration. The VMR rescaling should also be included in the iteration agenda, if the retrieval can change H2O close to the ground. That is, a VMR rescaling would not be something completely new, as I see it.



On 2021-09-16 15:01, Stefan Buehler wrote:

With our present definition of VMRs, we agree on that having 78% N2, 21% O2 and 
e.g. 3% H2O is unphysical? That with a lot of H2O (or any other non-fixed gas) 
the standard values of the fixed gases should be scaled downwards. In the 
example above, with 0.97. Do you agree?

Yes, I agree.

It seems a bit weird to me to use this definition at the (low) level of the 
absorption routines. Perhaps one solutions would be to have an option for this 
behaviour when ingesting concentration profile data? Perhaps by passing in a 
list of species that should be considered as not adding to the denominator for 
the VMR definition.

If we agree on the above, then this is the simplest (but not most theoretically 
correct) solution.

Why not correct?


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