Hi Stuart,

Yes, you cannot change VMR if you do this.  It is true you can also decide to 
divide by 0.20946 instead of vmr[I] to allow this change.  This would be more 
inline with what is done today, so still not in line with what is done in the 
original model.

About the other models, I copied this from one of them so the issues should be 
the same there as here.  I don’t have any strong opinion on these and how it 
should change.  Others will have to comment if they want to keep the current 
faulty version or not.  I think the newest one should change.

With hope,

> On 22. Oct 2021, at 12:52, Fox, Stuart <stuart....@metoffice.gov.uk> wrote:
> Hi Richard,
> I’m not sure about your proposed change to line 13421. As I understand it, if 
> you do this then you will calculate the same absorption for oxygen regardless 
> of what the user inputs for the O2 vmr. Is this desired behaviour? The 
> MPM-type coefficients and equations in the Tretyakov 2005 paper assume a 
> particular vmr (0.20946). Is it more correct to divide the calculated 
> absorption by this value, and then multiply by the user-supplied vmr at the 
> stage of the absorption calculation? 
> I also notice that the same VMRISO is in lots of the other complete O2 models 
> (MPM87, MPM89 etc), so this isn’t restricted to Tretyakov-05
> Stuart
> From: Richard Larsson <ric.lars...@gmail.com> 
> Sent: 22 October 2021 10:45
> To: Fox, Stuart <stuart....@metoffice.gov.uk>
> Cc: arts_dev.mi@lists.uni-hamburg.de
> Subject: Re: Another Tretyakov-05 O2 absorption question
> This email was received from an external source.   Always check sender 
> details, links & attachments.
> Hi Stuart,
> I think you have found a bug.  The isotopologue ratio is not fixed anywhere, 
> yet the VMRISO variable has been scaled down by its assumed value.  The 
> comment for this value is simply false today, it must have changed sometime 
> without anyone noticing.
> I think that VMRISO should be changed to simply be equal to the input vmr 
> value.
> The particular line for the VMRISO number today is legacy_continua.cc::13327. 
>  If that line is commented out and instead line legacy_continua.cc::13421 is 
> rewritten as "(...) / vmr[i]", I think you will get better results.
> Could you check if this change improves your results?
> With hope,
> //Richard
> Den mån 18 okt. 2021 kl 12:28 skrev Fox, Stuart <stuart....@metoffice.gov.uk>:
> Hi ARTS devs,
> I’ve got another question about the Tretyakov-05 complete absorption 
> implementation in ARTS. In the code for TRE05O2AbsModel, the line absorption 
> is calculated, and then converted in the final line into a partial 
> cross-section. When calculating this cross-section a constant equal to the O2 
> vmr assumed by Tretyakov 2005 (0.20946) multiplied by the main isotope 
> abundance is used, rather than just the O2 vmr. Should the isotope abundance 
> really be included here? Presumably it depends on whether it is also included 
> at the stage of the absorption calculation, or if the total O2 vmr is used 
> there, but I’m not familiar enough with the ARTS code to know which is the 
> case? I guess I would expect it should work properly for the case where the 
> user defines “abs_species_O2” without explicitly setting an isotopologue 
> ratio; the tests I’ve done suggest this is not currently the case.
> Regards,
> Stuart

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