Stuart,

I am also seeing the problem but I have no time to investigate in detail.
As a temporary solution, there exists another lineshape called "Mirrored
Lorentz" that you can use.  Testing your controlfile with it instead, the
results are more reasonable.  Simply change your

abs_lines_per_speciesAddMirrorLines

to

abs_lineshapeDefine(shape="Mirrored Lorentz", forefactor="VVW",
cutoff=750e9)

and you will see the results.  (Or just the original "Lorentz"
lineshape-call in your main controlfile.)

With hope,
//Richard

Ps. You are running an old version of arts.  The controlfile was not
working without changing the order of calls around.  I attach an updated
version.  I also recommend that you set ARTS_DATA_PATH to your
arts-xml-data path because this makes life easier (export
ARTS_DATA_PATH="PATH-TO-DATA" should do the trick).

I might have time to look at the details over the weekend and will get back
to you later but I hope the temporary solution is good for now.

2017-10-13 14:31 GMT+02:00 Fox, Stuart <stuart....@metoffice.gov.uk>:

> Hi all,
>
>
>
> I wasn’t sure whether to ask this on arts-user or arts-dev - I’m not sure
> whether there’s a bug in ARTS or something wrong with my inputs!
>
>
>
> I’m trying to do some clear sky brightness temperature simulations for
> Earth using water vapour lines from HITRAN-2016. However, I am getting huge
> (over 20K) differences in brightness temperature for a nadir viewing
> geometry depending on whether I include the mirror (negative frequency)
> lines or not. I don’t get the same thing using HITRAN-2012.
>
>
>
> Attached is a fairly minimal example controlfile, and extracts of H2O
> lines from the two versions of HITRAN. To see the difference then compare
> the two output files depending on which of the line sets is used. (The path
> to the data from arts-xml-data will need to be changed according to your
> setup). For HITRAN 2012 the numbers are pretty much the same. For HITRAN
> 2016 they are very different.
>
>
>
> I’ve done some digging and isolated the main cause of the issue to the
> lowest frequency H2O-162 line in HITRAN-2016 (at 21764932Hz). It seems to
> be a combination of the mirror of this line, the pressure shift and the VVW
> forefactor that causes the difference. My guess is that the pressure shift
> (at the highest pressures in the profile) is pushing the mirror line to a
> frequency very close to zero, possibly causing numerical issues in the
> forefactor/lineshape. (I also wonder if the pressure shift is being added
> in the wrong direction for the mirror lines?)
>
>
>
> Does anyone who knows the ARTS internals better than me fancy digging
> around to understand what is happening, or perhaps tell me that I’m doing
> something wrong!
>
>
>
> Cheers,
>
>
>
> Stuart
>
>
>
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>

Attachment: hitran_2012_2016_example.arts
Description: Binary data

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