Thanks Richard. The fact that changing to the mirrored Lorentz lineshape solves 
the problem suggests to me that it is highly likely that the pressure shift is 
being added in the wrong direction for the mirrored lines if using 
abs_lines_per_speciesAddMirrorLines, hence the positive pressure shift 
coefficient is shifting the mirror line closer to zero rather than further away.

I know I’m using an “old” version of ARTS – I’ll get round to updating it once 
I’ve had a chance to get round to somehow installing cmake 3 on our systems so 
that I can actually compile anything newer…

Cheers,

Stuart

From: Richard Larsson [mailto:ric.lars...@gmail.com]
Sent: 13 October 2017 14:01
To: Fox, Stuart <stuart....@metoffice.gov.uk>
Cc: arts_dev.mi@lists.uni-hamburg.de
Subject: Re: [arts-dev] Problem with line absorption in ARTS

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<mailto: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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