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

Attachment: hitran_2012_2016_example.arts
Description: hitran_2012_2016_example.arts

Attachment: hitran2012_H2O_161_162_0_200GHz.H2O.xml.gz
Description: hitran2012_H2O_161_162_0_200GHz.H2O.xml.gz

Attachment: hitran2016_H2O_161_162_0_200GHz.H2O.xml.gz
Description: hitran2016_H2O_161_162_0_200GHz.H2O.xml.gz

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