Hi Stuart, You are correct. The central frequency of the mirrored lines in arts are just set to be negative, letting all the other calculations go on as usual. So the addition of pressure shift is not handled correctly. (It even slows some functions down because we have to apply abs(F0), but this was not considered in regards to the shifting.)
Two possible solutions exists: 1) Change so F0 in line 1350 of absorption.cc has the absolute value applied to it before the shift and is then multiplied by its actual sign. 2) Remove the mirrored lines function and enforce F0>0, and tell people to use the existing "Mirrored Lorentz" instead when they want to use the transformation lineshape. Option 2 means that all simulations will be slightly faster because it reduces the number of calculations and we an remove some existing abs(F0) complications. Option 1 means people do not have to change controlfiles but it makes all code slightly slower. I argue we go with option 2. The present implementation of how we deal with the pressure shift is not really compatible with HTP, so it will have to change at some point soon anyways. What do others on the developer list say? With hope, //Richard 2017-10-13 15:07 GMT+02:00 Fox, Stuart <stuart....@metoffice.gov.uk>: > 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>: > > 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 > > > > > _______________________________________________ > arts_dev.mi mailing list > arts_dev.mi@lists.uni-hamburg.de > https://mailman.rrz.uni-hamburg.de/mailman/listinfo/arts_dev.mi > > >
_______________________________________________ arts_dev.mi mailing list arts_dev.mi@lists.uni-hamburg.de https://mailman.rrz.uni-hamburg.de/mailman/listinfo/arts_dev.mi
