Hi,
Thanks, I'll see if I can reduce that value by lowering the apriori cov for the polyfit order 0 then. So the "single continuum fit" was really just setting a single altitude and retrieving a singe H2O VMR value at this altitude whereas the profile case was the same retrievals (species, apriori cov) but with an apriori profile instead of a single value. I'll check the rel unit then, I was not aware this could be used to retrieved a single scaling value on a profile. Best regards, Eric ________________________________ De : Patrick Eriksson <patrick.eriks...@chalmers.se> Envoyé : vendredi 11 juin 2021 10:23:58 À : Sauvageat, Eric (IAP); arts_users...@mailman.rrz.uni-hamburg.de Objet : Re: [arts-users] Tropospheric continuum retrieval in ARTS Eric, It seems indeed that poly_order=0 is involved. I think you want this value to be small. Especially as it seems to only be positive. This behaviour would be OK if you thought that you have a calibration error that is random, but just one-sided. Sounds not reasonable. I think more realistic is to trust the calibration and let the tropospheric part take care of this part. poly_order=0 and the tropospheric correction have a very similar impact on the "baseline". The resulting tropospheric attenuation depends to some extent on the retrieved humidity profile, but that should be secondary. But you can of course take a look at how the tropospheric humidity profile look in both cases and compare. By the way, how do you do the single column case? I suggest to use the rel unit. So you scale the profile with a single relative value. Using VMR for this is less realistic (here you would change the full profile with the same VMR value, that is not realistic). Regards, Patrick On 2021-06-11 10:02, eric.sauvag...@iap.unibe.ch wrote: > Dear Patrick, > > > Thanks for you answer. > > > I did some further investigations on the value of the polyfit and as > expected the value of the constant terms is indeed quite different for > both options and very similar for 1st and 2nd degree terms (I appended > some basic representative plots to give an idea). > > > More specifically, it is smaller in the case of the "single continuum > fit" which is the one giving a +10% ozone profile. It would mean > that tropospheric attenuation is considered to be larger in the case of > the single continuum fit compared to the other option. I think this is > then consistent with the 10% higher ozone values obtained in this case, > would you agree ? > > > I remember having tried different constraint on the constant term but > only making it larger so I should redo some test reducing it more. I > believe one way to check if the tropospheric attenuation is correct is > to compare the opacity value computed from ARTS to one computed manually > on the spectra. > > I'll check that as well but thanks for your inputs already. > > > Best regards, > > Eric > > > > > > ------------------------------------------------------------------------ > *De :* Patrick Eriksson <patrick.eriks...@chalmers.se> > *Envoyé :* mercredi 9 juin 2021 23:39:33 > *À :* Sauvageat, Eric (IAP); arts_users...@mailman.rrz.uni-hamburg.de > *Objet :* Re: [arts-users] Tropospheric continuum retrieval in ARTS > Eric, > > I don't know about a fixed setup for dealing with the troposphere in > observations of this type. As your observations at max gives one piece > of information for the troposphere, I would say that the single > retrieval point setup makes most sense. In any case it is plausible option. > > I assume that you fit the measurements in both cases. > > If I get it right, your main worry is that you get a 10% difference in > the ozone profile between the options. This should likely originate in > that the retrievals give you a troposphere having a 10% difference in > transmission. > > My suggestion is then to look at the polyfit part. Is the fitted polyfit > the same between the options? My guess is that it differs. And that > gives room for retrieving a different tropospheric transmission. > > And it could be reasons to anyhow consider the polyfit part. I assume > you have a good calibration and the uncertainty in the overall > "baseline" level is due to the troposphere. Or expressed differently, > you want to fit the overall baseline level by changing H2O in the > troposphere, not by the polyfit. Or more exactly you want the first > polyfit coefficient to be small, the polyfit should just take care of > the "wiggling". To achieve this you should set the a priori uncertainty > for the first (0-order) coefficient to be very small, to effectively > enforce a low measurement response for the coefficient. With this, the > retrieval will have to fully adjust the overall baseline level by the > H2O profile, independently on the grid and a priori uncertainty you use > for H2O. > > Bye, > > Patrick > > > > > > On 2021-06-09 16:21, eric.sauvag...@iap.unibe.ch wrote: >> Dear ARTS community, >> >> >> I am doing stratospheric O3 retrieval with a ground-based radiometer >> (f=142GHz) and am trying to deal with the absorption contribution of the >> troposphere directly in the OEM implemented in ARTS (avoiding >> tropospheric correction prior to the retrievals). >> >> >> Up to now, I took inspiration from "qpack2_demo2.m" which suggests (if I >> understood it correctly) to implement a "H2O-PWR98, H2O" retrieval (main >> contributor of tropospheric opacity at these frequencies) on a lower >> atmosphere retrieval grid. This results in a water vapor profile >> retrieved together with my main ozone retrievals. Of course this profile >> has no good measurement response as my ozone radiometer is not designed >> to retrieve any H2O profile, but it seems that it provides the "right >> amount of opacity" needed to explain my spectrum.In addition, note that >> I am also performing a polyfit retrieval of degree 2 which is also >> fitting a constant term on my spectra which also probably contributes >> somehow to fit the global continuum absorption. >> >> >> I found out recently, that such a continuum retrieval was implemented in >> QPack1 (activated with "Q.CONTABS_DO") and from my understanding, it >> does not seem to retrieve any H2O profiles but only single values for >> the continuum (which somehow makes more sense to me). So I did try to >> provide a single grid retrieval point and single value for H2O cov >> matrix and it seems to work equally good as the retrieval including a >> full H2O profile (in the sense of convergence, correlation between both >> time series, ...) but it has a constant +10% VMR offset on my whole >> ozone profiles and I have no clue why. >> >> >> Also, I have made different tests to check the impact of the selected >> species (continuum vs full absorption model defined with or without H2O) >> but it did only produce slight changes in the results. As well, the >> height of the H2O grid or its altitude resolution does not seem to have >> significant impact on the retrievals. >> >> >> Sorry for this long email but I am really puzzled in what is the best >> way to deal with continuum absorption in ARTS and what I might be doing >> wrong. Therefore, any kind of feedback or help regarding this would be >> much appreciated. If needed, I can also provide examples plots of MR, >> AVK or profiles (not sure how it works for mailing list though). >> >> >> Many thanks in advance, >> Eric >> >> >> >> >> >> >> >> >> >> >> _______________________________________________ >> arts_users.mi mailing list >> arts_users.mi@lists.uni-hamburg.de >> https://mailman.rrz.uni-hamburg.de/mailman/listinfo/arts_users.mi > <https://mailman.rrz.uni-hamburg.de/mailman/listinfo/arts_users.mi> >>
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