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










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