Dear Eric,

We are working with a similar setup (Just at another frequency) for the 
radiometers in Kiruna.

For water vapour we use a rather loose constraint (large S_x/S_a value) to 
allow the fit of the profile for the H2O background (co-fit). Of course, H2O 
can be unrealistic as we just use it to fit the continuum/baseline

For O3 we use an altitude dependent constraint with a rather strong constraint 
below the tropopause (where we know the values are “negligible”) and above the 
sensitive area where we force the retrieval to the a priori knowledge. So we 
allow reasonable fitting only for the altitudes where the Jacobians show decent 
sensitivity. Note: We have good results with “rel” jacobians. “vmr”/absolute 
Jacobians caused problems.

We also co-fit a baseline  using “polyfit” of the lowest degree. However here 
we did not yet find a solution that we can start with a preselected value (e.g. 
an instrument-dependend baseline) so the a priori value is by default 0 and the 
a standard constrain constraint might cause difficulties with large large 
baselines due to high humidity or clouds with precipitation.

Maybe this helps.

Best regards,

Mathias




From: arts_users.mi-boun...@lists.uni-hamburg.de 
<arts_users.mi-boun...@lists.uni-hamburg.de> on behalf of 
eric.sauvag...@iap.unibe.ch <eric.sauvag...@iap.unibe.ch>
Date: Wednesday, 9 June 2021 at 16:22
To: arts_users.mi@lists.uni-hamburg.de <arts_users.mi@lists.uni-hamburg.de>
Subject: [arts-users] Tropospheric continuum retrieval in ARTS

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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