Hi Victoria,

Some comments, just in case ...

My question just referred to if the radiation field gets more flat or more varying (as a function of zenith angle) with increased scattering. And thus if the issue if angular interpolation gets more serious or not. The overall calculation accuracy should just increase with the number of streams.

At this moment I leave to Jana to comment on checks and warnings associated with the scattering data.

Bye,

Patrick



On 01/24/18 20:00, Victoria Sol Galligani wrote:
Hello everyone! Hi Jana!

I was aware of what you are saying about the number of streams. However .... in the context of the ARTS scattering methods + RTTOV-scatt inter-comparison I am working on, I have been testing the sensitivity of the output TBs to the number of streams chosen. In this regard, I think it would be interesting to run RT4 even if the interpolation induces large errors and the scattering matrix is not resolved well. If I run RT4 without this warning (actually error because arts doesn't run with its current checks) I could easily answer your answer Patrick (if the angle interpolation error increases with strong scattering), because I'm running with different profiles, some with much more scattering than others. At the moment its clear to me that more streams are needed for more scattering cases, so I guess that already answers your question Patrick ...

Do you have any advice regarding turning this warning/error off on my arts distribution?

Looking forward to sharing my preliminary results on this subject,
Hugs,

Victoria


On Mon, Jan 22, 2018 at 4:56 PM, Patrick Eriksson <patrick.eriks...@chalmers.se <mailto:patrick.eriks...@chalmers.se>> wrote:

    Hi Jana,

    I was aware of that the auto-increase does not change the number of
    output angles, but still thanks for the warning.

    That is, for me it was understood that you should check that the
    start number of streams is high enough, to make sure that angle
    interpolation does not induce too large errors. But not clear to me
    is if the angle interpolation error increases with strong
    scattering. This is something that I have never tested.

    Your comments seem to indicate that it is the case (i.e. increasing
    errors), but has you tested it? Without that it is a bit hard to
    decide what to do.

    Anyhow, my recommendation is that you focus on general cleaning and
    documentation. Leave any possible extension of RT4 on this point to
    us others, if we find it necessary.

    Bye,

    Patrick





    On 2018-01-22 17:46, Jana Mendrok wrote:

        Hi,

        With some of you we have discussed (even suggested) the use of
        the auto-increase number of streams feature of the RT4 interface.
        The background to this feature is that RT4 needs the scattering
        matrix to be properly resolved in order to conserve the
        scattered energy satisfactorily. Using the feature, the number
        of streams is internally(!) increased until the scattering
        matrix resolution is deemed sufficient.

        The crucial issue, i just got reminded of when i went through
        the code, is that this increase is only done internally. the
        output will remain on the original number of streams set!

        That means, *one should not start with a very low number of
        streams* and should not let the system completely self-adapt
        (as, strictly speaking, that's not what it is doing - the output
        dimensions won't adapt and will always remain the
        original/starting number of streams) (i'm going to add that info
        to the online doc).

        It should be kept in mind, that - unlike Disort - neither RT4
        nor ARTS itself have good interpolation options for "off-stream"
        angles, i.e. the number of streams RT4 is set up with does not
        only determine the RT4 solution accuracy (this is
        improved/adapted with the auto-increase feature), but also the
        number of output directions (not affected by auto-increase),
        hence the accuracy with which the field is known to and further
        applied within other WSM of ARTS.

        Best wishes,
        Jana


        Ps.  Something more for developers...

        Thinking about that, this seems quite inconvenient. So, the
        question what to do about it, how to change that. Two possible
        solutions pop into my head:

        (1) instead of interpolating the high-stream-solution to the low
        number of streams, we could interpolate everything to the
        highest number of streams and output the "high-resolution" field.

        (2) re-define (doit_)i_field from Tensor7 into a ArrayOfTensor6
        with one Tensor6 entry per freq. this would allow to have
        different angular dimensions per frequency (we'd need to also
        store the angles per each frequency). however, that would affect
        the output of other solvers, too, and the way (doit_)i_field is
        applied in (i)yCalc.

        so, option (1) seems less of a hassle.

        neither option will solve all issues (like, even if the
        radiation field is quite smooth, linear interpolation from
        low-resolution fields won't be very good and higher-order
        interpolation intrinsically requires, well, higher numbers of
        streams), but at least some (like better conserving the shape of
        the radiation field, where derived from higher number of streams).

        any opinions? do you consider this an issue at all, or is a
        cautionary note in the documentation enough? if an issue, any
        better ideas on solutions or opinions on "my" two options?

        and, anyone other than me willing to implement possible changes?


-- =====================================================================
        Jana Mendrok, Ph.D.

        Email: jana.mend...@gmail.com <mailto:jana.mend...@gmail.com>
        <mailto:jana.mend...@gmail.com <mailto:jana.mend...@gmail.com>>
        =====================================================================


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