Dear Ian, I also agree. In fact, I hope that we could use MC much more widely in some future version of ARTS, perhaps all the way down to MC sampling individual spectral lines.
/Stefan
On 7 Dec 2023, at 8:27, Patrick Eriksson wrote:
> Ian,
>
> Thanks for the input. Great that you have stress-tested MC. Too bad that it
> revealed a limitation.
>
> Good suggestion about iyMC. Today it would not be possible to do the random
> sampling from yCalc, it would require information on the sensor not at hand
> inside yCalc today. But we are planning to redesign the way the sensor is
> described, and this should be considered.
>
> Not totally sure exactly what you mean with using MC sampled antenna pattern
> more broadly, but I tend to agree. It would be good if there would be
> mechanisms to give monochromatic pencil beam calculations some width in
> frequency and space. It would speed up simulations of observations. As
> example, I have been playing around with a scheme to locally average the
> surface emissivity around the point you hit the surface, to make simulations
> in coastal areas faster.
>
> And yes, the most tricky part is finding the time for the work.
>
> Bye,
>
> Patrick
>
>
> On 2023-12-06 16:09, Adams, Ian S {he, him, his} (GSFC-6120) wrote:
>> Hi Stefan,
>>
>> I have been contemplating changes to the MC codes. One thing we have found
>> is that MCGeneral breaks down when Q starts to get large. We see unrealistic
>> results at 684 GHz when using horizontally aligned particles with high
>> aspect ratio. Yuli Liu, who is working with us now, did comprehensive
>> analysis, and we believe that the issue is the way the backwards algorithm
>> is using importance sampling to avoid the issue of inverting the extinction
>> matrix; however, this approach neglects the mixing of I and Q. I believe
>> this is a simple fix.
>>
>> The other issue is that MCGeneral is not very ARTS-like. Looking at the way
>> it is structured, I think a better approach would be to have an iyMC that
>> traces a single "photon," and yMC would integrate these individual results.
>> Random sampling of both the antenna pattern and the bandwidth could be
>> performed at this level. I also think that the MC sampled antenna pattern
>> could be more widely useful across ARTS.
>>
>> These papers provide an interesting curveball. The ARTS MC codes are
>> particularly slow, and they are not optimized for optically thin or
>> extremely optically thick atmospheres. We could look at using these
>> libraries, or at least techniques, but I'm not sure how intensive such a
>> restructuring of the code would be.
>>
>> Of course, the tricky piece here is finding someone with the time to do this
>> work. But, I think these changes would make the codes significantly more
>> usable, and hopefully therefore used.
>>
>> Cheers,
>> Ian
>>
>> On 11/29/23, 11:22 AM, "arts_dev.mi on behalf of Stefan Buehler"
>> <arts_dev.mi-boun...@lists.uni-hamburg.de
>> <mailto:arts_dev.mi-boun...@lists.uni-hamburg.de> on behalf of
>> stefan.bueh...@uni-hamburg.de <mailto:stefan.bueh...@uni-hamburg.de>> wrote:
>>
>>
>> Dear all,
>>
>>
>> I stumbled accross this interesting paper on an open C library for
>> particularly efficient MC calculations. Could this be the basis of ARTS 3D
>> MC flux and heating rate calculations? Using MC sampling also for the
>> spectral dimension, to be efficient, as in the second paper, which is also
>> impressive, I think. They use MC sampling even for the spectral lines, if I
>> got it right! (Basically treating each transition as if it were its own
>> absorption species.)
>>
>>
>> /Stefan
>>
>>
>> https://www.dropbox.com/scl/fi/smsisfgc2it3sx4gov970/J-Adv-Model-Earth-Syst-2019-Villefranque-A-Path-E2-80-90Tracing-Monte-Carlo-Library-for-3-E2-80-90D-Radiative-Transfer-in-Highly.pdf?rlkey=v5yvrm64fnljaf739j4ssllux&dl=0
>>
>> <https://www.dropbox.com/scl/fi/smsisfgc2it3sx4gov970/J-Adv-Model-Earth-Syst-2019-Villefranque-A-Path-E2-80-90Tracing-Monte-Carlo-Library-for-3-E2-80-90D-Radiative-Transfer-in-Highly.pdf?rlkey=v5yvrm64fnljaf739j4ssllux&dl=0>
>>
>>
>> https://www.dropbox.com/scl/fi/r1tm3jdzx57kb85nowmt0/Yaniss_ea_PNAS_2023_smi.pdf?rlkey=8d4a7rb4u8pehckawbfk08c9f&dl=0
>>
>> <https://www.dropbox.com/scl/fi/r1tm3jdzx57kb85nowmt0/Yaniss_ea_PNAS_2023_smi.pdf?rlkey=8d4a7rb4u8pehckawbfk08c9f&dl=0>
>>
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