Hi Patrick, I agree with your proposed changes.
/Stefan > On 09 Nov 2016, at 20:40, Patrick Eriksson <patrick.eriks...@chalmers.se> > wrote: > > Hi all, > > We (Jana and Patrick) have discovered several issues the last weeks > around the SingleScattering data format. > > 1. Definition of direction > - > A direction can be specified by how the photons move, or in what > direction you observe to detect the photons. The radiative transfer > functions in ARTS use the later definition, and call this a > line-of-sight (LOS). We have not found a clear statement if the > scattering data format assumes photon directions or LOS. In fact, > different assumptions have been made in DOIT and MC. In MC, LOS values > are mirrored before extracting scattering properties, while this is not > done in DOIT. > Our discussion of scattering data follows Mishchenko et al (2002) and we > should stick to it. With this interpretation, presently MC is doing the > right thing. As far as we understand, the issue has no influence on DOIT > for random orientation. For horizontally aligned particles, all is OK > for stokes_dim 1 and 2 (due to reciprocity), but there are issues > for higher stokes_dims (namely sign errors in the lower left and upper right > matrix blocks). > > > 2. Azimuth angle > - > In ARTS' definition of LOS the azimuth angle is counted clockwise, while > for scattering data the azimuth angle goes in the opposite direction > (Fig 6.1 in ATD, and is consistency with Mishchenko et al (2002)). This > is not considered by either MC and DOIT, and should give a sign error > for stokes_dim 3 and 4. > > > 3. Format for "horizontally aligned" > - > We have now realized that this format is not as general as we (at least > JM+PE) thought. It does not treat all horizontally aligned or azimuthally > randomly oriented particles. The (orientation averaged) particles must > also be symmetric around the horizontal plane. Such a symmetry will > rather be the exception when working with arbitrarily shaped particles > (and using DDA) and also, e.g., excludes realistically shaped rain drops. > We could introduce a new format for this, but that would make code and > documentation even more complicated. > Expressed simply and discussing the phase matrix, we currently store the > left part of the matrix holding data for incident and scattered zenith angles > (in table cols and rows, respectively). By making use of the reciprocity > theorem, > we could get away by storing just the upper triangle, i.e. with the same > amount > of data as now. But that would make the internal storage more complicated and > require more heavy calculations to extract the data (not just sign changes > are > needed, a transformation matrix, though simple, must be applied). So we just > simply suggest that we store the complete phase matrix. That is, the incoming > zenith directions will be [0,180] and not just [0,90] as now. And to keep > things as > simple as possible we suggest to do the same for abs_vec and ext_mat. > We don't need to change the fields in the data format, but this should still > be a > new version of the format. And when we are introducing a new format we would > also like to rename the "ptypes" as well, as "horizontally_aligned" is not a > good > name when we start to work with tilted particles. We suggest the names > totally_random > azimuthally_random > > (We are not 100% sure about some of the theoretical details, but the > three main remarks should still be valid.) > > Any comments or opinion? > > We (mainly Jana) plan to start attacking these things relative soon. If > anybody wants to help out in the revision, please let us know. > > Bye, > > Patrick and Jana > _______________________________________________ > arts_dev.mi mailing list > arts_dev.mi@lists.uni-hamburg.de > https://mailman.rrz.uni-hamburg.de/mailman/listinfo/arts_dev.mi _______________________________________________ arts_dev.mi mailing list arts_dev.mi@lists.uni-hamburg.de https://mailman.rrz.uni-hamburg.de/mailman/listinfo/arts_dev.mi
