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
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