Implement modifier for reducing signal as a function of sunz angle

[strandgren]

This PR Implements a new modifier that reduces the signal within a defined sunz angle interval in order to avoid the excessive white glow at the limb in geostationary VIS/NIR imagery. An example on how to define it and use it is shown below:

modifiers:
  sunz_reduced:
    modifier: !!python/name:satpy.modifiers.SunZenithReducer
    correction_limit: 55
    max_sza: 90
    strength: 1.5
    optional_prerequisites:
      - solar_zenith_angle

composites:
   true_color:
    compositor: !!python/name:satpy.composites.SelfSharpenedRGB
    prerequisites:
      - name: vis_06
        modifiers: [sunz_corrected, rayleigh_corrected, sunz_reduced]
      - name: ndvi_hybrid_green
      - name: vis_04
        modifiers: [sunz_corrected, rayleigh_corrected, sunz_reduced]
    standard_name: true_color

The ability to reduce the signal as a function of the sunz angle is already implemented as part of e.g. the SunZenithCorrector modifier which generally works very well for reducing the correction at high angles. The problem occurs when applying Rayleigh correction on imagery where the sunz correction has been reduced at high angles, since pyspectral assumes regular reflectance data normalized by the sun zenith angle when estimating the Rayleigh contribution. This issue is clearly demonstrated in the examples below:

It was discussed whether the Rayleigh correction can be adapted to account for this, but after another round of investigation and testing I still did not find a proper way to do this. Hence, I suggest to add the proposed modifier and then when users have had a chance to test it, we can see if we can find a better implementation. A better implementation would mean that the user defines the reduction in the sunz correction and then the Rayleigh correction properly accounts for this (meaning that there would be no need for a third modifier).

The results with the modifier implemented here, following the example recipe above looks like this:

Another example with the terminator across the disk, without and with the sunz reduction modifier in this PR:

The default values for the reduction (correction_limit: 55, max_sza: 90, strength: 1.5) have been chosen to give an overall good trade-off in having a nice transition into deep-space at the limb and not loosing too much information at the terminator. Nevertheless these values can of course be modified by any user that want to use the modifier.

As can be seen in the last example the modifier remove some information at the terminator. Nevertheless, it shall be noted that:

• Without the modifier the colors in this area are generally not well represented.

• The effect get's less evident/relevant when blending the composite with a night-time layer

• It should be possible to restore some of the information by reducing the Rayleigh correction at high sunz angles.

• Tests added

• Fully documented

[strandgren]

Another example for AHI:

[codecov]

Codecov Report

Merging #2556 (ba655df) into main (6326b03) will increase coverage by 0.00%.
Report is 43 commits behind head on main.
The diff coverage is 100.00%.

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Files Changed	Coverage Δ
satpy/modifiers/__init__.py	100.00% <100.00%> (ø)
satpy/modifiers/angles.py	99.21% <100.00%> (+0.03%)	⬆️
satpy/modifiers/geometry.py	90.32% <100.00%> (+2.56%)	⬆️
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... and 20 files with indirect coverage changes

[coveralls]

Pull Request Test Coverage Report for Build 6224673446

• 43 of 43 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 95.47%

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Totals
Change from base Build 6190601890:	0.004%
Covered Lines:	48768
Relevant Lines:	51082

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💛 - Coveralls

[mraspaud]

I'm wondering if changing the sunz cutoff would have a similar effect?

[strandgren]

I'm wondering if changing the sunz cutoff would have a similar effect?

That's what I try to demonstrate with the four images in the PR description. It works well as long as you don't apply any Rayleigh correction, but as soon as the Rayleigh correction is applied, you get issues at the limb since (I believe) pyspectral assumes reflectance data where no such reduction has been applied.

[mraspaud]

That's what I try to demonstrate with the four images in the PR description. It works well as long as you don't apply any Rayleigh correction, but as soon as the Rayleigh correction is applied, you get issues at the limb since (I believe) pyspectral assumes reflectance data where no such reduction has been applied.

Ah, indeed, sorry.

[mraspaud]

This is from another PR, right?

[strandgren]

hmm yes.. Seems like I may have branched out from the branch used for the other PR.. May I ask what's the best way to deal with this? Reset this branch or leave it as if for now and then pull in master and modify accordingly once the other PR has been merged?

[mraspaud]

I don't know if it's the right way, but I'd just remove here the code that is implemented in the other PR, and commit that. And then later merge the main branch.

[mraspaud]

LGTM, but I think there are two PRs mixed in here.

[strandgren]

LGTM, but I think there are two PRs mixed in here.

I think this should be resolved now :)