Add AHI airmass, ash, dust, fog, and night_microphysics RGBs

[djhoese]

The RGBs added in this pull request are common EUMETSAT recipes that are available for most other sensors. However, AHI doesn't have a 10.8um or a 12.0um channel (or channels whose ranges cover those wavelengths). This is my attempt at trying to balance all the information available for what the best AHI RGB recipes are. I've mostly taken this from various slides by JMA found by googling, from opinions by @ScottLindstrom at the SSEC which agreed with the JMA slides, and from existing ABI recipes. See ssec/polar2grid#258 for more discussion on this and why I'm doing it (to add them to CSPP Geo Geo2Grid).

The worst thing is I can't find a definitive guide for AHI recipes. The slides from conferences that I've found (2016 and 2017) are completely different from the training materials (2015?) which are also different from ABI recipes out there.

Note: None of these recipes were loadable for AHI data before this PR so this shouldn't be changing any existing functionality for users.

• Tests passed
• Passes flake8 satpy
• Fully documented

Examples

Airmass

Ash

Dust

Fog

Night Microphysics

[coveralls]

Coverage remained the same at 88.097% when pulling 7a3241a on djhoese:feature-ahi-rgbs into fbefad6 on pytroll:master.

[codecov]

Codecov Report

Merging #1065 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1065   +/-   ##
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  Coverage   88.09%   88.09%           
=======================================
  Files         191      191           
  Lines       29439    29439           
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  Hits        25935    25935           
  Misses       3504     3504

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[djhoese]

Here are some night examples. I noticed some odd artifacts in the night_microphysics but I think that is just in the data. Note these pictures are 10% the resolution of the original so the effect is made more apparent. I'll investigate this next.

Airmass

Ash

Dust

Fog

Night Microphysics

[djhoese]

Yep this is the B07 having bad data in it:

@mraspaud If the night_microphysics looks OK to you otherwise I think this is good to go.

[ScottLindstrom]

That doesn't look like bad B07 data to me, rather it's a manifestation of the poor precision of B07 at very cold cloud top temperatures. So a single radiance value might span several tenths of degrees at cold temperatures. Thus you get speckled values. All of the RGBs look reasonable to me, although I don't see a lot of obvious fog in the NightTime microphysics but that might be a function of the weather on that day.

[djhoese]

I got approval from @mraspaud on slack. Merging now...