Adjust default blending in DayNightCompositor

[pnuu]

This PR adjusts the default Sun zenith angle (SZA) interval where the overlapping day and night products are blended from 85.0 ... 95.0 to 85.0 ... 88.0.

The default limit of SZA correction in Satpy is the same 88.0 degrees. If the upper limit is higher, there will be a dark band between the products.

The original version:

Proposed version:

• Tests adjusted
• Tests passed
• Passes flake8 satpy
• Documentation updated

[coveralls]

Coverage decreased (-0.004%) to 88.094% when pulling 484590b on pnuu:adjust-dnc-default-blend-limits into fbefad6 on pytroll:master.

[codecov]

Codecov Report

Merging #1064 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1064      +/-   ##
==========================================
- Coverage   88.09%   88.09%   -0.01%     
==========================================
  Files         191      191              
  Lines       29439    29438       -1     
==========================================
- Hits        25935    25934       -1     
  Misses       3504     3504              

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[djhoese]

My gut reaction was "why change it, it looks fine already" until I actually saw the comparison. Yeah, this looks better.

Have you tried it with other composite combinations? Like with a true_color on the day side? I'm wondering if the edges are worse...wait this change just gives more of the night image, right?

[mraspaud]

LGTM, thanks for fixing this.

[pnuu]

Yes, this just shifts the night time product a bit into the day side. I've made 11 separate DNC composites that I've passed to our forecasters for reviews, and all of them look nicer with these new limits. Here's a composite with true color and ash RGBs.

[djhoese]

Good image. That's what I was hoping for; an image with a very dark image on one side and a bright image on the other. Looks good.

[mraspaud]

How does it look like if you take a night-only composite on the night side, eg night_microphysics, does it still look ok ?

[pnuu]

Here's one with night microphysical:

I'll still run one test with higher SZA limit for the day side.

[pnuu]

And here are the three above composites with low_lim=87.0 and high_lim=88.0, so only one degree SZA range where the blending is done.

Natural color / colorized IR clouds:

True color with ash:

Natural color with night microphysical:

[mraspaud]

Looks really good @pnuu !