Reproduce geo/true_color from CIRA (green channel simulation)?

[guidocioni]

I'm looking for a way to reproduce the GeoColor/TrueColor composite that CIRA publishes. This is a comparison of the true color from the RAMMB slider app and one that I produced myself.

I know the differences are most likely due to the way the missing green channel is simulated, however is there no way to reproduce the same method CIRA is using inside satpy?
Using true_color_reproduction instead than true_color seemst to improve the colors, but it still not the same as the one from CIRA.

[djhoese]

Sorry for the late reply. I thought someone had answered and then I was off of work for a bit. I think @strandgren may be the best person to answer. To clarify though, what instrument are you using and concerned about?

[guidocioni]

Sorry for the late reply. I thought someone had answered and then I was off of work for a bit. I think @strandgren may be the best person to answer. To clarify though, what instrument are you using and concerned about?

To be fair this is not a life-and-death situation but more of a question that probably others had in the past.

Sorry for the missing information but I was referring to GOES (16-19) ABI.

[strandgren]

Hi, so both images are from the same satellite and instrument, i.e. GOES/ABI?

I have not really worked with ABI green channel simulation, but rather correction of the available green channels on FCI and AHI. That being said I think that different methods and LUTs are used for all the three methods:

• Satpy true_color
• satpy true_color_reproduction
• CIRA geo_color

and I don't think that the implementations in satpy are necessarily derived from or directly related to the CIRA approach, making it hard to re-produce.

Have you compared against co-located MODIS/VIIRS overpasses to see if it's actually desirable to replicate CIRA? To me it seems like the CIRA geo/true_color image above suffers from the typical issue of a mis-placed green band for true color imagery (0.51um instead of 0.55um) with desert surfaces being pinkish and vegetation being brownish. I'm quite sure that CIRA should be using a two-step approach for the ABI green band:

1. Simulate a 0.51um green channel (similar to ABI and FCI)
2. Correct the weak vegetation channel of the 0.51 channel by mixing in signal from the VIS0.8 channel

but to me it seems like the second step is missing in the image above, although I cannot say for sure and it's just my impression.

[djhoese]

Ah sorry, I should have read this more closely. I was distracted by the geo color mention. If we limit the discussion to primarily a "generic" true color RGB then I might have more opinions but I'm also not familiar with how CIRA makes theirs.

[guidocioni]

i did not compare to LEO satellites, and I indeed think CIRA is exacerbating a little bit the saturation, however in "flat" conditions like this one, their version makes it easy to recognize subtle differences to distinguish the sand storm from the background desert surface. Shouldn't their method be published somewhere?

[strandgren]

They have published it here https://journals.ametsoc.org/view/journals/atot/37/3/JTECH-D-19-0134.1.xml and in references therein. Especially Section 5. GeoColor 2.0: Application to GOES-R ABI, 1) Dayside) explains the green band simulation and correction:

The need for a synthetic green band, required along with the blue and red bands for rendering true color imagery, arises from its omission on the ABI (Miller et al. 2006a) in favor of other bands. Despite the general popularity of true color, it was not an NWS requirement, and the decision to omit the enabling green band was made under the constraint of limited space on the ABI focal plane array. Miller et al. (2012) describe a spectral correlation approach that relates MODIS 0.55 μm “green” reflectance to 0.469 μm (blue), 0.645 μm (red), and 0.858 μm (near-infrared), all bands atmospherically corrected in preprocessing. Their analysis, conducted on a diverse assortment of mesoscale scenes, shows absolute differences in real and synthetic green reflectance of 0.1 (on a scale of [0, 100]) and relative differences of 5%–10%, depending on scene properties. The greatest uncertainty in this synthetic green method occurs in shallow-water zones where the correlation between green and near-infrared chlorophyll-a reflectance is small.

The original transition plan was to develop a synthetic green correlative relationship using Himawari-8 AHI (which has a green band), and apply it directly to GOES-R ABI, since the blue, red, and near-infrared bands in common between AHI and ABI are very similar (the instruments were built by the same vendor). However, upon first light of AHI the need for an unanticipated additional step became apparent. The AHI true color imagery revealed a suboptimal response to vegetation (missing the spectrally narrow chlorophyll-a reflectance feature centered on 0.55 μm) in its native 0.51 μm “green” band compared to the 0.55 μm band of MODIS and VIIRS. The spectral misalignment produces a low bias compared to the 0.55 μm reflectance for certain land surface types, with the effects manifested in true color imagery as brown jungles/forests and deserts that were too red.

To provide an improved sensitivity to these surfaces, Miller et al. (2016) introduce a hybrid green method, which blends in a fractional component (~7%) of the AHI near-infrared band (0.86 μm), determined objectively from a minimization of the hybrid band against the 0.55 μm green band of SNPP VIIRS. For ABI GeoColor V2.0, a 0.51 μm synthetic green band is first produced from AHI-derived lookup tables (following Miller et al. 2012), and then the hybrid green step is applied using ABI’s own 0.86 μm band.

I think that the methodology has slightly evolved since then, including using a similar approach for the green band correction that we developed and use for FCI, i.e. using the NDVI to determine the amount of signal from the VIS0.8 band to blend with the 0.51um band (rather than a constant value of 7%). But I wouldn't expect significant changes.

[guidocioni]

OK so it may not be as easy as I thought