Adds tech note about longwave radiation

[bishtgautam]

Adds documentation about ELM's longwave radiation model.

[BFB]

[github-actions]

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[bishtgautam]

@thorntonpe, @daleihao, The rendered technote chapter is available at https://docs.e3sm.org/E3SM/pr-preview/pr-7133/ELM/tech-guide/longwave_radiation/

[daleihao]

Great job! I just leave few minor comments.

[daleihao]

longwave radition module in ELM?

[daleihao]

I think we can provide the values of snow, soil emissitivity?

[bishtgautam]

Good catch. I will add it.

[peterdschwartz]

Some clarifying questions for my benefit.

Where do UrbanRadiation calculations fit in here? Eqs (16)-(17) or are those for bare ground?
Some potential confusion about the "longwave radiation module" on my part. I assume this isn't meant in the sense of a fortran module like SurfaceRadiationMod or CanopyFluxesMod. Is the vision to have the fortran/code details in the ELM Developer Guide which will cite the relevant Technical Guide docs?

[bishtgautam]

@peterdschwartz,

Where do UrbanRadiation calculations fit in here? Eqs (16)-(17) or are those for bare ground?

That's a good question. The equations in this chapter only apply to naturally vegetated and crop land units; they do not apply to urban land units. For Urban landunits, we need a new chapter that will correspond to CLM's chapter on the urban model, and this will written for ELM in the future.

Is the vision to have the fortran/code details in the ELM Developer Guide which will cite the relevant Technical Guide docs?

I don't have a complete vision for this, but here are my thoughts and reasoning, and I welcome feedback. It would undoubtedly be helpful to document these implementation details. However, I'm not sure if we need a new Implementation Guide for it or if we should add a new section at the end of each technical guide chapter summarizing these details. These details would be helpful when converting the Fortran code to Kookos (at some point in the future), but I think we should not spend a lot of effort meticulously documenting the implementation in the Fortran implementation because we are going to leave that implementation behind.

[bishtgautam]

@peterdschwartz, I'm adding you as a reviewer because we need the approval of one reviewer that have write access to the repo.

[peterdschwartz]

Merged to next and master