RealClimate: Unforced variations: July 2023

(see also my series of comments from Sept 2020 and my Update/Progress on How to See the Greenhouse Effect; Diagram ideas, diagrams done so far (including Seeing Cross Sections – Screen & pinhole camera views); all of part 12?; portions of parts 2 and 13. – this introduces part 12)

my series of comments from Sept 2020, combined, with some editing, and another comment:

I’ve been working on some adjustments to my color guesstimates, but first… re 108 Philippe Chantreau
If we could see infra-red, I imagine that the GH effect would cause an ambient “glow” of IR light, most intense close to the surface, and decreasing in intensity with altitude, until reaching a threshold altitude. With an increase in GH effect, I expect that the glow would intensify and the threshold altitude would increase. What I can’t quite put in words is what happens at the threshold altitude. …

… I believe what you’re refering to is the concept of an effective emitting level, which, by analogy with the Sun, is a vertical position that is representive of the Earth’s own photosphere…

…

Imagine the opacity is produced by many opaque particles; they are blackbodies, absorbing whatever light reaches them and emitting according to their temperatures [hotter = brighter].   For each, you see a cross-sectional area source of radiance [σa = absorption cross section].  You can’t see all of them because the closest ones hide some of those farther away, etc.  The more densely packed or bigger they are, the less far you can see, and so the light you see matches temperatures closer to you.

Generally the size of the blackbodies depends on the material/substance they represent, frequency, and pressure and temperature (via line broadening, and the ratios of different energy states).

(… PS, more generally, there can also be ‘little mirrorballs’ (scattering cross sections) – in this case consider the reflections (and reflections of reflections…) of blackbodies. Also they may vary with direction and polarization, but that’s not of much concern here…)

You need to be able to see temperature variations in order for there to be a net radiant flux of heat through where you are; it has to look brighter in one direction than it’s opposite.  If it is transparent where you are, the flux passing by you depends on conditions somewhere else (and there can be no net radiant heating or cooling at your location).  Adding opacity gives the material influence on the radiation, and the potential to radiantly warm or cool.  At a certain point, increasing opacity hides the temperature gradient and so everything looks the same where you are; there is no net flux.

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… anyway, a distribution of all the blackbody cross-section area that you can see is called an emission weighting function* [EWF], and looking down from space, that would be the Earth’s photosphere.

caveat: emission weighting function* may be defined for a single direction; for a whole hemisphere of directions up or down, you have to weight by the cosine of the angle from vertical and integrate over solid angle.

Anyway, due to various potential nonlinearities (Planck function not linear over temperature, temperature not linear over optical path,…?), the temperature of the centroid of the emission weighting function won’t necessarily match the brightness temperature of the radiance or irradiance – even at just one frequency.

ignore this: The concept of an effective emission/radiating level most easily applies for a greybody atmosphere, where the opacity is constant over the thermal IR band…

see this instead:
From my: https://www.realclimate.org/index.php/archives/2023/06/unforced-variations-jun-2023/#comment-812164

The effective emitting level (roughly speaking, a centroid of the emission weighting function (EWF) – because what you see is coming from a range of heights), looking down from Space, varies greatly over the spectrum; In the atmospheric window ~ 8-12 microns (or 8-13?) (interupted by the ozone band around … I think it’s 9.6 microns), it can get close to the surface in the absence of clouds (some of the EWF is on the surface); it goes up into the stratosphere within the CO2 band centered around 15 microns (~667 cm-1). Most OLR (LW, ie. ~terrestrial, flux to Space) is emitted from within the troposphere.

See links here as a guide:
https://www.realclimate.org/index.php/archives/2023/05/cmip6-not-so-sudden-stratospheric-cooling/#comment-811964
(PS for a flux, effective emitting level is at an optical depth of 2/3 units (it’s 1 unit for radiance in a single direction), based on my Calculus work – hopefully I didn’t mess it up; – this won’t necessarily align the flux with the temp. at that specific height, though.)

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cont. from my series of comments from Sept 2020:

(re 178 patrick027 – there are a few unstated caveats in all that, in case anyone wants to get nitpicky (ie. wouldn’t the closer objects look bigger? Well that matches up with contributing to a larger range of directions reaching your pupil…))

The net upward LW (thermal IR) flux at any level, in the global time average for an equilibrium climate, must combine with the net upward convective flux (which is [global time average: relatively small] above the tropopause) to balance the net downward SW (solar UV, visible, solar IR) flux.

… and so the divergence of the net upward LW flux (increasing with height), which is LW cooling, must balance the solar heating and the convergence (decrease with height) of the upward convective flux (convective heating).