I know I don't know, but I'll throw out a guess that the window is dynamic and one expects variability with the model, unlike the static IR absorptions of molecules. At this time the model is giving less, at another time it may be seen to give more.
Actually no, it's not a dynamic difference... it's more simple. The model is a very simple one-dimensional (height only) one in which a fixed atmospheric temperature profile is assumed (within the troposphere, a wet adiabatic lapse rate profile IIRC). Very simple, and inappropriate for this case. Does that help?
It's much simpler. Remember that the model assumes a simple vertical temperature profile throughout the atmosphere. The temperature for ground level is derived from this profile, by substituting height=0. The radiation from the window is computed using this temperature.
In reality however, the ground has a higher temperature: this spectrum was collected from the Sahara in full sunlight. The ground is directly heated by the Sun, and a boundary layer of a few metres above it is formed in which the temperature gradient is much higher than in the general atmosphere.
From the graph one can estimate what this temperature difference is: model, 315 K = 43 C, measurement, 320 K = 48 C, for a difference of five degrees centigrade. All the time assuming that both atmosphere and ground are Planckian black bodies, probably about right for this spectral range.
This boundary-layer effect is well-known to geodesists: it is the reason why triangulation networks are measured from high places, in Finland measuring towers of 10-40 m height, which stick outside the boundary layer. And then there is levelling refraction (yes, those dastardly Finns again), which requires special modelling as the optical path with levelling runs inside the boundary layer.
Because that's the only place where you can actually see down to the ground? At other wavelengths you're looking at atmospheric layers at various non-zero heights. They are also all colder than the ground.
The funny thing that I learned the basics of these things in an earlier life, when studying astrophysics.
The atmosphere of the Sun is similar to that of Earth, in that it is heated from below. And there is a negative temperature gradient with height within the atmosphere. This is why the solar spectrum, just like the Earth's thermal infrared spectrum I linked to, is an absorption spectrum, as found by Fraunhofer.
Now, as all temperatures are 20x higher, the Planck curve is in the visible light; and also, instead of bands caused by molecular vibrations and rotations, you see lines associated with electron jumps in atoms, as there are no intact molecules left.
If you look at the Sun in the continuum, you see the solar "surface", or photosphere. If you look at the Sun in the core of an absorption line, you look at a higher level, where the gas is cooler and shining more darkly. You can do that with a spectral filter. With a tunable Fabry-Perot filter you can even move up and down in the solar atmosphere like with an elevator!
Now you also understand the phenomenon of limb darkening :-)
All this can be confirmed by looking at the Sun on those rare occasions when the Moon blocks out the light from the photosphere, and we see the "dark" cores of the absorption lines shine on their own against the dark sky: as this is monochromatic radiation from spectral lines of the various elements present in the Sun -- especially the red hydrogen line --, you may now understand why this layer is called the chromosphere...
Just goes to say, in denying the greenhouse effect one throws away a sizable chunk of astrophysics as well.
The cartoon is from xkcd. And a good one it is!
ReplyDeleteBTW talking about Planck, try this on:
The greenhouse effect -- it works, bitches.
Question for advanced students: why is, in the "ATM Window", the model giving less radiation from the ground than the satellite observed?
I know I don't know, but I'll throw out a guess that the window is dynamic and one expects variability with the model, unlike the static IR absorptions of molecules. At this time the model is giving less, at another time it may be seen to give more.
ReplyDeleteActually no, it's not a dynamic difference... it's more simple. The model is a very simple one-dimensional (height only) one in which a fixed atmospheric temperature profile is assumed (within the troposphere, a wet adiabatic lapse rate profile IIRC). Very simple, and inappropriate for this case. Does that help?
ReplyDeleteYou Fiend! Why do the Fins always cause us cognitively-challenged Americans such mental anguish!?!? (sips beer....ah!).
ReplyDeleteOkay, how about IR associated with the satellite or something else which super-imposes in that region?
It's much simpler. Remember that the model assumes a simple vertical temperature profile throughout the atmosphere. The temperature for ground level is derived from this profile, by substituting height=0. The radiation from the window is computed using this temperature.
ReplyDeleteIn reality however, the ground has a higher temperature: this spectrum was collected from the Sahara in full sunlight. The ground is directly heated by the Sun, and a boundary layer of a few metres above it is formed in which the temperature gradient is much higher than in the general atmosphere.
From the graph one can estimate what this temperature difference is: model, 315 K = 43 C, measurement, 320 K = 48 C, for a difference of five degrees centigrade. All the time assuming that both atmosphere and ground are Planckian black bodies, probably about right for this spectral range.
This boundary-layer effect is well-known to geodesists: it is the reason why triangulation networks are measured from high places, in Finland measuring towers of 10-40 m height, which stick outside the boundary layer. And then there is levelling refraction (yes, those dastardly Finns again), which requires special modelling as the optical path with levelling runs inside the boundary layer.
But then why is the boundary-layer effect only within that window?
DeleteBecause that's the only place where you can actually see down to the ground? At other wavelengths you're looking at atmospheric layers at various non-zero heights. They are also all colder than the ground.
DeleteOk, I get it! Thanks so much! You really are ruining the fiendish Finnish reputation!
ReplyDeleteThe funny thing that I learned the basics of these things in an earlier life, when studying astrophysics.
ReplyDeleteThe atmosphere of the Sun is similar to that of Earth, in that it is heated from below. And there is a negative temperature gradient with height within the atmosphere. This is why the solar spectrum, just like the Earth's thermal infrared spectrum I linked to, is an absorption spectrum, as found by Fraunhofer.
Now, as all temperatures are 20x higher, the Planck curve is in the visible light; and also, instead of bands caused by molecular vibrations and rotations, you see lines associated with electron jumps in atoms, as there are no intact molecules left.
If you look at the Sun in the continuum, you see the solar "surface", or photosphere. If you look at the Sun in the core of an absorption line, you look at a higher level, where the gas is cooler and shining more darkly. You can do that with a spectral filter. With a tunable Fabry-Perot filter you can even move up and down in the solar atmosphere like with an elevator!
Now you also understand the phenomenon of limb darkening :-)
All this can be confirmed by looking at the Sun on those rare occasions when the Moon blocks out the light from the photosphere, and we see the "dark" cores of the absorption lines shine on their own against the dark sky: as this is monochromatic radiation from spectral lines of the various elements present in the Sun -- especially the red hydrogen line --, you may now understand why this layer is called the chromosphere...
Just goes to say, in denying the greenhouse effect one throws away a sizable chunk of astrophysics as well.
Absolutely wonderful points worthy of further intellectual digestion!
Delete