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Location: Tennessee, USA | The atmosphere is in thermal equilibrium, at least until you get up to 70 km or so and collisions become infrequent. From Grant Petty, A First Course in Atmospheric Radiation, second edition (paperback) page 126:
For all common applications in atmospheric radiation, Kirchhoff's Law can be taken as an absolute [emphasis added]. It is therefore only for the sake of completeness that I point out that Kirchhoff's Law only applies to systems in local thermodynamic equilibrium (LTE). This condition applies, for example, when the molecules in a substance exchange energy with each other (e.g., through collisions) much more rapidly than they do with the radiation field or other sources of energy. LTE, and thus Kirchhoff's Law, breaks down at extremely high altitudes in the atmosphere, where collisions between molecules are rare. He goes on to talk about non-LTE emissions which include lightning and the aurora. If Kirchhoff's Law applies, which it does, then a balance of emission and absorption is required. My statement jumped over a step or two in the logic.
In the meteorology texts that I have seen, sensible and latent heat transfer are measured separately, but that's neither here nor there. The statements are not inconsistent because the temperature of the atmosphere controls emission/absorption, not the other way around. Because emission and absorption are in balance they do not cause net heating or cooling. Remember, we're talking about global annual means here. Diurnal, seasonal and geographical variations in insolation are much more complex.
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Location: Tennessee, USA | I see someplace else I left something out. Sensible and latent heat transfer affect the surface temperature by removing heat and making it cooler than if there were only radiative transfer. The surface temperature and humidity then determine the structure of the atmosphere with respect to the variation of temperature, pressure and density with altitude.
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Location: Sydney Australia | hey where are Madmartigan and Coolhansnl?
DeWitt I've tried making sense of what you have been saying but it's 1:00 am and I've come home from work a little over an hour ago my brain is numb. I'll take a look maybe in the morning but certainly Monday. However I would like to remind at this point that the first question describes a perpetuum mobile of the second kind, and that is why I query that 324 watts doing the rounds between the surface and the GHG. I am not overly keen on MODTRAN because I don't know the guts of it. I would rather some empirical data. |
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Location: Tennessee, USA | Jan,
I can try scanning some observed IR spectra from Grant Petty's book and then calculate spectra using similar conditions with MODTRAN and post both figures. However, I can tell you that they look very similar to me. They y axis scales are different, but there are reference curves of the Planck function in each that can be used for comparison. The figures in the book are in mW/m2 sr cm-1 and the MODTRAN spectra are W/m2 cm-1. I think this means they are different by a factor of pi (and 1000 for the mW vs W). The tropical atmosphere emission spectrum is coincident with a 300K Planck curve for part of the spectrum in both cases.
Edited by DeWitt 29/9/2007 19:02
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Location: Tennessee, USA | aha,
After consideration, I think I can see where we differ. I'm using global annual mean energy balance figures. That is a static case so fluxes don't change and radiative balance applies. If you look at what happens during the day, that is not static, fluxes are continuously changing and radiative balance doesn't apply. During the day, emissivity and absorptivity are equal, but emission and absorption are not. In general, absorption dominates from sunrise until the temperature peaks in the afternoon, then emission dominates until the sun comes up again. Clouds and rain add additional complications. However, if I want to calculate the effect of doubling CO2 concentration, I think using the static global annual mean fluxes makes it easier to see what's happening. |
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Location: Sydney Australia | DeWitt
I can try scanning some observed IR spectra from Grant Petty's book and then calculate spectra using similar conditions with MODTRAN and post both figures.
thank you that will be interesting the thing I'm most curious about is what is going on in the real world rather than model world in the range from about .5 km up to where NIMBUS3 did observations from. I haven't been able to find any. Any way I must be off my neighbours were up making loud noise to 4:00 am and I have lawns to mow. |
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Location: Tennessee, USA | ...I have lawns to mow.
Haven't had to mow my lawn in many weeks. No rain to speak of here.
Now I have to figure out how to make my multi-function printer/scanner do something other than print. |
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Location: The Hague, The Netherlands | nimbus is here (note the temperature inversion over Antarctic!)  here is observed temperature inversion over the arctic 
Edited by coolhansnl 30/9/2007 16:49
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Location: Tennessee, USA | As promised here are the images.
This figure is from page 219 of A First Course in Atmospheric Radiation, Second Edition (paperback) p219.
This figure is from page 223.
MODTRAN calculated spectra for conditions as close as possible to the observed spectra.
The sub-arctic looking up 245K calculated spectrum doesn't look quite the same as the Barrow, AK spectrum because there was a temperature inversion at the surface. The 667 wavenumber band is depressed because it's coming from close to the surface where the temperature is much lower than the air above it. You can't adjust the lapse rate in the Archer MODTRAN interface to duplicate this effect.
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Location: The Hague, The Netherlands | DeWitt, we're on the same wavelength..  |
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| DeWitt - 30/9/2007 02:01aha,After consideration, I think I can see where we differ. I'm using global annual mean energy balance figures. ... If you look at what happens during the day, that is not static, fluxes are continuously changing and radiative balance doesn't apply. During the day, emissivity and absorptivity are equal, but emission and absorption are not. OK. Here we agree.I'm using global annual mean energy balance figures. That is a static case so fluxes don't change and radiative balance applies. ... However, if I want to calculate the effect of doubling CO2 concentration, I think using the static global annual mean fluxes makes it easier to see what's happening. I dont think that considering the mean actually changes the situation. For every layer the temperature, now mean, still defines absorptivity and emissivity. And emissivity defines emission. Absorption, however, is not the property of the layer. It is defined by absorbtivity and radiation from outside. So, it doesn't have to equal emission. So, no guaranteed radiative balance. Instead energy balance must always come true. Fuxes, now constant, move the heat so that there is enegry balance. This way the sensible and latent heat from the surface can speard into the atmosphere. A large protion of atmosphere can be in radiative balance, but it is not required. It is consequence of energy balance and local conditions. |
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Location: Tennessee, USA | aha,
I see what you're saying and I stand corrected. Energy is in balance, otherwise there would be heating or cooling, but longwave radiative emission can be higher than absorption if there is an energy input from shortwave radiative absorption of incoming sunlight, sensible, and latent heat transfer.
I saw a recent comment from Nasif/Madmartigan on CA and according to that F0 is not the solar constant. I guess I'll have to bite the bullet and go to his web site. |
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Location: Southeast Essex | There is no such thing as an energy balance.
A clear demonstration of this is the fact that climate is in constant flux. |
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Location: The Hague, The Netherlands | IMHO there is, but we don't know the energy flux changes and we don't know the frequency response of the system in sufficient enough detail to make valid predictions. Moreover the future load of the balance is really crystal ball staring. ("Ultraglaskugelbereich", as the germans can express it so beautifully) |
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Location: Southeast Essex | Hmmm,
I am of the firm belief that balances, norms and averages play no part in the real world. The real world is in constant unrest and cannot be explained in these terms IMO. |
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Location: Machynlleth, Mid Wales | Village - 1/10/2007 11:18 There is no such thing as an energy balance. A clear demonstration of this is the fact that climate is in constant flux. All natural systems strive to attain equilibrium. This is elementary physics. Fluxes in weather and climate are manifestations of this principle. Cheers - John |
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Location: The Hague, The Netherlands | So we must make our policy decisions on:
Unknown past (Hockestick controversy)
Unknown response (spread in climate sensitivity values)
Unknown future (spread in SRES emission scenarios)
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Location: Southeast Essex | John Mason - 1/10/2007 14:54 Village - 1/10/2007 11:18 There is no such thing as an energy balance. A clear demonstration of this is the fact that climate is in constant flux. All natural systems strive to attain equilibrium. This is elementary physics. Fluxes in weather and climate are manifestations of this principle. Cheers - John Thanks for backing me up John. I know you like to word things in your own way. |
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Location: Southeast Essex | coolhansnl - 1/10/2007 15:04 So we must make our policy decisions on: Unknown past (Hockestick controversy) Unknown response (spread in climate sensitivity values) Unknown future (spread in SRES emission scenarios) This is the sad reality of climate science which is bold enough to try to predict the future. There are no facts, there is nothing firm that one can grasp to base long term climate. The biggest problem to the synthetic aproach is the very reason that nature has no equalibrium. That is systemic real shock. There is no way to predict shock to the system and therefore constant unrest remains a reality. |
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Location: Sydney Australia | Thanks Dewitt the pictures gel pretty well with the few I was able to obtain from another blog (John Daly's Waiting for greenhouse) that were taken by NIMBUS 3 (I would just love to get hold of the raw numbers).
The thing I notice is that where there is no temperature inversion the atmosphere is opaque at a band centred on 15 micron because each show round about the same level of emission in that band as that expected from a black body at the same temperature of the surrounding air. I suspect this to be the case even though I don't have the actual temperatures to hand but it's consistent with Kirchoff's law.
This gets us back to what happens to that energy that's absorbed and how much does it heat the atmosphere? As far as I can figure it can only be analytically determined in the first instance by the total heat capacity of the column of air that's aborbing it and the energy it absorbs using dT = dq/mC where dT is the change in temperature, dq the heat absorbed 'm' and 'C' are the mass and specific heat respectively. One day I'll get to and calculate it out but in the meantime it's clear that all the atmospheric gases retain the radiated heat and in this first order case the actual concentration GHG is irrelevant as long as all the radiation is absorbed and this from Hans: If we talk in first order phenomena: The surface can warm only the CO2 and H2O molecules by radiation (as N2 and O2 don't absorb infrared), so direct radiation has a minimal effect on temperature change of the total atmosphere mass (CO2 and H2O being trace gases). is not correct given that also from Hans referring too my question regarding how long the thermal in-equilibrium between absorbing molecules and the remaining gases would last: Just one collision, there are sufficient cool O2 and N2 and Ar molecules around: approx 999620/380 . |
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A debate between Hans Erren and Nasif Nahle
