In the The Amazing Case of “Back-Radiation” series, which included Part Two and Part Three, someone commented that it would have been good to see more than a few days of DLR (downward longwave radiation, aka “back radiation”) data. There were some monthly summaries from a number of locations, but the BSRN (baseline surface radiation network) data that I selected and plotted was quite limited.
At the time I was using Excel to load up the data and with values recorded every minute it wasn’t easy to plot more than a week of data. Armed with some new tools, here’s the data from Darwin, Australia from 2003 from the BSRN network:
Long, Charles (2009): Basic measurements of radiation at station Darwin
Click on the image for a larger view
The mean = 409 W/m² and the standard deviation = 27 W/m². (I don’t know what happened in July, I expect it is more likely to be instrument / data collection issues than the DLR taking vacation for the month).
Here is the expanded data on January through to June. The vertical axis is the same for each for easier comparison. Click on any of the graphs below to get a larger view.
January:
Long, Charles (2009): Basic measurements of radiation at station Darwin
February:
Long, Charles (2009): Basic measurements of radiation at station Darwin
March:
Long, Charles (2009): Basic measurements of radiation at station Darwin
April:
Long, Charles (2009): Basic measurements of radiation at station Darwin
May:
Long, Charles (2009): Basic measurements of radiation at station Darwin
June:
Long, Charles (2009): Basic measurements of radiation at station Darwin
The atmosphere cools down a lot slower than the land, which is why the difference between DLR for day and night is generally quite small. The way to think about any “body” heating or cooling is to consider two factors:
- its specific heat capacity (how much heat is needed to lift 1kg of that substance by 1K or 1°C
- its ability to radiate (or conduct) heat
99% of the atmosphere is composed of gases that can’t radiate any significant heat – N2 and O2. As shown in CO2 – An Insignificant Trace Gas? the absorption and emission ability of these gases is more than a billion times less than water vapor and CO2.
So the result is that the atmosphere takes a long time to heat up and to cool down when radiation is involved.
What is important to understand is that the DLR value measured at any one time is dependent on two important factors:
- the temperature profile of the atmosphere above the measurement location
- the concentration of gases that can radiate longwave
So lateral air movements have the ability to cause larger DLR changes. A strong wind blowing colder drier air can reduce the DLR significantly, and a hotter moister wind can increase DLR significantly.
how can you prove that DLR is the back-radiating portion?
i.e. the radiation which originated at the earths surface and got absorbed by some GHG and then being emitted towards the earth surface?
Why do you say that the difference between Day and Night time is quite small in DLR. Isn’t it about 40 W/m2 on these graphs. Seems quite significant at first glance (I don’t know the day-night temperature differences in Darwin though).
PS. Do you have data on local surface temperature to plot on these graphs.
gnbatt
I don’t like the term “back radiation” myself and the general term in use in atmospheric physics is “downward longwave radiation” or DLR.
The DLR is simply a consequence of these 2 points:
1. The atmosphere is above absolute zero
2. The atmosphere includes gases that can radiate at these temperatures
So to track one specific measurement and identify cause and effect.. not particularly worthwhile, or even possible.
The DLR from atmosphere that we measure at the surface is around 300 W/m^2.
If you have a theory about the reason why the atmosphere radiates this amount of energy, you are welcome to explain it (easier than me trying to guess). And then we can review your explanation.
ok, the ‘green house’ or the ‘glass house’ analogy, necessarily says that… it is the ‘backradiation’ which causes the extra warming that we see on the earth’s surface?… so somebody needs to prove that it is indeed ‘backradiation’ and not the radiation ‘trapped’ by GHG’s due to any number of extra-terrestrial sources?
How well established is the sd of the DLR?
Would it be correct to say that GHGs make the atmosphere less transparent to long wave radiations and increasing the GHGs levels in the atmosphere warms it by making it capture a larger part of the upward long wave radiations?
Is the increased DLR just a consequence of the warmer athmospheric temperatures and not the ’cause’ of the warming?
Or do both parts of the process (the increased capture of ULR by the atmosphere AND its increased ability to radiate heat downward) contribute to the warming?
(I am just trying to get my mind around those radiative processes and I am interested in your insights. Thank you for the outstanding work you are doing through this website.)
hunter:
For each dataset on a graph I used the Matlab command to calculate the standard deviation.
SoD,
You said local variations of GHG can affect local DLR significantly.
Can it affect local temperatures as well or is the GHE essentially global in its effects, as a result of the global energy budget?
propater:
The last one is most correct. If a gas can absorb at a wavelength it can also radiate at that wavelength.
If the atmosphere was transparent to the longwave radiation it would not be able to absorb any heat from radiation and the only heat would come from convection/conduction from the surface (see The Hoover Incident ). And it would not lose any heat by radiation.
Because the atmosphere can absorb radiation from the earth it heats up and therefore also radiates. Without the downward portion of this radiation the earth would be much cooler.
Alexandre:
It is always, first of all, a “local” effect on temperatures.
But then heat flows from higher to lower temperatures which is what redistributes heat around the globe – so, for example, the poles radiate more than they receive in radiation, while tropical regions radiate less than they receive. For a little about that last point, see Predictability? With a Pinch of Salt please.. Part One
gnbatt
You won’t find references to “greenhouse analogies” in any articles on this site except for using “greenhouse” gases as a well-known term.
Take a look at the CO2 – An Insignificant Trace Gas? series for more detailed explanation.
s of d,
Thank you.
It would appear that the sd is much larger than the signal of warming that CO2 is creating.
How is this reconciled in the science of doom?
I wouldn’t worry about the standard deviation in that context here. Most of the deviation value here comes from day-and-night cycles. Measuring an increase shouldn’t be much of a problem.
Mait,
That seems a bit convenient, and does not seem to actually reconcile anything.
hunter:
It’s an issue of frequency bands. The sd is from the diurnal cycle, which is easy to separate from a slow multi-decadal secular trend like CO2 increase.
Is the second part actually true for the emission side of DLR. I would think that the DLR value wouldn’t be much different for atmopshere with 90% CO2 concentration and 0,0009% CO2 concentration if the temperature of the gas is the same. Only the amount emitted by a single CO2 molecule would be much higher for lower concentration. It would borrow the energy from other gases through collisions.
Emissivity = absorptivity. Would you say that the absorption of radiation wouldn’t be much different for 90% or 0.0009% CO2 because the CO2 molecules can just transfer the energy to the other components by collision? If you did, you would be wrong. Emission will change little in the center of the band once the absorptivity gets close to 1, but the emission band itself will get wider and the total emission will increase as the concentration of CO2 increases.
You are right, I wrote it in quite a bad way. Actually the amount radiated by an CO2 molecul would be same, but they would just occupy a larger amount of space. The DLR should still stay the same though.
hunter:
Perhaps you are misunderstanding the meaning of “standard deviation”. I think I have confused people by putting it on the graph.
In any case I suspect you are thinking of it as some kind of error estimate?
But in this case it isn’t. It is just a measure of the “spread” of the signal.
SoD,
If the natural spread is much larger than the signal being looked for what is the significance of the difference?
As Spencer points out, small changes in cloud cover can explain the observed changes, for example.
Mait,
Only at the center of the band where emission and absorption are saturated or emissivity = absorptivity = ~1. The width of the emission band would change drastically from 0.0009% to 90%. Since the total emission is the integral of emission as a function of wavelength over all wavelengths, the total DLR from an atmosphere with 90% CO2 would be much higher than for an atmosphere with 0.009% CO2. A quick and dirty calculation using MODTRAN ( http://geoflop.uchicago.edu/forecast/docs/Projects/modtran.orig.html ) at an altitude of 0 km looking up with the 1976 U.S. Standard Atmosphere gives the following net increases in DLR from 0 ppmv CO2 for order of magnitude increases in CO2 starting at 9 ppmv or 0.0009%
concentration(ppmv) DLR (W/m2)
9 12.9
90 22.2
900 32.8
9000 47.3
The assumptions used in the MODTRAN calculations start to break down for high concentrations of CO2 so I didn’t bother trying to calculate 9% and 90%. But I assure you that DLR would continue to increase as the concentration of CO2 increased.
True, I probably added way too many zeros to my numbers. Though is modtran calculator particularly good for this kind of calculation (I think it recalculates surface temperature as well)?
A more refined argument would be that for bandwidths where the atmosphere is nontransparent at concentration1, increasing the concentration doesn’t have any effect on DLR as long as the temperature is constant?
Disclaimer: I’m not saying that increasing CO2 concentrations on earth wouldn’t result in an increase in DLR. My hypothesis is rather that DLR on the surface is highly dependant on the temperature of the lower layers of the atmosphere. That’s why I don’t quite understand why the DLR measurements are not coupled with temperature measurements.
If the natural spread is much larger than the signal being looked for what is the significance of the difference?
sorry- hit the wrong reply button.
Mait:
Increasing the concentration does have an effect on DLR. Emission at any given wavelength is proportional to:
- the concentration of emitters at that wavelength, and
- the Planck function (i.e. proportional to the 4th power of temperature)
Yes, but aren’t we talking about bodies with a contsant volume and mass here. If we decrease the concentration, radiation from particles higher up will reach the surface (so to speak), which means that the radiating “body” would become larger as well.
“If the atmosphere was transparent to the longwave radiation it would not be able to absorb any heat from radiation and the only heat would come from convection/conduction from the surface (see The Hoover Incident ). And it would not lose any heat by radiation.”
Doesn’t that last statement apply to infrared radiation only? As long as the temperature is above absolute zero, the atmosphere must radiate at some wavelengths, no?
Rocco:
There’s no fundamental requirement that a gas has to radiate. In practice, as far as I know, all gases do radiate even though the intensity for some is so low as to be unmeasurable.
For example, N2 has an absorption factor (which equally applies to emission) of 10 billion times less than CO2 in the 4um band. And nothing outside.
So in this area of the band (which is infrared) N2 will radiate at best at 1/5,000,000 of the intensity of CO2. (This takes into account the relative concentration of the 2 gases).
There’s nothing special about infrared/non-infrared. And a gas at atmospheric temperatures will only be able to radiate anything of significance in the infrared bands (>0.7um). Here is a log plot of blackbody spectral intensity for some typical atmospheric temperatures plus higher temperatures we never see (325K/52′C and 350K/77′C):
As you can see the intensity of radiation is extremely low below 1um. For example, for 300K (23′C) the intensity is less than a million million times lower at 1um compared with 4um.
The significance of the planck curves is that if a gas has strong emission lines at these low wavelengths the intensity would still be so low as to be unmeasurable.
scienceofdoom,
You’re neglecting collisional induced absorption (CIA). When, say, a nitrogen molecule collides with another molecule, the electrostatic field of the molecule can be distorted resulting in a dipole moment. A photon can be either emitted or absorbed by this process. The process results in a continuum spectrum. Peak absorption is about 100 micrometers. CIA for oxygen and nitrogen can be neglected at low altitudes where there is significant water vapor present. The CIA for water vapor, which is likely responsible for what’s called the water vapor continuum, is many orders of magnitude higher than for oxygen and nitrogen. But if you have a limb path through the upper atmosphere, CIA for oxygen and nitrogen can’t be neglected.
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