We have mostly looked at the upward spectra at the top of atmosphere (TOA) as various conditions are changed. There’s a good reason for this focus – the outgoing longwave radiation (OLR) determines how much the climate system cools to space.
Over a given timescale this either matches absorbed solar radiation or the planet is heating or cooling. So it is changes in OLR (or absorbed solar) that really affect the heat balance in the climate.
By comparison, the trend in downward longwave radiation (DLR) at the surface is more a result of overall planetary heating and cooling. But of course, the climate is a lot more complex than indicated by that last statement.
Let’s take a look. Note that Part Four – Water Vapor already has some graphs of how the DLR or “back radiation” changes with water vapor concentration.
Here is the DLR for 4 different surface temperatures. In each case there is a lapse rate of 6.5 K/km, the boundary layer humidity (BLH) = 100%, the free tropospheric humidity (FTH) = 40% and there were 10 atmospheric layers in the model with a top of atmosphere at 50 hPa. More about the model in Part Two and Part Five – The Code.
The top graph is the real case, the bottom graph is without the effect of the water vapor continuum:
Figure 1
The continuum operates over the whole range of terrestrial wavelengths of interest, but its main impact is in the “atmospheric window region” between 800-1200 cm-1. This window region doesn’t have many strong absorption lines so absorption from any other cause has a big effect.
As we can see, the “window” is very dependent on temperature – which is mainly a result of the amount of water vapor. It’s clearer when we look at the spectral difference between the two cases for each of the temperatures:
Figure 2
Notice that the 273 K (0 °C) condition is almost unaffected by the continuum. This is because the effect is dependent on the amount of water vapor squared. And the amount of water vapor is strongly dependent on temperature.
Let’s look at the total flux for both cases and compare with a reference of blackbody emission from the bottom layer of atmosphere (in this case 400m above the surface so about 2.6°C cooler than the surface, and see note 1):
Figure 3
This shows that once we are above a surface temperature of 300 K (27 °C) with high boundary layer humidity the radiation from atmosphere to surface is getting close to blackbody emission. The graph also demonstrates that most of that change is due to the continuum.
Now good emitters are also good absorbers. So here is another way of looking at the same effect – the % of surface radiation in the 800-1200 cm-1 window region that makes it to the top of atmosphere (without being absorbed anywhere along the way):
Figure 4
These were all with CO2 at 360ppm (and N2O at 319 ppbv, CH4 at 1775 ppbv and no ozone).
Let’s look at how changing CO2 concentration affects these results.
Figure 5
This is a very important graph – what does it show?
- while different surface temperatures have quite different TOA radiation to space – the change in CO2 causes a fairly constant change in this radiation
- changing CO2 has much less effect on the DLR (radiation from the atmosphere to the surface), and as the temperature increases this effect is even more reduced
Let’s look at the “delta”:
Figure 6 – [Corrected Jan 23]
This shows clearly how the change in atmospheric DLR due to doubling CO2 is very much a function of surface temperature. And at the same time, the change in TOA radiation (“OLR”) is almost independent of surface temperature.
From the information presented in this article on how DLR is affected by water vapor at high temperatures the first point shouldn’t be surprising. And from the explanation in Part Four – Water Vapor both points shouldn’t be surprising.
For interest, here are the two DLR spectrum for 280 ppm & 560 ppm at 288 K, and below, the difference:
Figure 7
Conclusion
The surface energy balance is very important for determining the dynamics of surface heat transfer, including initiating convection. As the temperature gets up to 30°C the ability of the surface to radiate to space is reduced to a very low value.
“Deep convection” which drives the tropical circulation is mostly initiated in these very hot surface conditions.
The effect of changing CO2 on atmospheric radiation to the surface (DLR) is small. With high boundary layer relative humidity, water vapor masks out most of the effect of changing CO2 in hotter surface conditions.
But the effect of increasing CO2 on the TOA radiation balance is completely different. High surface humidities have little or no effect on this TOA balance. And there, doubling CO2 has a significant impact (all other things being equal) as shown in figure 12 of Part Seven – CO2 increases.
Working out radiation balance through the atmosphere in your head is difficult. Most people attempting it don’t have the right “calibration points”.
The fundamental physics is straightforward, at least in terms of the values of absorption and emission of radiation (not the “why”). But calculating the result requires computing effort and an integration (summation) across:
- multiple layers at different temperatures and concentrations
- the hundreds of thousands of absorption/emission lines of multiple GHGs
- a large range of wavenumbers
Related Articles
Part One – some background and basics
Part Two – some early results from a model with absorption and emission from basic physics and the HITRAN database
Part Three – Average Height of Emission – the complex subject of where the TOA radiation originated from, what is the “Average Height of Emission” and other questions
Part Four – Water Vapor – results of surface (downward) radiation and upward radiation at TOA as water vapor is changed
Part Five – The Code – code can be downloaded, includes some notes on each release
Part Six – Technical on Line Shapes – absorption lines get thineer as we move up through the atmosphere..
Part Seven – CO2 increases – changes to TOA in flux and spectrum as CO2 concentration is increased
Part Eight – CO2 Under Pressure – how the line width reduces (as we go up through the atmosphere) and what impact that has on CO2 increases
Part Nine – Reaching Equilibrium – when we start from some arbitrary point, how the climate model brings us back to equilibrium (for that case), and how the energy moves through the system
Part Eleven – Stratospheric Cooling – why the stratosphere is expected to cool as CO2 increases
Part Twelve – Heating Rates – heating rate (‘C/day) for various levels in the atmosphere – especially useful for comparisons with other models.
References
The data used to create these graphs comes from the HITRAN database.
The HITRAN 2008 molecular spectroscopic database, by L.S. Rothman et al, Journal of Quantitative Spectroscopy & Radiative Transfer (2009)
The HITRAN 2004 molecular spectroscopic database, by L.S. Rothman et al., Journal of Quantitative Spectroscopy & Radiative Transfer (2005)
Notes
Note 1: This model looks at the range of wavenumbers 200-2,500 cm-1, which equates to 4-50μm, to ease up the calculation effort required. This means that when we sum up the contribution from all calculated wavelengths we are missing some bits. So for example, if we calculate the emission of thermal radiation by a surface at 288K with an emissivity of 1.0 we calculate 390 W/m² – the “blackbody flux”.
But with our “restricted view” of the spectrum we will instead calculate 376 W/m².
Almost all of the “missing spectrum” is in the far infra-red (longer wavelengths/lower wavenumbers), and is subject to relatively high absorption from water vapor.
SoD: The sentence “There’s a good reason for this focus – the outgoing longwave radiation (OLR) deter mines how much the climate system cools to space” is incorrect. The outgoing longwave radiation (OLR) determines how much longwave radiation (OLR) come directly from the surface to All. A cooling is not available.
Micro-physically makes the term “back radiation” sense, not thermo- physically. Thermo-physically is the “Back Radiation” only a mathematical value.
Thermo- physically make only sense the heat flow from the hot surface to the cooler atmosphere (and only this heat flow can be measured), which depends only on the temperature difference between the surface and the absorbing layer of the atmosphere. These low heat flow can be converted with the Pyrgeometer-equation in the microphysical value of the radiation.
SoD: Figure 5 shows what is described in the text. But in Figure 6 it seems that the green and blue line are switched.
Many figures show a kink at 288K. Have you done the calculations only at three temperatures or is there an other source of this kink. It seems not very real to me.
Uli,
Thanks, I have corrected the graph (figure 6).
About the “kink” – yes this is the result of a limited number of runs. The calculations were done at 4 temperatures – 273, 288, 300, 305K.
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[…] Part Ten – “Back Radiation” – calculations and expectations for surface radiation as CO2 is increased […]
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[…] For the tropics, the hot humid atmosphere radiates quite close to a blackbody, even in the “window region” due to the water vapor continuum. We can see this explained in detail in Part Ten – “Back Radiation”. […]
SoD
Looking at the radiation models, I see that you are always calculating radiation from a surface heated to a certain degree through the atmosphere.
However looking at the radiation budget, it gives a different picture: about 1/3 of absorbed incoming solar radiation is happening directly in the atmosphere. Only remaining 2/3 are warming the ground.
This is a big difference and shows the warming of the atmosphere coming from the sun, not the surface?
The net exchange with the surface are in comparison small, the net exchange by radiation being about equal to the net exchange by conduction (excluding the water cycle)
Lars,
The energy balance of the troposphere includes LW, SW, convection, and latent heat transfer. In none of the calculations presented in this and the related threads any realistic attempt is done to calculate all those. What’s done is to use some other method for fixing the temperatures, either standard atmospheres or assumptions about the lapse rate. For this reason solar SW does not affect the calculations if IR in the troposphere.
For stratosphere solar SW is essential as discussed in part twelve.
Lars,
What you seem to be missing is that we can measure the temperature, pressure and humidity profiles in the atmosphere with radiosondes attached to balloons. We can measure the atmospheric emission spectrum with an FT-IR spectrophotometer at the same time. Then, using the same data, we can calculate the emission spectrum using a radiation transfer model such as a line-by-line program. Guess what, the measured and calculated spectra agree closely. The exact energy balances in the atmosphere are unnecessary to this calculation.
Payne,
If I correctly understand models work from radiating – cooling – of the heated ground. However the “ground” temperature is not that of the ground but of the air at 2 meters.
The ground itself does have in reality a different temperature.
The measured and calculated spectra might agree, but only a proper energy balance would give proper results which (proper energy balance) seem to be missing in the models?
From the abstract of the 2012 paper by Stevens et al we can read
Another 2012 paper on Earth’s Energy Flows is this by Stevens and Schwartz.
From both of these papers we can learn that the details of the energy balance of the surface and the interior of the troposphere are not as well known than the balance at TOA. The reason is that at TOA we have only radiation (SW and LW) while many different mechanisms contribute at surface and in the troposphere.
The exact level where the temperature is measured is one small issue related to these uncertainties, but my impression is that it’s not a particularly important one.
SoD may correct me, if I have missed something, but I think that these issues have not been a subject of posts on this site. Perhaps they might soon have their turn.
I have a misprint in the first reference. The correct name is Stephens, not Stevens as in the second.
Lars P.
SURFRAD
And AERI
At the TOA, a significant fraction of the photons emitted by the surface are observed mainly in the “window” region from ~8-12 μm, and then only if there is no cloud cover. Almost everywhere else, the transmittance through the atmosphere is zero so emission is determined by the temperature of the atmosphere at the effective height of emission, which is equivalent to the altitude where the optical depth measured from infinity down equals 1. The optical depth is a function of wavelength.
Pekka, Payne, thank you for your answers! Will need to find some time to get through, not really easy to find those free minutes… however, at first sight I have some comments:
Pekka Pirilä,
From Stephen’s paper: “Specifically, the longwave radiation received at the surface is estimated to be significantly larger, by between 10 and 17 Wm –2, than earlier model-based estimates” is actually one of the points I was trying to address with my post.
If we take for example a desert, the sand may be heated to 70°C however the air above has 45°C (as example).
The radiation from 70°C is something completely different then the 45°C radiation…
The way how this behaves is not correctly covered with a model averaging solar radiation?
Correct at TOA we have only radiations, however the data that we have from there is only recent and with huge error margin.
The climate might be happening completely under the TOA?
http://www.bbso.njit.edu/science_may28.html
Payne
Thank you for the links. I would still say that SURFRAD with the 7 station is still only the beginning of the story, but good to see that data is being collected.