Friday, May 6, 2011
Back to the Tropopause, Again
I photo shopped (poorly) the radiation spectrum for the atmosphere trying to layer it in a way that is something like it changes with altitude. Now the bottom panel represents what IR "sees" from the Earth's surface. Water vapor is next up followed by the other stuff. Oxygen-ozone should be on top also because of the ozone layer in the stratosphere, but I missed that.
Looking at the bottom total spectrum you can see the "window" that continues from the surface. This window releases the majority of the surface radiation/heat to space. In this chart the window is the blue part at the top.
I am now going to add a skillfully edited version of the shopped spectrum with two red lines that boarder the main window and a thin black line showing the main portion if CO2 in the IR spectrum (the right third roughly of the chart).
I managed to mangle the size but hopefully you can see the lines. The height of the spectrum lobes indicate the percentage of absorption/emission. The individual gases do not capture the infrared radiation, per se, they absorb and the re-emit, which delays the rate of radiative flow both into and out of the atmosphere. The into part of that is a little bit confusing. Radiation doesn't have to exactly follow the laws of thermodynamics. If it has a clear window it can go any direction it likes. The radiation does have to follow the laws of thermodynamics if it heats anything. The Sun can only heat the Earth because it is warmer than the Earth. The Stratosphere is warmer than parts of the Earth and a good deal of the upper troposphere, so it can heat the things it is warmer than regardless of the direction. With the exception of ozone, infrared radiation has a clear picture window in the tropopause.
I used a traffic analogy to describe the Greenhouse effect. Increased CO2 delays or restricts outgoing radiation in its spectrum. That delay allows molecules below the CO2 absorption become warmer because they cannot release radiation/heat as easily due to the traffic jam, if the radiation/heat is on the backed up road or spectrum of CO2. The molecules can also gain or lose heat via collisions with other molecules. Those molecules that gain heat can lose the heat via more collisions with other molecules or in some cases relax and release a photo of infrared energy in their spectrum.
Collisional heat transfer between the gas molecules dominates in the more dense atmosphere with water vapor, the main greenhouse gas with the largest spectrum, absorbing most of the outgoing infrared heat radiation. The diatomic, two atom molecules, like oxygen and nitrogen, absorb insignificant amounts of heat via infrared radiation. They are for all practical purposes clear window for infrared as shown in the spectral charts. Ozone, O3, does absorb and emit infrared in its spectrum, as well as the other greenhouse gases.
From the Earth's surface, heat is transferred to the atmosphere by convection, radiation and conduction. Both convection and conduction do their thing with collisions. Convection, latent heat, accounts for about 23% of the atmospheric warming, with 15% via infrared radiation and about 7% of the warming is by conduction according to the NASA energy budget cartoon. 6% of infrared in over the 15% absorbed by the atmosphere, passes directly to space through the window.
All of these percentages are dynamic. Molecules gain and lose heat which slows the progression of heat through the atmosphere, which allows the Earth to have its warmer temperature. While there is significant water vapor in the air, the convection or latent heat transfer is the big player at 23% decreasing with altitude. Water vapor is also a major player in radiative transfer because of it higher concentration and broader spectrum. At some point in the upper troposphere, water vapor decreases to the point where its combined latent and radiative heat transfer equals that of CO2, methane, NOx, O3, etc. combined. I don't know if this point has a name, so I will call it the Greenhouse radiative impact equilibrium (Grie) layer. This Grie layer at the top of the troposphere is dynamic. It is higher at the tropics, lower at the poles and varies with season, planetary waves, upper air turbulence and convection. Because it is dynamic, the conventional two dimensional radiative models do not do it justice.
Above the Grie layer, the anthropogenic greenhouse gases begin to dominate. As we move higher above the Grie layer, collisional heat transfer reduces, increasing the impact of spectral absorption and emission at fix wavelengths according to the spectrum of the involved gases. Cooling by pure radiative emission is the source of the tropopause. At the Tropopause, more than half of the emitted infrared has a path both radiatively and based on the laws of thermodynamics, to space. If it were not for the dynamic nature of the upper troposphere, the percentage over half estimated in the simple two dimensional models would be small enough to neglect. My theory of the tropopause sink is that the percentage is not only not negligible, but an important part of the regulation of atmospheric temperature. Water vapor can emit in wavelengths outside of the other GHGs, so it is capable of releasing a higher percentage of heat in the form of infrared at the Grie layer.
Proving the significance of the Tropopause sink is not a piece of cake. Temperature and pressure data for this region is noisy and not finely tuned to the layer. The RSS TTS product, is a rough indication, but far from adequate as the band pass filter is so broad it encompasses more of the upper troposphere and lower stratosphere than needed. It does show a few interesting things. One is that the temperature trend is virtually zero. The second is that the 1998 super El Nino had a major impact with a very rapid reversal of temperature.
The atmosphere has a typical pattern of more rapid warming than cooling below the middle troposphere. This pattern is pretty clear in all the surface temperature averages and is most predominate in the higher latitudes. There can be a variety of explanations for this, lower convective heat transfer to the tropopause is a possibility. Rossby waves and other planetary waves, quasi-oscillations, appear to be less efficient at transferring heat above the Grie layer. Cooling is then more gradual unless a severe storm season occurs. Convection in the tropics is very effective at releasing heat.
There is some confusion if a warmer atmosphere will increase total convection, but it is indeed likely. Increased convection will not only increase the heat loss above the Grie layer it will also increase reflection of short wave radiation with increased convective clouding. It is unlikely that the Tropopause sink will eliminate the impact of increased GHGs, but it appears to be capable of limiting the sensitivity to the lower half of the estimated IPCC range.
There is a lot more work to be done to determine the impact of the Tropopause Sink, if it indeed has a major impact greenhouse gas related climate. It is interesting though.
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- ▼ May (27)