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Saturday, May 15, 2010

Structure of the Atmosphere

This post is part of a primer on infrared spectroscopy and global warming. The previous post introduces a three-layer model  in the context of developing a radiative-transfer model of the atmosphere.  From that post it should be apparent that one needs to include an understanding of the structure of the atmosphere to understand radiative transfer through the atmosphere. This post provide a summary of the structure of the atmosphere.  It is intended to be a quick introduction, rather than a detailed treatise.  Some of the sources listed go into more detail for the interested reader.


The earth can be conveniently divided into four layers: the troposphere, the stratosphere, the mesosphere, the thermosphere.  The region outside the thermosphere is sometimes referred to as the exosphere.

The Troposphere

The troposphere is the lowest layer and the layer in which we live.  By mass it accounts for 80% of the atmosphere.  It is the increase of carbon dioxide in the troposphere that is cause of global warming.  The height of the troposphere  varies by latitude and other effects.    It is thinnest at the poles and thickest at the equator and can vary from about 7-17 km in thickness.

Generally speaking, temperature decreases with height through the troposphere. The surface temperature of the earth is about 288 K,  and it decreases about 6.5 K per km in altitude.   There are thin regions within the troposphere in which the temperature increases with altitude.  These temperature inversions inhibit mixing and can exacerbate smog by inhibiting dilution.

At the top of the troposphere, the decrease in temperature stops in a region called the tropopause.  the temperature in this region averages about 218 K. 

The Stratosphere

The stratosphere extends from the tropopause to about 50 km in altitude.  Within the stratosphere temperature increases with height, slowly at first and more drastically at higher altitude.  At the top of the stratosphere, the stratopause, the temperature is about 270 K.

The stratosphere is very dry compared to the troposphere.  For the same reason that temperature inversions inhibit mixing, the stratosphere also inhibits mixing.

The stratosphere has a high concentration of ozone.  The ozone layer absorbs ultraviolet light.  Chlorinated fluorocarbons (CFCs) have been implicated in the depletion of the the ozone layer and are being phased out under a treaty called the Montreal Protocol.  CFCs are long lived in the atmosphere and are able to migrate to the stratosphere.  In the stratosphere, chlorine atoms are liberated and participate in heterogeneous reactions catalyzed by polar stratospheric clouds (PSCs) that deplete ozone.

The media often confuse the issue of ozone depletion with global warming, but they are really two different processes that occur for two different reasons.  Ozone depletion occurs because of chemical reactions within the stratosphere involving chlorine from CFCs, whereas global warming occurs because of the increased absorption of infrared radiation in the troposphere by increasing amounts of carbon dioxide emitted from burning fossil fuels.  Although there are potential feedback mechanisms that may cause these phenomena to interact with one another, to first order, they are separate issues.

One effect of global warming is that the stratosphere is actually cooling!  (The troposphere is absorbing radiation that would normally be absorbed in the stratosphere.  Because less radiation gets to the stratosphere, it cools. See comments for more discussion).

At the top of the stratosphere, in stratopause, the increase in temperature stops.

The Mesosphere and Thermosphere

The Mesosphere extends from the top of the stratopause at about 50 km to about 85 km.  The temperature decreases with altitude from about 270 K to about 180 K.  The Thermosphere extends to the exobase, which varies significantly from about 350-800 km.

Weather and Geography

Of course the temperature varies according to location on the earth as well as local and temporal weather patterns.  The values of temperature (as well as pressure) cited here are approximate mid-latitude mean values.  Accounting for variation, of course, makes the model more complex.






Pressure and Density

As altitude increases through the atmosphere, pressure decreases.  As pressure decreases there are fewer absorbers in the same volume of air.  Additionally, the frequency of collisions between molecules decreases and thereby changes the infrared spectra.

Pressure, p,  through the atmosphere decreases approximately exponentially.  If p0 is the pressure at sea level (or other reference point), then:

     p = p0*exp (-z/H)

where z is altitude in meters and H is a quantity called the scale height.  Scale height is a function of temperature at 288 K, it is about 8435 m.  Density decreases with altitude with the same relationship.

Composition

The following table shows the composition of the atmosphere by volume, relative to a dry atmosphere.  Infrared active species are noted.



Radiative Transfer

The next logical step to take in building a radiative-transfer model of the troposphere is to expand the three-layer model to a multi-layer model, in which I account for the temperature and pressure variation.  The next post in this series builds such a model for carbon dioxide alone.  After that step, I can start to discuss more sophisticated models of the atmosphere.  It is perhaps worth mentioning the natural tension between keeping the model understandable to the lay person, and including more detail into the model.  By going step-by-step, I hope to at least make some of the more sophisticated models understandable, but first I extend the simple model.


Sources

11 comments:

Fred Moolten said...

Hi Rich - I find your articles very informative, but having commented in a RealClimate blog on the mechanism responsible for CO2-mediated stratospheric cooling, I felt motivated to suggest a correction here. If atmospheric CO2 is increased and sufficient time is permitted for a steady state to be restored, the stratosphere will be found in a persistently cooler state than before the CO2 increase. However, this is not because less radiation is reaching the stratosphere. In a steady state, the outgoing radiation at the tropopause will equal the solar energy absorbed by the troposphere and the Earth's surface, regardless of CO2 concentration - the only effect of CO2 will be to have elevated the temperature at which surface and troposphere emit that same amount of radiation.

The best quantitative explanation for stratospheric cooling mediated by rising CO2 entails a role for stratospheric ozone (this is unrelated to the cooling effect of ozone depletion, which is a separate entity). Ozone, by absorbing solar UV and some visible light, warms the stratosphere. CO2, which does not absorb in the UV or visible range, increases the emissivity of the stratosphere, enabling it better to radiate the ozone-derived heat into space - this is because the IR emission rate from CO2 is determined by temperature. Because ozone contributes much of the heat to the stratosphere, this cooling effect outweighs the rather small warming effect that occurs from the additional IR absorption the extra CO2 would incur. In essence, the CO2 serves as an escape mechanism for ozone-derived warming. In the absence of ozone, added CO2 is predicted to warm both the troposphere and stratosphere, because CO2 cannot emit more radiation than it aborbs if there is no other heat source available to excite its emissions. If CO2 is the only radiatively active moiety in an atmospheric layer exposed to a given level of incoming IR, adding more CO2 will result in more IR absorption, followed by a temperature increase until a higher temperature is reached sufficient to restore IR emissions to the previous level.

Fred Moolten said...

The last sentence of my previous comment was confusing. Added CO2 will absorb more IR, raising the temperature until it reaches a level at which the CO2 present now emits IR at a higher rate sufficient to once again balance the rate of IR absorption.

Rich said...

Fred,

Thanks for your comment. "In essence, the CO2 serves as an escape mechanism for ozone-derived warming." That argument makes sense to me.

Rich said...

It is worth referencing the discussion in the comments to this post: http://www.realclimate.org/index.php/archives/2010/07/a-simple-recipe-for-ghe/

I think that both you and Gavin make some good points.

Because the stratosphere is warmer (owing to uv absorption by ozone and other species), it makes sense to me that the effect you are discussing must be operative. Until the tropopause actually warms significantly, however, the effect I discussed is also operative.

Fred Moolten said...

Hi Rich - I just saw your latest comment. If you visit the RC discussion, my comment #333 there summarizes the logic behind the claim that stratospheric cooling requires the presence of an absorber of sunlight, such as ozone. Comment #14 quotes from the true expert in this field, with additional details. At equilibrium, a warmer troposphere must (by the Second Law and the principles of radiative transfer) warm the layers above it, unless some additional warming source (ozone) is also contributing. Although it may seem counterintuitive at first, the same principle also applies during radiative forcing after a CO2 rise, when the temperature has not yet reached equilibrium. As long as any layer is warmer than before (even if only slightly), it will emit more heat, including emissions upward, and so it would still be a thermodynamic violation for a cooler layer above it to cool further rather than warm also. Note that these conclusions are based on a strictly radiative model, and might be altered by convective changes, but given that the main heat tranfer effect of convection is to move latent heat upward, it is unlikely that convection would reverse the sign of the changes in upper atmosphere temperatures.

Rich said...

Fred,

I get the point. Thanks.

Rich

Rich said...

It's probably worth mentioning that a similar mechanism might be involved with what's happening to the thermosphere: http://www.csmonitor.com/Science/2010/0715/Earth-s-upper-atmosphere-collapses.-Nobody-knows-why

Keith_J said...

What about the presence of noctilucent clouds in the mesosphere? These are observed during a hemisphere's summer.

Since the mesosphere exhibits a lapse rate like the troposphere, it is plausible a water vapor cycle exists in this layer. Such a cycle would transport far more thermal energy than that of radiation, creating a negative feedback with respect to temperature.

The fact that water vapor latent heat transport in the troposphere far exceeds that of radiation heat loss from the surface should be noted. This is easily quantified by average global precipitation (precipitation equals evaporation+sublimation) and noting the 42 kJ/mole enthalpy of vaporization.

The hydrologic cycle drives the mass exchanges in the troposphere. Enthalpy of dry air remains about constant since mean free path in sinking air masses essentially makes this process adiabatic.

Rich said...

Keith,

Not really sure what your point is. Such clouds have been observed in the mesosphere, but they are rare. The increased frequency may be related to climate change.

Charles nelson said...

I would think the table would better serve readers with PPM equivalence of h2o instead of 0-5%...My calc: 5% of 1 million ppm would be 50,000 ppm o water could be represented in chart as zero ppm to 50,000 ppm. but I am not sure zero is ever achieved in the Atmosphere. 1% h20 would be 10,000 ppm.... I think this shows a better relationship...

Rich said...

5 percent by volume is the same as 50,000 ppm by volume.