Re: atmospheric blanket / greenhouse effect

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

I'm no expert in radiative equilibrium, but I'll try to address the
questions that were raised by my post:

Bob LaMontagne wrote:
> A further refinement would recognize the importance of 
> convection in the lower atmosphere. The average temperature 
> profile in the lower atmosphere is exactly that followed by a 
> parcel of air warmed by contact with the Earth's surface and 
> then rises adiabatically through the troposphere. That has to 
> be more than just coincidence.

Actually, the average temperature profile in the lower atmosphere is
about 6.5 K/km (according to the US Standard Atmosphere) whereas the
temperature profile due to adiabatic cooling (assuming vertical balance
and no latent heating) is about 10 K/km.

John S. Denker wrote:
> [snip]
> Having said all that, evidence suggests that that
> this process [radiative equilibrium] is not dominant in
> our atmosphere.
> The non-isothermal nearly-isentropic profile of
> the troposphere is set by convection, swamping
> the effect of radiative transport.  (The height
> of the tropopause corresponds to the height of
> the tallest thunderstorms.)  Furthermore, the
> stratosphere is pretty much isothermal,
> indicating that in the absence of convection,
> radiation and re-radiation aren't sufficient to
> set up a large temperature gradient.

I agree that radiation balance is not sufficient to explain the
temperature structure of the atmosphere.  Indeed, in my simple model, I
assumed a single layer, i.e., the atmosphere had no structure at all.

Except for variations in the absorbtivity of solar radiation (e.g. the
ozone layer), I agree that the vertical structure of the atmosphere
depends a great deal on convection and transport.  That is why the lower
atmosphere is so close to isentropic and why the stratosphere is able to
maintain a negative lapse rate (it is isothermal only near the
tropopause).

However, I don't see how this affects the net balance, which is what
leads to the average surface temperature.  Somehow we have to get the
energy from the surface to space.

John S. Denker wrote:
> Are you claiming the magnitude of your effect is
> independent of the thickness of the atmosphere?

Yes and no.  The assumptions of absorption are certainly dependent on
the thickness of the atmosphere.  A very thin atmosphere won't absorb
100% of the outgoing IR, for example.  However, if you can meet my
assumptions (or close to it - say, 80%), then in a sense it is
independent of the thickness of the atmosphere.

Simply with my assumptions, I am able to get a surface temperature of
303K (assuming 30% albedo).  Assuming 10% absorption of solar by the
atmosphere and 80% absorption of terrestrial by the atmosphere, I get a
surface temperature of 286K.  Without an atmosphere, I get a temperature
of 255K.

Doesn't this seem to imply that the model captures the major physics?

Admittedly, there is at least one problem with my scenario - it doesn't
explain why Venus has such a high surface temperature.

Bob LaMontagne wrote:
> Unfortunately, CO2 is only a player in a few narrow bands of 
> the IR radiated upward from the Earth. Within those narrow 
> bands it is basically opaque and between the bands it is 
> almost totally transparent. Doubling the CO2 content of the 
> atmosphere produces no significant change in the IR radiation 
> budget.

I knew water contributes more but I must admit that I wasn't aware that
the atmosphere was already opaque to IR in the bands associated with
CO2.  Do you have a source for this that I can reference?

Does this mean that Venus' temperature profile is not due to its CO2
content?

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301