Sunday, August 16, 2015

Why greenhouse gases accelerate convective cooling in the troposphere

Stephen Wilde has alerted us to a new chapter on planetary atmospheres published in Treatise on Geophysics, 2nd Ed, Volume 10, 2015, which underlies a number of points made in his prior HS post

Erasing AGW: How Convection Responds To Greenhouse Gases To Maintain The Hydrostatic Equilibrium Of The Atmosphere.

Stephen writes,
"Amongst much else he says this:

“At high pressures, collisions generally occur before an excited atom or
molecule undergoes radiative decay and emits a photon in a
random direction, with the result that vibrational or rotational
energy is converted into KE [Kinetic Energy] and heat, called thermalization
or quenching.”

So, the higher the pressure at a surface beneath an atmosphere, the more
likely is energy transfer by collisions in place of photon emission.

Thus the denser the air at the surface the more readily energy will be
passed from GHGs to non GHGs.and the less photon emission there will be
relative to temperature.

The 'ideal' adiabatic lapse rate slope set by mass and gravity marks the steady decline in the probability of photon emission relative to collisional activity as one descends through atmospheric mass. All the actual lapse rates have to average out to that 'ideal' lapse rate for an atmosphere to be retained.

I've been saying that for a while.

Just as I have been saying all along, atmospheric mass reduces photon
emission in favour of collisional activity so that the surface temperature
can rise without destabilising radiative equilibrium with space.

Since the surface is heated unevenly, convection then stores that additional
surface energy in the form of potential energy within rising and falling
columns of air. That potential energy is taken up and returned towards the
surface in a never ending cycle for so long as there is an atmosphere.

The descending column inhibits convection beneath it and so continuing insolation can raise surface temperature beneath the column above the Stefan-Boltzmann prediction.

Earth, as a whole, radiates 255k to space but 33k is recycled within convective
overturning and it is that 33k which provides the necessary energy for the
motion involved in convective overturning."
The author also notes "convection dominates lower tropospheres" in the radiative-convective equilibrium of Earth's troposphere and other planets with thick atmospheres. In other words, if greenhouse gas "radiative forcing" [a term not used by the author one single time] increases, a compensatory increase in convection (and evaporation) will negate and erase any such warming at the surface from greenhouse gas "radiative forcing."

Section of the chapter discusses convective stability and provides derivations of the same mathematics underlying the HS 'greenhouse equation.'

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