lördag 29 december 2012

The Earth-Atmosphere System as Blackbody

Data suggests that the Earth including the troposphere can be viewed as a blackbody heated by 140 W/m2 absorbed by the Earth surface and emitting 140 W/m2 at a bolometric temperature of about -55 C  (according to Stefan-Boltzmann's Law) attained at the tropopause at 0.1 - 0.3 bar as its "outer boundary".

The 140 W/m2 absorbed by the Earth surface comes from 240 W/m2 absorbed by the Earth-atmosphere with an albedo of 0.3 out of a total of 340 W/m2 incoming from the Sun, minus 60 W/m2 absorbed and re-emitted by the atmosphere minus 40 W/m2 directly radiated from the Earth surface to outer space through the "atmospheric window".

Including the troposphere and the stratosphere in the Earth-atmosphere system makes the stratopause  at 0.001 bar the outer boundary with an observed temperature of 0 C with corresponding blackbody radiation of 320 W/m2, which is close to the total of 340 W/m2.

The internal thermodynamics/raditation of the Earth-atmosphere system is very complex and difficult to accurately model, but it thus appears that it is possible to view the system as a single blackbody in two ways:
  1. Earth + troposphere 
  2. Earth + troposphere + stratopshere 
with an observed temperature of the "outer boundary" in reasonable correspondence in both cases with the bolometric temperature as the blackbody temperature of the same irradiance.

Viewing the Earth + troposphere as a blackbody with outer boundary or top of the atmosphere TOA temperature determined as bolometric temperature, makes the surface temperature depend on the lapse rate, and with the lapse rate mainly determined by thermodynamics, the surface temperature would be insensitive to small changes in the internal radiative composition of the atmosphere. In other words, there would be no detectable greenhouse effect.

For Venus the observed temperature at 0.25 bar is -20 C which is again equal to the bolometric temperature.

We thus find evidence that for both the Earth and Venus including atmospheres up to about 0.3 bar, the TOA temperature is the bolometric temperature determined by distance to the Sun, and the surface temperature is determined by a lapse rates mainly set by thermodynamics. 

1 kommentar:

  1. Hi Claes,

    Would you agree with this post by Clive Best?