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QUANTITATIVELY
Resevoirs in units of 10^12 kg of carbon, rates of transfer in units of 10^12
kg of carbon per year.
... And what happens to the water depends on the Temperature which depends
on the amount of greenhouse gasses.....
Temperature
where hPa is the same as mbar
THE LAYERS & PAUSES - these plus thermosphere, ionosphere,
homosphere,
heterosphere, exosphere, homopause, exobase, etc.
NOTE the size of a scale height in each case compared with the height of the
troposphere - the troposphere is about 2-3 scaleheights.
Radiative Transfer - absorbtion/radiation
How do we QUANTIFY this greenhouse effect? How do we MODEL the absorption/
radiation?
First start with a very, very simple model..... see Chapter 3 pages 52-61
of Goody & Walker
NOTE - Te = Teff is the EFFECTIVE temperature - that's the &
quot;temperature"
that is derived by measuring all the (blackbody - or thermal radiation) IR flux
from a planet (e.g. by flying over it or from an IR telescope on Earth) and
saying what temperature object would be emitting this amount of flux?
The OPAQUE SLAB MODEL - sometimes called "grey model" - does not
depend on wavelength.
Assumptions:
- Each layer totally absorbs all light coming into it and radiates at its
own temperature
- There are no additional heat sources (no heating from the inside of the
planet, no additional absorption in one of the layers)
Starting at the top of the atmosphere and working down layer by layer, equating
the flux into each layer with the flux out
Layer 0 - T1 = Teff
Layer 1 - sT24 = sT14
+ sT14 ....so, T
24
= 2T14
Layer 2 - sT34 + sT14
= sT24 + sT24
....so, T34 = 2T
24
- T14 = 4T1<
sup>4
- T14= 3T1<
sup>4
.....
Layer N - TN4 = NT
14
So, if you imagine you keep going down the layers until you reach the surface
of the planet - where the temperature is Tg - then you can envisage an expression
Tg4 = (1+t) Teff4
where we call t = OPTICAL THICKNESS (why
(1+t)
rather than t? Good question. I have no idea - probably
due to some alternative derivation).
Earth: t = (Tg/Teff)4 -1 = (288K/255K)<
sup>4 -1 = 0.6 - the Earth's atmosphere is thin - not even one totally absorbing
layer.
Venus: t = (Tg/Teff)4 -1 = (750K/238K)<
sup>4 -1 = 98 - Venus atmosphere is very thick (G&W quotes a surface
temperature
of 700 K and gets t = 68 - but the book was before
the Soviet landers on Venus).
NOTE: This quantity t = (Tg/Teff)4 -1
= OPTICAL THICKNESS - we shall introduce a related quantity
e = OPACITY in the next class.
The next step would be to add heating and cooling due to absorption and radiation...
and make a much more complicated model.
Rates are in units of Kelvin per day
In reality.... it's further complicated....by the fact that there are horizontal
variations - some places have clouds, some do not. And there are vatiations
with latitude.... etc. But, when you average over the globe, for Earth, this
is what you get for a NET RADIATION BUDGET - visible light in (sunlight)
- and IR light out (thermal radiation).
units are Watts per square meter. Here's a color version....
And the Sun varies with time....
.... now that we have heated up the atmosphere - what about moving
it around - next.....
.
.
.
.
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