Friday, November 30, 2012

Thermodynamic Reference Layers - A Visual Aid

When discussing the Greenhouse Effect, radiant energy tends to dominate the discussion and most folks have  some difficulty visualizing some of the concepts.  This visual aid will not be perfect, but hopefully, will resolve some of the problems.  The numbers in Watts per meter squared correspond to temperatures if each of the colored layers were ideal radiant surfaces.  Radiant energy emits isotropically, or in all directions equally.  Since the drawing is in two dimensions you have to imagine that each of the colored layers are emitting towards, away, up, down and to each side.  The arrows for the red 500 Wm-2 point to what would be the poles of the Earth.  From the Equator, the center of the red, temperature drops by about one degree for each degree as you get closer to the points of the arrows.  316Wm-2 is the energy of a surface at 0 degrees Celsius, the freezing point of fresh water.  240Wm-2 is about -18 C degrees which is what the temperature of the Earth looks like from space.  200 Wm-2, about -30 C degrees (28.4 C actually, but 30 is a nice round number) is approximately the effective radiant layer of the atmosphere and 65 Wm-2 is roughly the effective temperature of the tropopause, which may require some explanation.

Since the coldest temperature ever measured in the Antarctic is about -90 C degree (~65Wm-2)  it is a good reference temperature in my opinion.  In my opinion just means it is an assumption, there is no known physical property that restricts the tropopause to any fixed temperature.  The -30 C degrees is an approximation based on the Stephens energy budget estimates which tend to make reasonable sense.  The 240 Wm-2, -18 C is used often in the literature as the no greenhouse gas temperature of the surface of the Earth based on what it would appear to be from space.  The freezing point of water is a solid physical standard.  Even the freezing point of water can vary, so all these layers are just rough references.

Below the red layer is supposed to be the deep oceans.  334Wm-2 is roughly the radiant energy of a surface at 4 C degrees, the approximate average temperature of the oceans.  The oceans though are liquid and much denser than the atmosphere.  Because of the density of the characteristics of liquid water, radiant energy is not typically used in liquid energy flow calculations.  Water has a density of about 1 kilogram per liter at sea level.  Air has a density of roughly 1 kilogram per meter cubed.  Since there are 1000 liters in a cubic meter, the density of air at sea level is about 1/1000 of the density of water.  As altitude increases, the density of the air decreases which causes a great deal of the confusion.  Density and heat capacity are related.  Lower density, lower heat capacity.

As density decreases, the utility of radiant energy transfer makes more sense.  There is less conduction and convective energy transfer because there are few molecules to collide with each other transfer energy by contact, conduction.  Since convection is driven by differences in density, the lower the density the less difference there can be, so convection decreases.  The only means to transfer energy in a vacuum is through radiant energy.