Radiative Energy Transfer in the Atmosphere
Dai Davies, 190106
In this note I attempt to clarify the role of radiative transfer of energy in the Earth's atmosphere.
Since I first studied physics I've been aware that any solid object above 0˚K radiates heat. It was only recently that I became aware that the atmosphere does too via radiatively active gasses (RAGs) such as water vapour and carbon dioxide. It took me a while to get used to the fact, and I've noticed that others also find the idea a little strange.
RAGs are bumped into internal excited states during collisions with other molecules then, if they don't lose the excitation energy in a subsequent collision (the usual case) they emit it as an infrared photon. This is usually absorbed by another RAG molecule. Some reach the Earth's surface or escape to space.
Figure 1: Earth's Energy Budget with my additions
This is explicitly shown in Figure 1 in the upper troposphere (“emitted by atmosphere”) but less obviously so near the surface.
Figure 2: Re-radiation of energy absorbed from surface radiation
The 400W/m2 radiated by the Earth's surface is nearly all absorbed in the lower few hundred metres of the atmosphere. In the example of Figure 2 the energy is re-radiated with one sixth going down. Going from this simplified 1D example to a full 3D collisional model could have up to half going down, but most of the excitation energy is lost in collisions increasing the molecular kinetic energy (temperature) of the air. Only a few W/m2 will be directed at the ground. The 340W/m2 labelled “back radiation” in Figure 1 is almost totally generated by collisions.
Missing in the original NASA version of Figure 1 was radiative transfer on the mid-troposphere. I have added that in dark red.
The temperatures of the surface and lower atmosphere will tend to equalise with the air, on average, cooler because it's losing heat to space1. If heat is transmitted rapidly up to space and not significantly accumulated there is no significant greenhouse effect. In my Radiative Delay articles I look at upward radiative transfer in detail and quantify the delay. It does not create significant heating.
If the greenhouse effect is not significant then how does the atmosphere warm the Earth's surface? That it does is not generally disputed. These diagrams are time averages. During the day, with the Sun heating the surface, net energy flow will be from surface to air, with the reverse being the case at night. The Diurnal Smoothing Effect (DSE), which I describe elsewhere2, provides the answer.
1: It is sometimes claimed that the second law of thermodynamics prohibits the transfer of heat from a cool atmosphere to a hotter surface. This is incorrect. The law refers to net transfers which will always be from hot to cold. This doesn't prevent individual photons from travelling either direction.
2: The DSE is described with an example calculation in http://brindabella.id.au?f=EAR, and modelled in detail in the Open Climate Modeller (OCM) software package.