ATMO 171 Introduction to Meteorology and Climatology

9/27/99

LONGWAVE RADIATION

Of the 100 "units" of solar radiation falling on the top of the atmosphere, only about 50 units, on average, are absorbed at the earth's surface. This energy goes into heating the surface of the earth. The earth then emits radiation upward as a black body, with most of the energy emitted at infrared wavelengths (i.e. "longwave radiation"). As Figure 3.24 in Danielson indicates, the amount of energy emitted upward by the earth's surface is about 105 units, on average. How can the earth's surface emit more energy than it absorbs? Does this mean the earth's surface should be getting colder?
Remember that the atmosphere absorbs much of the longwave radiation emitted upward by the earth's surface. Some of this energy absorbed by the atmosphere is emitted back down to the earth's surface (the greenhouse effect). The value of 105 units represents how much energy the earth emits after absorbing both shortwave radiation (from the sun) and longwave radiation (from the atmosphere). To understand how this all balances out, we have to look at the energy "budget" of each component in the earth/atmosphere system (i.e. the earth's surface, the atmosphere, and the system as a whole).

Energy Budget of Combined Earth/Atmosphere Sysytem
To perform an analysis of the energy budget, we compare the amount of energy entering the system to the amount of energy leaving the system. Figure 3.31 shows all the energy input and output of each component in the system. For the combined earth/atmosphere system, we are interested in the amount of energy going into the system (i.e. downwards across the upper boundary in the figure) versus the toal amount of energy leaving the system (upwards across the upper boundary). The total amount of energy input is simply the 100 units of energy from the sun. The total amount of energy output is the sum of all the individual contributions (i.e. the arrows crossing the upper boundary, pointing upwards).
 6 units shortwave radiation scattered by air molecules 
20 units shortwave radiation reflected by clouds
 4 units shortwave radiation reflected by the surface 
 6 units longwave radiation emitted by earth's surface
38 units longwave radiation emitted upwards to space by the atmosphere
26 units emitted longwave radiation emitted upward by clouds

 TOTAL = 100 units. 
Therefore, we can conclude that the radiation budget of the earth/atmosphere system as a whole is in balance - energy in equals energy out.

Energy Budget of the Earth's Surface
Now look at the energy input at the earth's surface (i.e. the bottom boundary in Figure 3.31).
 50 units of shortwave radiation absorbed from the sun. 
 85 units of longwave energy absorbed from both air and clouds

 TOTAL =  135
As pointed out above, we see from Figure 3.31 that the earth emits 105 units upwards. The budget tells us that 135 units in minus 85 units out leaves 30 units unaccounted for - the earth's surface is now absorbing more than it is emitting. Does this mean the earth's surface should now be warming? No. To understand how the budget balances at the earth's surface, we have to look at the additional processes in Figure 3.31 - namely, sensible and latent heat fluxes. But first, let's look at the budget for the atmosphere.

Energy Budget of the Atmosphere
Now we look at how much energy goes into the atmosphere (i.e. what gets absorbed) versus how much energy gets emitted by the atmosphere. In Figure 3.31, the arrows pointing into the atmosphere represent energy inputs:
 16 units of solar radiation absorbed by air molecules
  4 units of sunlight absorbed by clouds
 99 units of longwave terrestrial radiation

 TOTAL = 119 units of energy INTO the atmosphere. 
The arrows originating in the atmosphere and pointing out of it represent the energy leaving the atmosphere via emission:
 38 units emitted upward from air molecules
 26 units emitted upwards by clouds
 85 units emitted downward by clouds and air molecules 

 TOTAL = 149 units of energy OUT of the atmosphere
(Note the 85 units of energy emitted by the atmosphere was counted as 85 units of energy INTO the surface in the surface budget). The total budget is 119 units in versus 149 units out - a net loss of 30 units. Does this mean the atmosphere is getting cooler, since more energy is being emitted than is being absorbed? No, instead we have sensible and latent heat fluxes that transport energy from the surface to the atmosphere, balancing their respective budgets

9/29/99

Sensible and Latent Heat Flux - Balancing the Budget

The results of the budget analysis above show that the earth's surface has a surplus of 30 units of energy, while the atmosphere has a deficit of 30 units (rmember these energy amounts are all relative to the 100 units of incoming solar radiation at the top of the atmosphere). This excess of energy at the earth's surface is transferred to the atmosphere via sensible and latent heat flux. (Note: "flux" here refers to the transport of energy by atmospheric processes). When we include these fluxes in the budget of the earth's surface and the budget of the atmosphere, they will both be balanced.
Sensible heat flux is made up of two different processes - conduction and convection. Together these processes transport, on average, about 6 units of energy from the surface to the atmosphere.

Conduction - air coming into contact with a warm surface will be heated through collisions between the air molecules and the surface. This is a relatively slow, inefficient process - only a very thin layer of air near the surface will become heated. However, once this air becomes warm enough, it may become buoyant and rise - which brings about convection.
Convection - occurs when air over a surface "hot spot" becomes heated by conduction. This air, being hotter but at the same pressure, will become less dense (ideal gas law). Thus it will be "lighter" than the surrounding air, and begin to rise. As it rises, it will transfer its heat to the cooler air surrounding it. Meanwhile, cooler air will also flow in to replace the rising air - this air will then be heated by conduction, and the process repeats itself. Convection acts to mix the air.

Latent heat flux is due to the evaporation of liquid water at the earth's surface. In order for liquid water molecules to break free of the molecular bonds and become free water vapor molecules (i.e. evaporation), energy must be supplied - this energy is given the term latent heat. The latent heat of water is 600 calories per gram - that's how much energy it "costs" to evaporate one gram of water. These 600 calories of energy come at the expense of the surface's thermal energy. Since temperature is an indicator of the average kinetic energy of molecules in a substance, this means that when liquid water evaporates from a surface, the surface will cool down. Eventually, water vapor in the atmosphere will condense (revert to liquid form) - when this occurs, it gives back those 600 calories to the surrounding air - thus the air is heated. Overall, for each gram of water vapor that evaporates from the surface and condenses in the atmosphere, you take away 600 calories of energy from the surface, and deliver 600 calories to the atmosphere. If you add up the net effect of this latent heat flux, it averages out to be about 24 units of energy, relative to the 100 units coming in from the sun. Thus latent heat flux transports more energy FROM the surface TO the atmosphere than sensible heat flux - about four time more energy.
Plant life is a major contributor to latent heat flux from the earth's surface through the process of transpiration.
If we take into account the energy transported from the surface to the atmosphere by both sensible heat flux (6 units) and latent heat flux (24 units), the total amount of energy transported is 30 units - this exactly balances the energy budgets for the surface and the atmosphere.

Radiative Forcing

If we re-examine Figure 3.31, we can see that when we take all these different energy fluxes into account, the energy budget for each part of the system, and the system as a whole, will balance. However, it is important to realize that the numbers used in our budget represent global averages - that is, they are averaged over all locations and all times of year. If you were to pick one particular location and time of year, the numbers wouldn't necessarily balance. Figure 3.30 shows how each part of the budget varies depending on whether you are over the ocean or the desert or a vegetated area.
In fact, the net incoming solar radiation and the net outgoing longwave radiation are rarely equal for a given location and time of year. This is shown in Figure 3.32. An imbalance between the absorbed solar radiation and the outgoing longwave radiation is referred to as radiative forcing. It is this imbalance that "drives" our climate, so to speak, because processes like sensible and latent heat flux, advection by wind and ocean currents, and heating or cooling of an area throughout the year (i.e. storage), are all set in motion by the radiative forcing. We will return to this concept later when we talk about climate change and global warming.