At the Earth's surface, positive quantities of radiant energy are available to be transformed into non-radiative energy forms. These non-radiative energy transfers are important components in helping to maintain the energy balance of the Earth-atmosphere system as shown in Figure 2.3. The largest portion of radiant energy absorbed at the Earth's surface is used to evaporate water. The radiant energy is transformed into latent heat, which is attached to the water molecules and travels with them until they return to the liquid or solid state in the atmosphere. This represents a significant transfer of energy from the Earth's surface to the atmosphere. The second largest allocation of net radiant energy is to the atmosphere in the form of sensible heat. Air is a poor conductor of heat, and atmospheric warming by conduction from the Earth's surface is limited to a depth of a few centimeters. However, warming the bottom of the atmosphere imparts a bounce to the air that supports transferring the heated air by convection. The smallest quantity of net radiant energy goes to underlying soil layers or is used to melt snow and ice (Peixoto and Oort, 1992). For a land area, the partitioning of net radiation, Rn, among alternative energy fluxes is expressed as
where LE is the latent heat flux into the atmosphere resulting from evapotranspiration at the surface and condensation within the atmosphere, H is the sensible heat flux to the atmosphere by conduction and convection resulting from the temperature difference between the surface and the overlying air, and G is the soil heat flux into the ground by conduction.
The energy balance for water is more complex because the depth where the energy storage flux is considered negligible can be as much as several kilometers for areas where deep water is formed. In addition, horizontal energy flux may be a greater consideration in water due to the greater mobility of water compared to a land surface (Rosen, 1999). Even for land areas, Equation 2.12 does not include terms such as latent heat of fusion, oxidation, and heat transfer by precipitation that can be important locally or for limited time intervals. Another significant characteristic of the energy balance is that it is negative at night and during winter at high latitudes. During these periods, thermal radiation dominates the radiation balance and non-radiative fluxes provide energy to the surface to compensate for thermal radiation losses.
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