## Surface Energy Budgets

We first review the process of energy exchange between the atmosphere and an unvegetated surface. The surface energy budget equation, discussed in Chapter 3D, is usually written as follows:

n where Rn, the net all-wavelength radiation, = [S(1 - a)] + Ln

S = incoming short-wave radiation, a = fractional albedo of the surface, and L = the net outgoing long-wave radiation.

R" is usually positive by day, since the absorbed solar radiation exceeds the net outgoing long-wave radiation;

at night, when S = 0, Rn is determined by the negative magnitude of L.

The surface energy flux terms are:

G = ground heat flux,

H = turbulent sensible heat flux to the atmosphere,

LE = turbulent latent heat flux to the atmosphere

(E = evaporation; L = latent heat of vaporization).

Positive values denote a flux away from the surface interface. By day, the available net radiation is balanced by turbulent fluxes of sensible heat (H) and latent heat (LE) into the atmosphere and by conductive heat flux into the ground (G). At night, the negative R" caused by net outgoing long-wave radiation is offset by the supply of conductive heat from the soil (G) and turbulent heat from the air (H) (Figure 12.2A).

• MESOSCALE OCEAN EDDY • LARGE CUMULUS Figure 12.2 Energy flows involved in the energy balance of a simple surface during day and night (A) and a vegetated surface (B).

Source: After Oke (1978).

Figure 12.2 Energy flows involved in the energy balance of a simple surface during day and night (A) and a vegetated surface (B).

Source: After Oke (1978).

Occasionally, condensation may contribute heat to the surface.

Commonly, there is a small residual heat storage (AS) in the soil in spring/summer and a return of heat to the surface in autumn/winter. Where a vegetation canopy is present there may be a small additional biochemical heat storage, due to photosynthesis, as well as physical heat storage by leaves and stems. An additional energy component to be considered in areas of mixed canopy cover (forest/grassland, desert/oasis), and in water bodies, is the horizontal transfer (advec-tion) of heat by wind and currents (AA; see Figure 12.2B). The atmosphere transports both sensible and latent heat. 