Geoscience Reference
In-Depth Information
temperature and the temperature of air is something that humans can more
readily 'sense' than its latent heat content.
Energy balance of an ideal surface
Figure 4.1 illustrates the energy budget of a sample volume with unit horizontal
area that intersects a horizontal, uniform 'ideal' terrestrial surface that comprises
soil with overlying vegetation permeated by air. Outgoing vertical energy fluxes
are defined at some level above the vegetation called the
reference level
, and at
some depth below the soil surface. Horizontal fluxes are defined parallel to the
wind at the edges of the sample volume. The several energy components involved
in defining the energy balance of this sample volume are as follows.
Net radiation,
R
n
The driving input to the surface energy balance is the net flux of radiant energy,
over all wavelengths, at the upper surface of the sample volume. This flux is called
the
net radiation, R
n
. The net radiation is itself a balance between four compo-
nents: specifically incoming and outgoing radiation in the shortwave band called
solar radiation
, and the incoming and outgoing radiation in the wavelength band
determined by temperatures typical of the Earth surface and the lower atmosphere
called
longwave radiation
, see Fig. 4.2. Because the position of the Sun changes, the
strength of incoming solar radiation varies greatly with time of day leading to a
marked diurnal variation in the net radiation flux. Daytime net radiation is domi-
nated by the solar radiation balance (except at high latitude in winter), while
nighttime net radiation is determined by the longwave radiation balance.
The nature of the surface radiation balance and how net radiation can be quanti-
fied is discussed in greater detail in Chapter 5.
R
n
Figure 4.1
Energy balance of
a sample volume selected to
lie through a horizontally
uniform land surface.
R
n
is
the net radiation, λ
E, H
and
G
are the latent heat, sensible
heat, and soil heat fluxes,
respectively;
S
t
and
P
are the
physical and biochemical
energy stored within the
sample volume; and
A
in
and
A
out
are horizontally advected
energy entering and leaving
the volume, respectively.
l
E
H
A
in
A
out
P
S
t
G
