Geoscience Reference
In-Depth Information
10% of the latent heat flux. An example of the fluxes of carbon dioxide and water vapor,
estimated by means of the energy budget method with the Bowen ratio (cf. Equation (4.11))
on a day of intense photosynthetic activity, is shown in Figure 2.27.
The advected energy term
A
h
comprises all the energy changes resulting from water
flowing in or out of the system to which Equation (2.72) is applied. Precipitation is a source
of advection; rainfall may be important in the case of the energy budget of a snow cover,
and snowfall may affect the energy balance of a warm lake. Advection by river flows may
sometimes have to be considered in the energy budget of a lake, especially when it is shallow.
The term (
∂
W
/∂
t
) can be omitted from (2.72) when it is applied to a thin layer of water,
soil or canopy. In the case of tall vegetation, however, it may have to be considered; for
example, it has been observed (Stewart and Thom, 1973) that this term can be especially
significant after sunrise and near sunset, when it may be of the same order of magnitude as
the net radiation
R
n
. Still, on a daily basis it can be safely neglected. When the layer under
consideration is a snow pack, this term is generally important (McKay and Thurtell, 1978)
since, as formulated in Equation (2.72), it includes the energy used in fusion. In the case
of an entire lake, (
∂
W
/∂
t
) can be determined from successive water temperature profile
surveys.
2.6.4
Global climatology of the surface energy budget
To give a rough idea of their magnitude, Figure 2.28 shows the global long term averages
of the main components of the energy budget calculated by Ohmura (2005) from the
Short-wave
radiation
Long-wave
radiation
342
102
240
+96
−
200
+104
−
104
19
85
169
25
345
385
HL
e
E
α
s
R
s
ε
s
R
ld
R
lu
s
Fig. 2.28 Mean global energy budget fluxes in W m
−
2
, estimated from an extensive observational data base by
Ohmura (2005). The incoming short-wave radiation at the top of the atmosphere is 342 W m
−
2
, which
is one quarter of the solar constant.
