Geoscience Reference
In-Depth Information
involved in plant photochemistry is usually comparatively small (on the order of
2% of the incoming solar energy). For this reason this energy term is often
neglected in hydrometeorology.
Advected energy,
A
d
The net advected energy,
A
d
A
in
-
A
out
(see Fig. 4.2), is also usually neglected for
homogeneous ideal surfaces, but can become significant in 'oasis' situations.
=
Flux sign convention
The sign convention most often used for the several terms in the energy budget
summarized above is biased toward the value of fluxes being positive in daytime
conditions. Consequently:
(a) all radiation fluxes are defined as being positive when directed toward the
surface (this applies to
R
n
and its component fluxes
S, S
d
, S
r
, L
u
and
L
d
, see
Fig. 4.2).
(b) all the other vertical energy fluxes are defined as being positive when
directed away from the surface (this applies to
E, H
, and
G
)
(c) the storage terms are defined as being positive when they take in energy
(this applies to
S
t
and
P
)O
(d) if considered, the net advected energy,
A
d
, is defined as positive when it
brings energy into the sample volume.
λ
Consistent with this sign convention, the overall energy budget for the sample
volume through an ideal surface shown in Fig. 4.1 is written as:
(
) (
)
RGASP EH
−+ −− =
l
+
(4.2)
n
t
t
The terms on the left of Equation 4.2 are usually grouped together in this way
because in daytime conditions they together define the energy available in the
energy budget that is shared between the latent and sensible heat on the right. For
this reason this set of terms is sometimes referred to as
the available energy, A
,
which is defined by:
(
)
ARGASP
=−+−−
(4.3)
n
t
t
The magnitudes and, in the case of vertical energy fluxes, the direction of terms
in the surface energy budget are characteristically different in daytime and
nighttime conditions, and they also depend strongly on whether the surface is
bare soil or covered with vegetation and also on how much moisture is available
in the soil.
