Geoscience Reference
In-Depth Information
Hence, a layer thickness that is correct to represent the diurnal cycle, will not
be able to reproduce variations with a shorter period (the top layer is too sluggish),
or a longer period (the binding to the lower layer is too strong). In weather predic-
tion models this problem is partly circumvented by the introduction of a number of
stacked layers, where the thickness of each layer is tailored to take care of variations
with a certain period (e.g., minutes, daily, seasonal and yearly, see also
Sections 2.3.6
and
9.2.6
).
To analyse the role of the turbulent luxes further, we describe the sensible heat lux
in terms of the difference between the surface temperature
T
top
and the air temperature
at some height,
T
a
(see
Chapter 3
). Furthermore, if we omit the evaporation term, or
linearize it in terms of (
T
top
−
T
a
) (see
Chapter 7
), the sum of the turbulent luxes can
be parameterized as:
HLE
, where
α
FR
is a combination of a turbu-
+=
( )
α
TT
−
top
a
v
FR
∂
T
t
Q
Cd
*
2
−
( )
−
−
( )
top
T
−
T
∂
=
α
TT
lent diffusivity,
C
s
and
d
top
. This gives:
,
FR
top
a
top
bot
P
s op
which shows that the turbulent luxes can be interpreted as a restoring term as well
(leaving only
Q
* as the forcing term).
2.3.6 Vegetated Surfaces
A vegetation cover moves away the active surface (where the interaction with radia-
tion and turbulent luxes takes place) from the soil to the top of the vegetation. First
the vegetation interacts with the atmosphere, and subsequently the energy is trans-
ferred by the plant parts and the air between the plants to or from the soil.
From the perspective of the soil heat lux, the effect of this partial decoupling is
that the amplitude of the soil heat lux is damped (see
Figure 2.26
). In the same igure
it can also be seen that the short-term variations due to clouds on May 23 are visible
only in the bare soil data. From the perspective of the vegetation, the partial decou-
pling implies that the vegetation layer has its own temperature, which is only loosely
coupled to that of the underlying soil. This has important implications, as it is the sur-
face temperature that interacts with the atmosphere through the upwelling longwave
radiation and the sensible heat lux (see
Chapter 3
).
A method to incorporate the effect of the vegetation layer in meteorological mod-
els (without dealing with all the details of the transfer of radiation and heat through
the vegetation) is the use of an extra vegetation layer, on top of the soil (e.g., Viterbo
and Beljaars,
1995
). The vegetation layer is considered to have no heat capacity, as
vegetation is mainly made up of air (between the plant parts) and the heat capacity
of air is rather low. As a result, all energy supplied to the vegetation layer is instantly
transferred. The exchange of heat between the vegetation layer (with temperature
T
veg
) and the upper soil layer (with
T
top
) is treated empirically as:
G
=
Λ
veg eg
(
T
−
T
)
(2.41)
top
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