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(a)
200
bare soil
grass
150
100
50
0
-50
-100
0
3
6
9
12
15
18
21
24
Time (hour UTC)
(b)
200
bare soil
grass
150
100
50
0
-50
-100
0
3
6
9
12
15
18
21
24
Time (hour UTC)
Figure 2.26
Observations of soil heat lux (at 5 cm depth) at de Haarweg
Meteorological Station under grass and under bare soil: May 22, 2007 (
a
) and May
23, 2007 (
b
).
where
Λ
veg
is called the skin layer conductivity (compare the empirical description
of
G
in the force-restore method, Eq. (
2.40
)). A typical daytime value of
Λ
veg
for low
vegetation is 5 W m
-2
K
-1
, but it does depend on the vegetation type and the fraction
of soil covered by vegetation. Furthermore, the value for
Λ
veg
appears to be different
between day and night (see ECMWF,
2009
).
Another simple model for the soil heat lux, often used in the case that no observa-
tions of
G
are available, is based on the fact that under vegetation the soil heat lux
follows a diurnal cycle that is comparable to that of the net radiation (at least during
daytime). This leads to the model:
GcQ
=
G
*
(2.42)
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