Geoscience Reference
In-Depth Information
Table 8.6
Observed and derived atmospheric conditions and surface properties
for grass and forest during 'normal' conditions (climatology) and during heat
wave days
Quantity
Source
Normal
Heat wave
Grassland
Forest
Grassland
Forest
T
s
(°C)
Observed
21.6
18.8
30.9
26.9
H
(W m
-2
)
Observed
93
133
105
254
L
v
E
(W m
-2
)
Observed
171
149
254
158
L
v
E
/(
H
+
L
v
E
) (-)
Observed
0.65
0.53
0.71
0.38
r
a
(s m
-1
)
Assumed
40
10
40
10
RH (-)
Assumed
0.7
0.7
0.7
0.7
T
a
(°C)
Derived
18.6
17.7
27.4
24.8
r
a
+
r
c
(s m
-1
)
Derived
116
93
138
158
r
c
(s m
-1
)
Derived
76
83
98
148
VPD (hPa)
Derived
6.3
6.0
10.8
9.3
∆
T
a
(°C in 4 h) in
BL of 750 m
Derived
1.5
2.1
1.7
4.0
The values for the aerodynamic resistance
r
a
and relative humidity were assumed; the other
variables have been derived (see the text for details).
the forest only increases the sensible heat lux. What could be the explanation for
these opposite responses?
Table 8.6
(two rightmost columns) provides the data for an
analysis of this behaviour, where the same assumptions were used as for the normal
conditions. The irst difference between normal data and the heat wave conditions
is the increase in air temperature of about 8 °C, indicating that indeed heat wave
conditions occurred (maximum temperatures more than 5 K above climatology).
As a result of this increase in air temperature, and assuming an unchanged relative
humidity, the vapour pressure deicit increases signiicantly, and hence the evapora-
tive demand of the atmosphere does as well. Whereas the grass completely yields to
this demand (with only a small increase in canopy resistance), the forest responds
strongly to the increased demand by nearly doubling its canopy resistance. This
response is found not only in the data analysed here, but is - at least qualitatively -
consistent with results of, for example, Kelliher et al. (
1993
). The data clearly show
that due to the strong aerodynamic coupling of forests to the atmosphere (and sub-
sequent cooling) they do not need much evaporative cooling to keep the surface
temperature in bounds.
The analysis shows that under heat wave conditions forests remain more conser-
vative than grass as it comes to the use of water. But the reverse side of that medal
is that forests supply more heat to the atmosphere, thus increasing the intensity of a
heat wave. If the sensible heat luxes given
Table 8.6
are fed into a boundary layer
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