Geoscience Reference
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30
25
20
15
10
z
d
=5cm
z
d
=20cm
z
d
=50cm
z
d
=100 cm
5
0
-5
0
90
180
270
360
Time (day of year)
Figure 2.19
Seasonal cycle of the soil temperature of a bare soil plot at four depths
(different levels than in
Figure 2.18
) at Haarweg Meteorological station in the year
2007. Note that the lowest two levels are actually below grass, but the difference will
be insigniicant.
•
The time at which the maximum temperature is reached shifts to later times when going
further below the surface. On May 23 the maximum temperature at 5 cm depth occurs at
14 UTC (2 hours after local noon; see
Figure 2.6
), whereas the maximum at 20 cm depth
occurs around 19 UTC, i.e., 5 hours later.
On the cloudy day, the effect of broken clouds is visible in the observations at 5 cm
•
depth, but the temperatures at greater depth are smooth.
At 50 cm depth there is no diurnal cycle but a very slight linear increase is visible. The
•
data show an increase of 0.15 K in one day.
Apart from the diurnal cycle, there is also a yearly cycle in soil temperatures. The
day-to-day variation of the daily mean soil temperature is shown for the same loca-
tion in
Figure 2.19
. Similar features occur as observed for the diurnal cycle:
•
The amplitude of the yearly cycle decreases with depth, but is still clearly visible at 50
and 100 cm depth.
The peak of the temperature shifts to later dates with increasing depth (at 100 cm depth,
•
the maximum temperature is reached in September).
The short-term (day-to-day) variations are clearly visible at 5 and 20 cm depth, but hardly
•
affect the temperatures at 50 and 100 cm depth. Only the longer cold and warm spells in
the end of the year penetrate down to 100 cm.
Although the diurnal and yearly cycles appear to behave very similarly, there is one
important difference: the depth at which the cycle is no longer visible (this is where
it has been damped). For the diurnal cycle this is somewhere between 20 and 50 cm
deep, whereas for the yearly cycle this level will lie well below 100 cm.
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