Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 33

0.4

0.3

0.2

0.1

22 May, cloudy
23 May, sunny

0.0

4 8 12 16 20

Time (hour UTC)

Figure 2.10 Albedo for two consecutive days in 2007: May 22 (cloudy) and May 23
(clear). Observations from Haarweg meteorological station (grass).

not cancel. A simple example to demonstrate this is to look at the time dependence
of the albedo on a single day. During one day, the Sun traverses a range of zenith
angles and azimuth angles. Figure 2.10 gives an example of the diurnal variation of
the albedo for a grass surface in the Netherlands for a clear day and an overcast day.
For the clear day (May 23) there is an obvious diurnal cycle with higher albedos in
early morning and late afternoon (at large values of the solar zenith angle, or low
solar altitudes): in that case the radiation does not penetrate into the grass, but instead
is relected by the top of the leaves. At midday the radiation penetrates into the grass
and is trapped due to multiple relections. In contrast, the albedo on the overcast day
is rather constant in time, with a small reduction in the morning. In the late afternoon
the clouds have cleared and the albedo is similar to that of the clear day at that time
of day.
Finally, as higher canopies have more layers in which radiation can be trapped (see
Figure 2.9 ), the albedo roughly decreases with vegetation height (see Figure 2.11 ,
where aerodynamic roughness has been used as a proxy for vegetation height). This
relationship is useful when comparing different vegetation types. However, when one
considers the development of vegetation height and albedo for one speciic vegeta-
tion type (e.g., during a growing season), the relationship may be different, or even
reversed (for maize see, e.g., Jacobs and van Pul, 1990 ).
Typical values of the albedo for different surface types can be found in Table 2.1 .

Question 2.10: Consider again the row crop of question Question 2.9 . If the sky is
completely overcast, and all incoming solar radiation is diffuse radiation, how large will
be the difference in albedo between the situation in which the Sun shines in a direction
parallel to the rows versus the situation in which the Sun shines in a direction perpen-
dicular to the rows?