Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 41

lower. This in turn is due to the fact the plants use a large part of the available energy
for evapotranspiration, rather than to heat up their leaves and stems (see Chapter 6 ).
The partitioning of available energy between evapotranspiration and sensible heat
lux, as well as the coupling between that partitioning and the emitted longwave radi-
ation, is one of the important subjects of the forthcoming chapters.
Question 2.15: Determine the surface temperature at 12 UTC on May 22 and May 23,
using the data in Figure 2.14a and b .
a) Assuming a surface emissivity of 0.96 (see Table 2.1 )
b) Assuming a surface emissivity of 1.0
c) Assuming a surface emissivity of 0.96 but neglecting the relection of downwelling
longwave radiation
Question 2.16: Determine the difference in surface temperature between the bare soil
and the vine plants at 12 UTC for the data in Figure 2.15 . Use a reasonable assumption
on the surface emissivity. Incoming longwave radiation at 12 UTC is 410 W m -2 .
2.2.6 Net Radiation: Sum of Components
Now that all components of the net radiation at Earth's surface have been introduced,
they can be combined to yield the net radiation. First we examine the net shortwave
and net longwave radiation, K* and L* respectively. Figure 2.16a shows that K* closely
follows the diurnal course of the incoming solar radiation (the values at night are not
identical to zero owing to inite accuracy of the sensors; see 2.2.7 ). As compared to K*
and to the individual components L ↓ and L ↑ , L* is rather small: ranging between -100
and 0 W m -2 (see Figure 2.16b ). On the cloud-free day it has a distinct diurnal cycle.
On the cloudy day, however, L* is close to zero. This is due to the fact that with the
inclusion of clouds, the atmosphere acts as a nearly black body. Given the fact that the
surface also has an emissivity close to 1, L* is determined mainly by the difference
in temperature between the surface and the lower atmosphere. On overcast days this
difference will be small. Figure 2.16c shows that the diurnal cycle of net radiation is
dominated by that of K* . However, at night L* is the only determining factor.
2.2.7 Measurement of Net Radiation
The measurement of net radiation is in principle straightforward. Sensors (radiom-
eters) exist that can measure either the short wave radiation lux density, or the long-
wave radiation lux density. Combination of two instruments of each (one facing
upward, the other facing downward) yields the four components of the net radiation.
This is the preferred way of measuring net radiation. Simpler instruments exist that
combine all four sensors in one, and yield the net radiation directly.
Shortwave radiation can be measured with a pyranometer, whereas the device used
to measure longwave radiation is called a pyrgeometer (see Figure 2.17 ). The measur-
ing principle of most radiometers is that radiation is absorbed at the top of a sensor. The