Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 43

3

3

4

1 2 1 2

Figure 2.17 Sketch of a pyranometer (left) and a pyrgeometer (right). 1, housing;
2, sensor, 3, transparent dome (left) or longwave transparent ilter (right); 4, radiation
shield.

temperature difference between the heated top and the cooler bottom then is a measure
of the heat lux through the sensor, which is under steady conditions equal to the radi-
ation input at the top. The distinction between a pyranometer and a pyrgeometer is
made by the cover (a dome or a lat plate) that protects the radiation absorbing surface
from the atmosphere. For a pyranometer this dome is usually made of glass or quartz,
limiting the spectral response to 0.29-2.8 or 0.29-4.0 µm, respectively (Gueymard and
Myers, 2008 ). The ilter of a pyrgeometer blocks shortwave radiation and transmits
longwave radiation (although there may be some absorption). Net-radiometers have a
dome that is transparent for both longwave and shortwave radiation.
Whereas a pyranometer absorbs only radiation that is external to the instrument
(except for the thermal offset; see later), the radiation received by a pyrgeometer
sensor is a balance between the radiation transmitted through the ilter, the radiation
emitted by the ilter (if it is not fully transparent for longwave radiation) and the emis-
sion of the sensor. Hence, to derive the incident longwave radiation, not only this net
effect (balance between incoming and emitted radiation) needs to be measured, but
the sensor's emission as well. This entails - at least - the additional determination of
the temperature of the instrument's housing.
For pyranometers two instrument-related error sources can be identiied. First, a
non-perfect cosine response implies that the absorbing surface does not act as a Lam-
bertian surface. This implies that the sensitivity is not equal for radiation from all
directions. Second, there may be thermal errors: the dome is generally cooler than the
heated absorbing surface (especially, but not exclusively, during night). Through radi-
ative and convective exchange this cools the absorbing surface, leading to a negative
offset (which is particularly visible at night when the instrument should give a zero
lux). The thermal offset can in part be suppressed by ventilating the instrument in
order that the temperature of the dome is closer to that of the housing. An additional
advantage of ventilation is that dew formation is suppressed. Other error sources
related to installation and use are insuficient cleaning of the domes and incorrect hor-
izontal alignment of the instrument (Kohsiek et al., 2007 ).
The main instrument-related error for pyrgeometers is that the ilter - which blocks
shortwave radiation - will heat up considerably under sunny conditions. If the ilter