The driving force for the Earth's hydroclimate is the supply of radiant energy from the Sun. Climate of the first kind and the atmospheric branch of the hydrologic cycle are intimately coupled with the character of solar radiation, how the Earth's atmosphere and surface respond to the incoming solar radiation, and how the Earth and atmosphere radiate energy to space. Measuring these energy fluxes is an important step in defining their time and space variations and understanding how they drive atmospheric circulation and energy, mass, and momentum transport.
Instruments for measuring radiant energy fluxes are deployed at a minority of regularly operated climate stations. The absence of radiation instruments is especially evident in developing countries due to the high maintenance requirements and maintenance expense of these instruments. Many radiation instruments require daily inspection and some require daily calibration. The demands of monitoring these instruments are major obstacles to the establishment of dense instrument networks providing routine observations. In recognition of the sparse data problem, the Baseline Surface Radiation Network (BSRN) proposed by the World Climate Research Program became operational in 1992 and now has about 20 fully operational stations. One of the purposes of the BSRN is to monitor long-term changes in radiation fluxes at the Earth's surface in a variety of climatic zones (Ohmura, et al., 1998).
Solar radiation is measured with a variety of instruments that can be broadly grouped as thermal or quantum sensors. These instruments are designed to measure the intensity of radiant energy over broad or narrow spectral bands. Thermal sensors absorb solar radiation and convert it into thermal energy in a form that can be measured. Thermal radiation quantities are expressed in terms of irradiance, which is a measure of the rate of energy received per unit area and has units ofWm~2. Quantum sensors utilize absorbed energy quanta to liberate electrons and produce an electric current (Guyot, 1998). A quantum sensor is sensitive to a specific solar spectral domain and has a high sensitivity and rapid response compared to a thermal sensor. Quantum sensors are used primarily to measure the photosynthetic light spectrum and are not deployed for routine meteorological or climatological observations.
Thermal solar radiation instruments are classified by the type of quantity measured. Pyrheliometers measure direct solar radiation. Pyranometers measure whole hemisphere solar radiation, reflected solar radiation, and diffuse solar radiation when screened from direct solar radiation by an equatorial ring. Pyrgeometers measure incoming longwave atmospheric radiation when facing upward and outgoing longwave terrestrial radiation when inverted. Pyrradiometers, or radiometers, measure all-wave radiation arriving on one plane.
Direct beam solar radiation with wavelengths of 0.3 to 4.0 mm is measured with a pyrheliometer oriented toward the Sun so the receptor surfaces are perpendicular to the incident solar beam. The instrument is attached to a mounting that permits it to follow the Sun for continuous recordings. A pyrhe-liometer has an aperture with an acceptance angle of 2.5° to 5° to limit its view to the solar disk and a narrow sky annulus (WMO, 1996).
The general design of pyrheliometers is a metal tube with a small opening at one end. The opening is covered with a quartz window to allow passage of the whole solar spectrum while protecting the sensor from wind and contaminants. A rotating wheel with different filters can be added to measure specific spectral bands. A blackened disk containing thermopiles is mounted at the closed end of the tube. A thermopile is an electrical loop of alternating lengths of wire where each length reaches from a surface warmed by radiation to a cool surface (Linacre, 1992). The solar radiation passing through the opening is directed to the rear disk containing the thermopiles. The incident solar radiation produces heating of the surface that is sensed by the thermopile and transformed into an electrical voltage interpretable as a radiation flux in Wm~2. Comparing the measured direct beam irradiance with global and diffuse irradiances requires obtaining the horizontal component of the direct solar irradiance. This is achieved by multiplying the direct solar irradiance by the cosine of the Sun's zenith angle. Pyrheliometers are the most accurate of all radiation instruments, and they are commonly used as calibration standards for working instruments (Bradley, 2003). However, they are usually found only at research stations or laboratories because of their need to track the Sun.
A pyranometer is an instrument for measuring global solar radiation, or both direct and diffuse solar radiation, onto a plane surface (Beaubien et al.,
1998). A horizontally mounted pyranometer detects solar radiation originating from all parts of the sky (Fig. 3.1). An inverted pyranometer measures reflected solar radiation. An upward facing pyranometer fitted with a ring device known as an occulting band shades the sensor from direct beam radiation so it measures only diffuse solar irradiance. Pyranometers are exposed continually in all weather conditions and must have a robust design that resists the corrosive effects of humid air (WMO, 1996).
The individual designs of pyranometers vary, but they normally use thermoelectric, photoelectric, pyroelectric or bimetallic elements as sensors (WMO, 1996). One of the most popular instruments is based on thermopiles that measure the thermal difference between a black surface and a white surface with both surfaces exposed to the sun in the same plane. Thermopile designs using alternating horizontal strips, alternating pie-shaped disks, or concentric circle arrays of the thermocouples are widely used in Australia, Europe, and North America. The principle is that radiant heating of the blackened surface of a thermopile relative to the reflectively white cool surface generates an electromotive force proportional to the energy flux received. An alternating surface design has the advantage that the environmental exposure of the surfaces is virtually identical, and changes in ambient air temperature have the same influence on both surfaces (Beaubien et al., 1998). The thermopiles are protected from the wind and from longwave radiation by a glass hemispherical dome. The spectral range of the pyranometer of 0.3 to 3.0 mm is limited by the transmission of the glass dome. Pyranometer sensitivity may change with time and exposure to radiation due to deterioration of the black paint. At least daily inspection of pyranometers and recorders is desirable, and regular calibration of pyran-ometers is especially important for achieving reliable measurements (WMO, 1996).
Measuring longwave radiation is a problematic pursuit and has often been abandoned in favor of inferring longwave radiation from measurements of shortwave and all-wave radiation. However, longwave radiation is a significant component of the surface energy budget, and there is a growing demand for good quality longwave radiation measurements (Philiponia et al., 1998). Improvements in materials used to block solar radiation have advanced the capabilities of instrument designs for longwave radiation measurements (WMO, 1996).
Measurements of longwave radiation are achieved using a pyrgeometer. This instrument resembles a pyranometer, but it has a polyethylene or silicon dome with good transmittance for the spectral domain of thermal infrared radiation in the range of 3.0 to 100 mm. Also, the pyrgeometer thermopile has one end connected to a blackened disk serving as the receiving surface and the other end is in thermal contact with the instrument casing. A circuit combining the output of a thermistor embedded in the instrument body and the thermopile gives a voltage proportional to the longwave flux. Consequently, the pyrgeometer works by determining the thermal balance of the instrument itself (Bradley, 2003). The reliability of a pyrgeometer is closely related to the transmittance of the dome, which must be cleaned daily, and polyethylene domes must be replaced frequently. Other factors contributing to instrument uncertainty are a lack of an absolute calibration standard, variation in calibration techniques, and uncertainties in the internal thermal balance of the instrument (Burns et al., 2003).
The surface energy budget is driven by net radiation, but net radiation remains among the most difficult atmospheric parameters to measure (Brotzge and Duchon, 2000). Each of the terms on the right side of Equation 2.9 must be measured. This can be accomplished with a pair of pyranometers and a pair of pyrgeometers with one instrument from each pair facing upward and one downward. The alternative is to use a pyrradiometer or net radiometer that produces a signal due to differential radiation absorption (Fig. 3.2). A large number of pyrradiometers are commercially available to measure the sum of shortwave and longwave radiative exchanges, but most instruments are for research applications and few weather or climate stations measure net radiation on a routine basis.
The pyrradiometer consists of two horizontal blackened plates placed back to back and separated by a thermal insulator to sense the downward and upward directed radiation fluxes. Each blackened disk has an internal thermopile, and
the temperature difference between the two sensor surfaces is proportional to net radiation (Rn). That is
where C is a parameter expressing the rate at which the sensor is warmed or cooled by conduction and convection, Tu is the temperature of the upper surface in °C, and Td is the temperature of the lower surface in °C. The instrument is mounted with the sensors horizontal so they measure the temperature differential produced between the plates by downwelling and upwelling radiative energy (Bradley, 2003).
Most pyrradiometers use polyethylene hemispherical domes to eliminate natural ventilation and reduce thermal convection from the instrument. Polyethylene is used for the domes because it is transparent to both shortwave and longwave radiant energy between 0.3 and 100 mm. However, the polyethylene domes degrade after a few months of exposure and require replacement of the domes and recalibration of the pyrradiometer (Brotzge and Duchon, 2000). The instrument is placed 1 to 2 m above the surface being examined to avoid shading the surface or averaging too large an area.
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