Pyrgeometer measurements from aircraft

Abstract

An Eppley Laboratory pyrgeometer was tested in several different modes of operation to determine its ability to measure infrared irradiance from an aircraft platform. During the initial tests, the instrument output varied by 30–40 W m−2 as the incident solar irradiance or air flow over the KRS-5 dome changed. The long pass filter (the KRS-5 dome) was found to be opaque to radiation of wavelengths shorter than 3.6 μ. Hence, the fluctuations described above may be attributed to changes in the temperature of the filter. The pyrgeometer output was studied as a function of the incident infrared irradiance and the temperature difference between the sensor surface and the KRS-5 dome. Laboratory tests verified this dependence and showed that by utilizing the thermopile cold junction and dome temperatures, infrared irradiances may be measured with a precision of ± 1.7 W m−2. Although not the original intent of this research, it is shown that the KRS-5 shielded pyranometer may be used to measure infrared irradiance with a precision of ± 2 W m−2 in a ground station installation. However, in order to realize the precision value mentioned above, two precautions must be taken. First, the temperature of the KRS-5 dome must be monitored; and second, the entire instrument should have sufficient air flow over it to minimize temperature differences between the KRS-5 dome and the thermopile cold junction. The pyrgeometer was also mounted in an upward-looking configuration on an aircraft platform and measurements were made at constant pressure levels under clear sky conditions to approximate a constant irradiance. It was found that the intense flow over the instrument minimized the effect of the solar heating of the dome. Infrared irradiances measured on a day/night flight comparison differed by an average difference of 3.5 W m−2. rms deviations about the mean value at any level were less than ± 2.5 W m−2. Observed downward irradiance divergences differed from values calculated using a radiative transfer model by less than 0.3 W m−2 mb−1. The ventilation, however, did not eliminate the sink-dome temperature differences because the temperature response of the thermopile heat sink is an order of magnitude slower than that of the KRS-5 dome; therefore, horizontal fluctuations of air temperature or abrupt changes of altitude result in erroneous output values. An attempt was made to determine an empirical correction for air temperature fluctuations from the aircraft air temperature data. No general correction factor relating temperature and output changes could be determined. However, individual applications of such an empirical relationship did reduce rms deviations of the output to less than 2 W m−2.