Pyrgeometer

Abstract: A detailed characterization is performed to calibrate pyrgeometers, using a newly developed apparatus that contains a blackbody radiation source and the means to vary the temperatures of the pyrgeometer under testing. Calibration measurements cover the parameter space of radiation and instrument temperatures that prevail during field measurements. Dome-temperature measurements, normally provided on pyrgeometers, are inadequate for accurate corrections of the dome emission. A new temperature measurement with three sensors inside the dome at 45° elevation is proposed and has been implemented on several test instruments. This modification and the detailed characterization measurements permit an improved evaluation, based on thorough analysis of the thermal balance of the instrument, leading to a sensitivity factor C and three correction factors, k1,2,3. Test measurements demonstrate the substantial improvement achieved on the accuracy of atmospheric and terrestrial long-wave radiation measurements, down to ±2 W m−2.

Abstract: The actual performance of an Eppley pyrgeometer is compared to the desired theoretical performance. Several systematic errors are identified and evaluated in detail. The three most significant errors identified are due to 1) battery voltage uncertainties, 2) nonlinearity of circuitry at extreme temperature and 3) differential heating of the instrument. The elimination of the error due to differential heating is found to be essential to the successful calibration of the instrument. A pyrgeometer laboratory calibration technique is described. Pyrgeometer measurements made from aircraft are shown to have potential errors as large as 60 W m−2. These errors, however, do not significantly affect the net radiation provided the upward and downward facing pyrgeometers are at the same equilibrium temperature, and may be largely eliminated by making accurate temperature measurements of the KRS-5 dome and the cold junctions of the thermopile. The corrections considered in this paper not only reduce the absol...

Abstract: Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison. The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insulation is larger than the measurements for clear-sky and total-sky condit...

Abstract: Because atmospheric longwave radiation is one of the most fundamental elements of an expected climate change, there has been a strong interest in improving measurements and model calculations in recent years. Important questions are how reliable and consistent are atmospheric longwave radiation measurements and calculations and what are the uncertainties? The First International Pyrgeometer and Absolute Sky-scanning Radiometer Comparison, which was held at the Atmospheric Radiation Measurement program's Southern Great Plains site in Oklahoma, answers these questions at least for midlatitude summer conditions and reflects the state of the art for atmospheric longwave radiation measurements and calculations. The 15 participating pyrgeometers were all calibration-traced standard instruments chosen from a broad international community. Two new chopped pyrgeometers also took part in the comparison. An absolute sky-scanning radiometer (ASR), which includes a pyroelectric detector and a reference blackbody source, was used for the first time as a reference standard instrument to field calibrate pyrgeometers during clear-sky nighttime measurements. Owner-provided and uniformly determined blackbody calibration factors were compared. Remarkable improvements and higher pyrgeometer precision were achieved with field calibration factors. Results of nighttime and daytime pyrgeometer precision and absolute uncertainty are presented for eight consecutive days of measurements, during which period downward longwave irradiance varied between 260 and 420 W m−2. Comparisons between pyrgeometers and the absolute ASR, the atmospheric emitted radiance interferometer, and radiative transfer models LBLRTM and MODTRAN show a surprisingly good agreement of <2 W m−2 for nighttime atmospheric longwave irradiance measurements and calculations.