Accounting for the Solar Radiation Influence on Downward Longwave Irradiance Measurements by Pyrgeometers

Abstract

easurements of broadband downward longwave (LW) irradiance are carried out at Lampedusa, Italy, in the Mediterranean, jointly with solar irradiance, since 2004 using shaded and unshaded Eppley Precision Infrared Radiometers (PIRs) and Kipp & Zonen CGR4 pyrgeometers. The calibrations of the different pyrgeometers are traceable to the Physikalisch-Meteorologisches Observatorium Davos World Radiation Center (PMOD/WRC). Nighttime measurements from intercalibrated pyrgeometers agree within ±2 W m?2. The performance of the pyrgeometers when operated unshaded has been analyzed by comparison with shaded instruments. For the PIR, the nonhomogeneous dome temperature in cloud-free conditions produces differences in LW irradiance within ±4 W m?2, depending on the time of the day and the season. The effect of the LW fraction of solar radiation reaching the sensor and a possible shortwave leakage of the dome interference filter produces differences between shaded and unshaded pyrgeometers during daytime. In cloud-free conditions, the unshaded PIR overestimates the LW irradiance measured by a shaded PIR by up to 10 W m?2, and by up to 12 W m?2 compared to a shaded CGR4. Conversely, the unshaded CGR4 LW irradiances agree with those of a shaded PIR within the measurements? uncertainties. An empirical formula to correct the solar effect of the PIR has been derived. Applying this correction, the average LW irradiance overestimate for cloud-free conditions decreases from 3.8 ± 1.8 to 0.0 ± 1.3 W m?2 in spring, and from 3.2 ± 1.7 to 0.0 ± 1.3 W m?2 in summer. Thus, LW irradiance from an unshaded PIR can be derived within about ±2 W m?2 of a shaded PIR.