Abstract
A new reference radiometer for downwelling atmospheric longwave irradiance has been designed and built. The new infrared integrating sphere (IRIS) radiometer is designed to acquire measurements with a time constant of less than 1 s. Based on a thorough characterization, the IRIS radiometer is able to measure longwave irradiance with an expanded uncertainty of ±1.8 W m−2 and ±2.4 W m−2 in the summer and winter seasons, respectively, which is equivalent to a temperature range between +15 °C and −15 °C for typical conditions at Davos, Switzerland. The long-term stability of the IRIS radiometer was determined over a one year period, yielding a calibration reproducibility in the laboratory of 0.5%. Outdoor measurements with four IRIS radiometers were performed during 13 clear nights in April 2011. The four radiometers measured atmospheric longwave irradiance with differences ranging from −1.1 W m−2 to +0.7 W m−2, which were well within their estimated uncertainties.
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