A transfer standard radiometer for atmospheric longwave irradiance measurements

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 degrees C and -15 degrees 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.