A new absolute reference for atmospheric longwave irradiance measurements with traceability to SI units

Abstract

developed by PMOD/WRC took part in these intercomparisons. The internal consistency of the IRIS radiometers and the agreement with the ACP were within ±1 W m −2 , providing traceability of atmospheric longwave irradiance to the international system of units with unprecedented accuracy. Measurements performed during the two field campaigns and over the past 4 years have shown that the World Infrared Standard Group (WISG) of pyrgeometers is underestimating clear-sky atmospheric longwave irradiance by 2 to 6 W m −2 , depending on the amount of integrated water vapor (IWV). This behavior is an instrument-dependent feature and requires an individual sensitivity calibration of each pyrgeometer with respect to an absolute reference such as IRIS or ACP. For IWV larger than 10 mm, an average sensitivity correction of +6.5% should be applied to the WISG in order to be consistent with the longwave reference represented by the ACP and IRIS radiometers. A concerted effort at international level will need to be implemented in order to correct measurements of atmospheric downwelling longwave irradiance traceable to the WISG.

Revising shortwave and longwave radiation archives in view of possible revisions of the WSG and WISG reference scales: methods and implications

“…The realisation of a more accurate standard group to determine irradiance (Reda et al, 2012) and a revision of the WISG scale have been ongoing issues in recent years (e.g. Gröbner et al, 2014Philipona, 2015). However, a replacement of the WISG as a transfer standard is not foreseen due to its all-weather and hence continuous measurement capabilities.…”

“…Current state-of-the-art research radiometers such as the Infrared Integrating Sphere radiometer (IRIS; Gröbner, 2012) and the Absolute Cavity Pyrgeometer (ACP; Reda et al, 2012) are windowless and are thus not suitable for continuous all-weather operation. The evidence for a reference scale revision comes from concurrent operation of the WISG alongside IRIS during night-time clear-sky conditions since 2008 which yielded an underestimation of the WISG clear-sky longwave irradiance by 2-6 W m −2 , depending on the amount of integrated water vapour (IWV) (Gröbner et al, 2014) in the atmosphere. These results have been confirmed in two intercomparison campaigns with the ACP (Gröbner et al, 2014) where the ACP and IRIS measurements were consistent to within ±1 W m −2 during both campaigns (which is within the instrumental uncertainties of ±4 and ±2 W m −2 , respectively), while the WISG measured lower values by an average of 5.6 W m −2 (Gröbner et al, 2014).…”

“…The latter is broadly defined as covering the wavelength range ∼ 0.3-3 µm (Petty, 2006), while the WSG pyrheliometers cover the range from less than 0.2 to above 50 µm (Reda et al, 2017). The corresponding standard group for longwave radiation, the World Infrared Standard Group (WISG) of pyrgeometers, was established in 2004 on the recommendation of the WMO (WMO, 2003) but is an interim working standard due to a number of ongoing issues (Gröbner et al, 2014;Philipona, 2015). Longwave radiation is broadly defined as covering the ∼ 4-100 µm range (Petty, 2006), while the WISG pyrgeometers cover the range ∼ 4-50 µm (Eppley Precision Infrared Radiometer (PIR) pyrgeometers) and ∼ 4.5-42 µm (Kipp & Zonen CG(R)4 pyrgeometers).…”

“…Longwave radiation is broadly defined as covering the ∼ 4-100 µm range (Petty, 2006), while the WISG pyrgeometers cover the range ∼ 4-50 µm (Eppley Precision Infrared Radiometer (PIR) pyrgeometers) and ∼ 4.5-42 µm (Kipp & Zonen CG(R)4 pyrgeometers). Recent measurements with newly developed high-precision ground-based radiometers have demonstrated that a revision of approximately −0.3 % and up to +5 W m −2 of the WSG and WISG scales, respectively, may be required (Fehlmann et al, 2012;Gröbner et al, 2014). Since a large number of shortwave and longwave radiation time series (e.g.…”

“…The implications of a revision, based on the study by Gröbner et al (2014), are then considered with respect to the BSRN archive, which is followed by an initial assessment of the effect on downward direct shortwave and downward longwave radiation (DSR, DLR) time series at three BSRN stations (Georg von Neumayer, GVN; Ny Ålesund, NYA; Payerne, PAY) and at Davos. Conclusions as well as recommendations for further studies are then presented in the final section.…”