Extending the Calibration Traceability of Longwave Radiation Time-Series (ExTrac)

Pyrgeometers are used to measure longwave radiation at the Earth’s surface that has been identified by the Global
Climate Observing System (GCOS) as an Essential Climate Variable (ECV). The Baseline Surface Radiation Network
(BSRN) has coordinated the measurement and archiving of radiation data at ground-based stations since the early 1990s.
However, raw pyrgeometer data has not been archived by BSRN, and only exists in station archives themselves. In fact,
data may no longer be available due to various legacy issues. This study reports a possible method to retrieve proxy values
of raw pyrgeometer data from BSRN archives for periods predating the time when raw measurements were recorded along
with processed data. Raw pyrgeometer datasets (1-min. resolution) and meteorological parameters from several BSRN
stations were used to train a station-specific model. Using this data to re-calculate longwave radiation gives a root-mean-
square-deviation of 1.7 – 4.6 and 1.9 – 4.8 Wm-2 (1-hr and 10-min data) with respect to original data that is less than the
measurement uncertainty of ~4 – 5 Wm-2