A new look at calibration and use of Eppley precision infrared radiometers. Part I: Theory and application

The calibration and accuracy of the Eppley precision infrared radiometer (P IR) is examined both theoretically and experimentally. A rederivation of th e fundamental energy balance of the PIR indicates that the calibration equa tion in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) fo r the emissivity is' used, then this leads to errors in the total flux of 5 -10 W m(-2). The basic precision of the instrument is found to be about 1.5 % of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer's optional battery-comp ensated output is used exclusively, then the uncertainties increase to abou t 1.5% of the total (20 W m(-2)). It is suggested that a modern radiative t ransfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the Rapid Radiative Transfer Model ( RRTM), from 55 radiosondes ascents in cloud-free conditions during the Trop ical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment field program, gave mean agreement within 2 W m(-2) of those measured with a shipborne PIR. PIR data from two sets of instrument intercomparisons were used to demonstrate ways of detecting inconsistencies in thermopile-sensit ivity coefficients and dome-heating correction coefficients. These comparis ons indicated that pairs of PIRs are easily corrected to yield mean differe nces of 1 W m(-2) and rms differences of 2 W m(-2). Data from a previous fi eld program over the ocean indicate that pairs of PIRs can be used to deduc e the true surface skin temperature to an accuracy of a few tenths of a kel vin.