Accurate radiometric measurement of the atmospheric longwave flux at the sea surface

The errors in pyrgeometer measurements of the atmospheric longwave flux at the sea surface due to differential heating of the sensor dome relative to the body and to shortwave leakage through the dome are evaluated. Contrary to the findings of Dickey et al., repeatable laboratory calibrations are ob tained for the error due to differential heating of the sensor. The magnitu de of the error due to this effect under typical seagoing conditions is sho wn to be up to 20 W m(-2) from measurements made with a precalibrated stand ard radiometer, for which the dome and body temperatures were recorded, dur ing a research cruise in the North Atlantic in late spring 1998. The error due to shortwave leakage is found to be similar in magnitude and to lead to a combined bias in the longwave flux of up to 40 W m(-2) under conditions of strong insolation. The error is reduced when averages are taken over a f ull diurnal cycle but remains at a typical level of 5-7 W m(-2) in the week ly mean flux, The differential heating of the radiometer is shown to be pri marily dependent on the incident shortwave radiation, moderated slightly by the cooling effects of airflow over the dome. An empirical correction is d eveloped for the differential heating error as a function of the shortwave flux and relative wind speed. Measurements of the longwave flux during the cruise from the standard radiometer and a second radiometer employed in the normal mode without logging of the component temperatures are compared. Ap plication of the empirical correction for differential heating to the secon d radiometer together with that for shortwave leakage leads to a reduction in the difference relative to the standard radiometer from -5.6 +/- 9.0 to -0.4 +/- 2.5 W m(-2). Iris suggested that this correction may be usefully e mployed as an alternative to recording component temperatures in future stu dies, particularly long-term buoy deployments, to improve the accuracy of t he measured longwave flux.