Accurate Radiometric Measurement of the Atmospheric Longwave Flux at theSea Surface

Abstract

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 obtained for the error due to differential heating of the sensor. The magnitude of the error due to this effect under typical seagoing conditions is shown to be up to 20 W m?2 from measurements made with a precalibrated standard radiometer, for which the dome and body temperatures were recorded, during 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 full diurnal cycle but remains at a typical level of 5?7 W m?2 in the weekly mean flux. The differential heating of the radiometer is shown to be primarily dependent on the incident shortwave radiation, moderated slightly by the cooling effects of airflow over the dome. An empirical correction is developed 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. Application of the empirical correction for differential heating to the second 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. It is suggested that this correction may be usefully employed as an alternative to recording component temperatures in future studies, particularly long-term buoy deployments, to improve the accuracy of the measured longwave flux.