Assessing the Quality of Shortwave and Longwave Irradiance Observations over the Ocean: One Year of High-Time-Resolution Measurements at the Lampedusa Oceanographic Observatory

Abstract

AbstractMeasurements of downwelling shortwave (SW) and longwave (LW) irradiance were carried out on an oceanographic buoy close to the island of Lampedusa (Italy), in the central Mediterranean Sea. Irradiance measurements on the buoy were acquired at high time resolution together with the radiometer pitch and roll angles. The measurements carried out during 2016 have been compared with ground-based observations made at the Lampedusa Atmospheric Observatory, about 15 km northeast of the buoy. The radiometers were compared before and after deployment on the buoy and are traceable to the World Radiometric Reference scale. The SW measurements were corrected for the thermal offset. A small bias (measurements over the sea are smaller than on land) of about −2 W m−2 is found in the daily mean SW, and a moderate bias of +6.2 W m−2 (irradiance over the sea is larger than on land) is found in the LW. Similar biases are found when instantaneous measurements obtained with horizontal radiometers, clean domes, and cloud-free conditions are selected, suggesting that impacts of the moving platform and poor dome cleaning are minor at this site. The effect of the mean tilt angle was also investigated. Deviations in the hourly mean SW irradiance are on the order of 20% for a mean offset of 4° with respect to the solar zenith angle; the effect of tilt angle on LW irradiance appears to be negligible. Radiative transfer calculations show that the observed biases may be ascribed to the differences in the instrument altitude (through radiation absorption, scattering, and emission by the atmospheric constituents in the lowest atmospheric layers) and in the SW surface albedo.