Assessing and Improving the Quality of Unattended Radiation Observations in Antarctica

Abstract

The quality of atmospheric radiation measurements made at automatic weather stations (AWSs) in Antarctica is assessed. The AWSs are placed on the coastal ice shelf in the katabatic wind zone and on the high Antarctic plateau, and they measure shortwave and longwave radiation fluxes using unheated/unventilated Kipp and Zonen (KZ) CM3/CG3 sensors. During three summertime Antarctic experiments, the AWS sensors were directly compared to instruments of a higher standard, the KZ CM11 for shortwave and Eppley PIR for longwave radiation. It was found that the single-domed KZ CM3 is less sensitive to riming than the double-domed KZ CM11. With an accuracy better than 5% for daily averages, the KZ CM3 and CG3 perform better than their specifications. Net shortwave radiation calculated from individual pairs of incoming and reflected fluxes shows large relative errors, and a method is presented to remedy this. Summertime longwave fluxes measured with the KZ CG3 show very good agreement with ventilated Eppley PIR measurements [root-mean-square difference (rmsd) about 1%], but a larger systematic difference is found when comparison is made with unventilated Eppley PIR measurements. Upward extrapolation of snow temperatures suggest that the unventilated Eppley PIR measurements have a systematic offset, but additional measurements are necessary to confirm this. Wintertime riming of the unventilated/unheated KZ CG3 sensor window leads to rejection of 25%?28% of the LW? data for the AWS on the ice shelf and the plateau. Replacing these data with parameterized values removes the systematic offset but introduces an uncertainty of 10%?15%.