Shipborne comparison of infrared and passive microwave radiometers for sea surface temperature observations

Abstract. In the spring of 2021, a shipborne comparison of sea surface temperature (SST) measurements was undertaken using thermal infrared (IR) and passive microwave (PMW) radiometers. The Danish Meteorological Institute (DMI) and the Technical University of Denmark (DTU) jointly deployed two IR and two PMW instruments aboard the Norröna ferry, which traversed between Denmark and Iceland for a week. The primary objective was to assess the proximity-based comparison of IR and PMW measurements, minimizing atmospheric influences and providing valuable insights into reconciling IR- and PMW-derived SSTs. A linear regression algorithm was developed using IR SST data as a reference to derive PMW SST from brightness temperature. The data analysis primarily focused on evaluating data variability, identifying discrepancies between IR and PMW SST, and assessing the overall uncertainty in the retrieval process. The overall root-mean-square error (RMSE) of the retrieved PMW SST was 0.88 K during the ship's motion and 0.94 K when the ship was moored. The analysis of the retrieved SST uncertainty budget involved the consideration of observed quantities and a forward model, accounting for factors like instrument noise, wind speed, incidence angle, and the RMSE of skin and sub-skin temperature. The resulting uncertainty budget in the retrieved PMW SST indicated 0.53 K for the data acquired during motion and 0.3 K for data collected during a port stay. Based on the analyses of the collected data and uncertainty estimations, recommendations are offered to improve future inter-comparisons and help reconcile IR and PMW measurements.