Impact of Microwave Data into the Copernicus Marine Service SST Products for the Baltic Sea and the Mediterranean Sea

Within the Copernicus Marine Service, the Danish Meteorological Institute and the Italian National Research Council (CNR) develop and deliver daily, gap-free (Level 4, L4) sea surface temperature (SST) analyses for the Baltic Sea (BAL) and the Mediterranean Sea (MED), based on infrared (IR) satellite observations. Unlike IR sensors, passive microwave (PMW) radiometers can observe the ocean surface through non-precipitating clouds, providing complementary SST measurements, albeit at coarser spatial resolution (approximately 50 km). This study presents an ongoing assessment of the ingestion of PMW-derived SST into the operational BAL and MED L4 SST production chains. Provided that product accuracy is preserved or improved, the inclusion of PMW data is expected to significantly enhance observational coverage, a critical benefit in cloud-prone regions such as the Baltic Sea. The analysis is based on two years (2020–2021) of daily collated (Level 3C, L3C) SST data from the Advanced Microwave Scanning Radiometer 2 (AMSR2). Following preliminary evaluations of PMW data quality, including validation against in situ measurements, the near-real-time operational chains for both BAL and MED products were run incorporating PMW observations through different ingestion strategies. These strategies were tested to account for the differing spatial resolution and accuracy characteristics of PMW and IR data. The impact of PMW data inclusion on the final L4 SST products was then quantified using statistical metrics such as bias and root mean squared error, which showed an improvement of the analyses accuracy. This work serves as a preparatory step toward the future integration of PMW SST observations from upcoming satellite missions, such as the Copernicus Imaging Microwave Radiometer (CIMR), which will provide microwave SST measurements at an unprecedented spatial resolution of approximately 15 km.