FUNDAMENTAL CLIMATE DATA RECORD OF MICROWAVE IMAGER RADIANCES, EDITION 5

K. Fennig¹, H. Konrad¹, M. Schröder¹

¹Deutscher Wetterdienst

The satellite-based HOAPS (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data) climatology, released by EUMETSATs Satellite Application Facility on Climate Monitoring (CM SAF), is primarily based on passive microwave measurements from the SSM/I (Special Sensor Microwave/Imager) and SSMIS (Special Sensor Microwave Imager / Sounder) sensor families. In order to derive reliable long-term trend estimates of the global water and energy cycle parameters it is strictly necessary to carefully correct for all known problems and deficiencies of the radiometers as well as to inter-calibrate and homogenize the different instruments. Moreover, all applied corrections need to be clearly documented to provide a complete calibration traceability for a Fundamental Climate Data Record (FCDR).

Following these recommendations, CM SAF also provides the underlying Brightness Temperatures as a stand-alone climate data record. The CM SAF FCDR of Microwave Imager Radiances comprises inter-calibrated brightness temperatures from the SMMR, SSM/I and SSMIS radiometers. The new 5th edition of this FCDR covers the time period from October 1978 to December 2024 including all available data from the SMMR radiometer aboard Nimbus 7, the SSM/I radiometers aboard F08, F10, F11, F13, F14, and F15 and the SSMIS radiometers aboard F16, F17, and F18. It provides homogenized and inter-calibrated brightness temperatures in a user-friendly data format.

The homogenization and inter-calibration procedures ensure the long-term stability of the FCDR for climate related applications. All available raw data records have been reprocessed to a common standard, starting with the calibration of the raw Earth counts, to ensure a completely homogenized data record. The data processing accounts for several known issues with the instruments and corrects calibration anomalies due to along-scan inhomogeneity, moonlight intrusions, sunlight intrusions, and emissive reflector. Also, an adjustment to the SSMIS geolocation has been implemented in order to improve the consistency.

The inter-calibration model incorporates a scene-dependent inter-satellite bias correction and a non-linearity correction to the instrument calibration. The data files contain all available original sensor data (SMMR: Pathfinder Level 1b) and metadata to provide a completely traceable climate data record. Inter-calibration and Earth incidence angle normalization offsets are available as additional layers within the data files in order to keep this information transparent to the users. The data record is complemented with radiometer sensitivities, quality flags, surface types, and Earth incidence angles.

While the SSM/I data record remains unchanged compared to the previous edition, the SMMR data quality is improved with an additional screening for calibration issues. An adjustment to the SSMIS geolocation has been implemented in the data record since version 4 in order to improve the consistency. Also issues with solar angle depending differences in F17 and F18 data have been identified and corrected, leading to an improved inter-satellite calibration and stability.

The consistency and homogeneity of the FCDR are statistically compared using the inter-satellite differences between the individual sensors. Also, differences between GMI and SSMIS and differences between observations and simulated brightness temperatures using ERA-5 and ERA 20c data are analyzed. The observed remaining variability in the inter-calibrated brightness temperatures is mainly caused by the natural variability due to differences in overpass times and sampling. Trends of the inter-sensor differences are below 0.03 K/decade and hence fulfill the stability requirements.

The public release of the 5th edition of the FCDR of Microwave Imager Radiances is planned in early 2026 and will be freely available from the CM SAF web user interface (https://wui.cmsaf.eu/).