Challenges in Interpreting Long Term Hyperspectral Infrared Sounder Records, and in Creating a Continuity Record from Multiple Instruments

Several hyperspectral infrared satellite sounding instruments have operated during the two decades of the new millennium. Observations from these instruments are a long and detailed record of weather, composition, climate processes, and climate trends. While that record has provided many insights into atmospheric phenomena, interpretation of the complete record from multiple instruments is an ongoing challenge. Some of the insights provided by the Atmospheric Infrared Sounder (AIRS) record, beginning in 2002, will be described. These include well-characterized trends in radiances, and in some retrieved quantities at large scales, such as significant Arctic warming and middle atmospheric cooling, and interdecadal changes in trace gases. Challenges to long-term monitoring by AIRS include understanding trends at medium spatial scale, and in characterizing long-term covariations of retrieved quantities. Multiple copies of other sounders, such as the Infrared Atmospheric Sounding Interferometer (IASI) and Cross-track Scanning Interferometer (CrIS) have acquired similar observations to AIRS. We will describe some of the accomplishments and challenges in creating a continuous record between these other sounders and AIRS. All instruments launched to date have very consistent radiometric and spectral properties, providing a foundational climate data record. However, no formal means to ensure single instrument long-term calibration and cross-instrument radiance consistency are in place. Interpreting retrieved quantities from different sounders is difficult for a variety of reason. These include retrieval systems with a variety algorithm formulations and prior state (‘first guess’) sources. All sounder retrievals suffer from incomplete uncertainty quantification, making long-term retrieval comparisons challenging. Reconciling and merging the retrieved data sets from multiple sounders will require a sustained future effort.