A Synopsis of AIRS Global-Mean Clear-Sky Radiance Trends From 2003 to 2020

Abstract

Atmospheric Infrared Sounder (AIRS) aboard the National Aeronautics and Space Administration (NASA) Aqua satellite has been operating since September 2002. Its information content, superb instrument performance, and dense sampling pattern make the AIRS radiances an invaluable data set for climate studies. The trends of global-mean, nadir-view, clear-sky AIRS radiances from 2003 to 2020 are studied here, together with the counterparts of synthetic radiances based on two reanalyzes, European Centre for Medium-Range Weather Forecasts Reanalysis V5 (ECMWF ERA5) and NASA Goddard Earth Observing System V5.4.1 (GEOS-5.4.1; a reanalysis product without assimilation of hyperspectral radiances such as AIRS). The AIRS observation shows statistically significant negative trends in most of its CO2 channels, positive but non-significant trends in the channels over the window regions, and statistically significant positive trends in some of its H2O channels. The best agreements between observed and simulated radiance trends are seen over the CO2 tropospheric channels, while the observed and simulated trends over the CO2 stratospheric channels are opposite. ERA5 results largely agree with the AIRS observation over the H2O channels. The comparison in the H2O channels helps reveal a data continuity issue in the GEOS-5.4.1. Contributions from individual variables to the radiance trends are also assessed by performing separate simulations. This study provides the first synopsis of the global-mean trend of AIRS radiances over all its thermal-IR channels.Plain Language SummaryUsing 18 years of satellite hyperspectral observations, we depict, for the first time, a synopsis of the trend of global-mean, clear-sky infrared radiances from ∼6 to ∼16 μm. Statistically significant trends are seen in the carbon dioxide and water vapor absorption bands but not over the infrared window region. The physical explanations of such trends are discussed and supported with simulation sensitivity studies. Such observed trends, channel-by-channel, are also compared with those derived from synthetic radiances based on reanalysis data sets. Discrepancies between observed and reanalysis-based trends are also discussed.Key PointsThe 18-year trends of global-mean, clear-sky radiances at all Atmospheric Infrared Sounder (AIRS) thermal-infrared channels are analyzedAIRS observation shows statistically significant negative trends in most of its CO2 channels and positive trends in some of its H2O channelsThe best agreements between observations and reanalysis are seen over the CO2 tropospheric channels