Establishment of AIRS climate-level radiometric stability using radiance anomaly retrievals of minor gases and sea surface temperature

Abstract

Abstract. Temperature, H2O, and O3 profiles, as well as CO2, N2O, CH4, chlorofluorocarbon-12 (CFC-12), and sea surface temperature (SST) scalar anomalies are computed using a clear subset of AIRS observations over ocean for the first 16 years of NASA's Earth-Observing Satellite (EOS) Aqua Atmospheric Infrared Sounder (AIRS) operation. The AIRS Level-1c radiances are averaged over 16 d and 40 equal-area zonal bins and then converted to brightness temperature anomalies. Geophysical anomalies are retrieved from the brightness temperature anomalies using a relatively standard optimal estimation approach. The CO2, N2O, CH4, and CFC-12 anomalies are derived by applying a vertically uniform multiplicative shift to each gas in order to obtain an estimate for the gas mixing ratio. The minor-gas anomalies are compared to the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL) in situ values and used to estimate the radiometric stability of the AIRS radiances. Similarly, the retrieved SST anomalies are compared to the SST values used in the ERA-Interim reanalysis and to NOAA's Optimum Interpolation SST (OISST) product. These intercomparisons strongly suggest that many AIRS channels are stable to better than 0.02 to 0.03 K per decade, well below climate trend levels, indicating that the AIRS blackbody is not drifting. However, detailed examination of the anomaly retrieval residuals (observed – computed) shows various small unphysical shifts that correspond to AIRS hardware events (shutdowns, etc.). Some examples are given highlighting how the AIRS radiance stability could be improved, especially for channels sensitive to N2O and CH4. The AIRS shortwave channels exhibit larger drifts that make them unsuitable for climate trending, and they are avoided in this work. The AIRS Level 2 surface temperature retrievals only use shortwave channels. We summarize how these shortwave drifts impacts recently published comparisons of AIRS surface temperature trends to other surface climatologies.

An Investigation of the Fengyun-4A/B GIIRS Performance on Temperature and Humidity Retrievals

“…Normally the bands in absorption spectra of CO 2 and H 2 O are used for retrieving temperature and humidity profiles, respectively. For the sake of the retrieval accuracy, those channels that are affected by 'foreign' gas and have a significant contribution to temperature or humidity should be removed [38].…”

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

“…Its stability was initially estimated to be ∼4 mK yr −1 (Aumann & Pagano, 2008). Strow and DeSouza-Machado (2020) evaluated the stability of more than 400 AIRS channels and showed the stability of these channels is ∼2 mK yr −1 .…”

An empirical solution for the AIRS scene mirror contamination from adjacent footprints

“…A bottoms-up approach on the AIRS calibration equation estimated the uncertainty of the radiances to be between 0.10K and 0.60K [3], depending on detector module and scene brightness temperature. One study, which used AIRS brightness temperature anomalies to create various retrieved geophysical anomalies over 16 years, estimated that most AIRS channels have a stability better than 0.02K/decade [4]. Two similar studies that compared AIRS window channels to sea surface temperature products estimated the stability of these window channels to be 0.002-.006 K [5,6].…”