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.
Highlights
The Atmospheric Infrared Sounder (AIRS) on NASA’s Aqua satellite platform (Aumann et al, 2003) measures 2378 high-spectral-resolution infrared radiances between 650 and 2665 cm−1 with a resolving power (λ/ λ) of ∼ 1200
If AIRS is to contribute to climate-level trend measurements, uncertainty estimates for the time stability of the AIRS radiances are a prerequisite before using AIRS Level 2/3 products for climate-level trending
The results presented here conclude that the AIRS shortwave channels are drifting positive by about 0.058 K per decade relative to the longwave channels, which appear to be in extremely good agreement with established sea surface temperature (SST) climate products as discussed above
Summary
The Atmospheric Infrared Sounder (AIRS) on NASA’s Aqua satellite platform (Aumann et al, 2003) measures 2378 high-spectral-resolution infrared radiances between 650 and 2665 cm−1 with a resolving power (λ/ λ) of ∼ 1200. The atmospheric CO2 concentration is especially important for AIRS retrievals since most of the radiance measured in the temperature sounding channels is due to CO2 emission. We use the high accuracy of the trends in these in situ measurements of minor gases to determine the stability of a large number of AIRS channels. AIRS stability is referenced to trends in these minor gases and SST by performing 1D-Var retrievals of clear scene radiance anomalies averaged into 40 equal-area latitude bins and 16 d time periods. Comparisons of the retrieved gas concentrations and SST trends, combined with examination of the retrieval residuals, provides a number of powerful tests of AIRS radiometric stability as well as detailed information on AIRS performance changes due to several minor instrument shutdowns that took place occasionally over the mission. We examine the time series of the anomaly retrieval residuals (BT observed – fit) time series since, together with the anomaly geophysical retrievals, they provide detailed information on AIRS radiances over time, especially the instrument response to various short shutdowns that occurred during the mission
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