This paper evaluates the absolute accuracy and stability of the radiometric calibration of the Atmospheric Infrared Sounder (AIRS) by analyzing the difference between the brightness temperatures measured at 2616 cm−1 and those calculated at the top of the atmosphere (TOA), using the Real‐Time Global Sea Surface Temperature (RTGSST) for cloud‐free night tropical oceans between ±30° latitude. The TOA correction is based on radiative transfer. The analysis of the first 3 years of AIRS radiances verifies the absolute calibration at 2616 cm−1 to better than 200 mK, with better than 16 mK/yr stability. The AIRS radiometric calibration uses an internal full aperture wedge blackbody with the National Institute of Standards and Technology (NIST) traceable prelaunch calibration coefficients. The calibration coefficients have been unchanged since launch. The analysis uses very tight cloud filtering, which selects about 7000 cloud‐free tropical ocean spectra per day, about 0.5% of the data. The absolute accuracy and stability of the radiometry demonstrated at 2616 cm−1 are direct consequences of the implementation of AIRS as a thermally controlled, cooled grating‐array spectrometer and meticulous attention to details. Comparable radiometric performance is inferred from the AIRS design for all 2378 channels. AIRS performance sets the benchmark for what can be achieved with a state‐of‐the‐art hyperspectral radiometer from polar orbit and what is expected from future hyperspectral sounders. AIRS was launched into a 705 km altitude polar orbit on NASA's Earth Observation System (EOS) Aqua spacecraft on 4 May 2002. AIRS covers the 3.7–15.4 micron region of the thermal infrared spectrum with a spectral resolution of ν/Δν = 1200 and has returned 3.7 million spectra of the upwelling radiance each day since the start of routine data gathering in September 2002.

We evaluate the stability of the radiometric calibration of the Atmospheric Infrared Sounder (AIRS) by analyzing the trend in the time series of the difference between the brightness temperatures measured in the 1,231‐cm−1 atmospheric window channel, corrected for atmospheric transmission, relative to the Real‐Time Global Sea Surface Temperature (RTGSST) for oceans between 30S and 30N. The observed bias relative to the RTGSST between 2002 and 2019 was less than 250 mK, with a 2–3‐mK/yr trend. Establishing the stability of the 1,231‐cm−1 channel at tropical ocean temperatures at the 2–3‐mK/yr level is a necessary but not sufficient condition of establishing the calibration stability of all AIRS channels over the full dynamic range at a comparable level. Our analysis indirectly establishes the stability of the RTGSST for the 2002–2017 time period and region at the 2–3‐mK/yr level, with a degradation since 2017.

The analysis of the response of the Earth Climate System to the seasonal changes of solar forcing in the tropical oceans using four years of the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) data between 2002 and 2006 gives new insight into amplitude and phase relationships between surface and tropospheric temperatures, humidity, and convective activity. The intensity of the convective activity is measured by counting deep convective clouds. The peaks of convective activity, temperature in the mid‐troposphere, and water vapor in the 0–30 N and 0–30 S tropical ocean zonal means occur about two months after solstice, all leading the peak of the sea surface temperature by several weeks. Phase is key to the evaluation of feedback. The evaluation of climate models in terms of zonal and annual means and annual mean deviations from zonal means can now be supplemented by evaluating the phase of key atmospheric and surface parameters relative to solstice. The ability of climate models to reproduce the statistical flavor of the observed amplitudes and relative phases for broad zonal means should lead to increased confidence in the realism of their water vapor and cloud feedback algorithms. AIRS and AMSU were launched into a 705 km altitude polar sun‐synchronous orbit on the EOS Aqua spacecraft on May 4, 2002, and have been in routine data gathering mode since September 2002

The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on 4 May 2002. The AIRS is designed to measure atmospheric temperature and water vapor profiles and has demonstrated exceptional radiometric and spectral accuracy and stability in orbit. The International System of Units (SI)-traceability of the derived radiances is achieved by transferring the calibration from the Large Area Blackbody (LABB) with SI traceable temperature sensors, to the On-Board Calibrator (OBC) blackbody during preflight testing. The AIRS views the OBC blackbody and four full aperture space views every scan. A recent analysis of pre-flight and on-board data has improved our understanding of the measurement uncertainty of the Version 5 AIRS L1B radiance product. For temperatures greater than 260 K, the measurement uncertainty is better than 250 mK 1-sigma for most channels. SI-traceability and quantification of the radiometric measurement uncertainty is critical to reducing biases in reanalysis products and radiative transfer models (RTMs) that use AIRS data, as well as establishing the suitability of AIRS as a benchmark for radiances established in the early 2000s.Remote Sens. 2020, 12, 1338 2 of 25 measure surface temperature and emissivity, cloud top height, cloud liquid water and cloud fraction. The data from the IR sounders assimilated into Numerical Weather Prediction (NWP) models have demonstrated amongst the highest forecast improvement in NWP models of all data types. The data are regularly used in studies of processes affecting weather [7] and climate as well as the validation of weather and climate models [8]. The data are also used in applications including volcano alerts [9], drought prediction [10], and air quality and pollution transport [11]. The AIRS instrument has shown exceptional radiometric stability to date, making it a valuable tool for climate trending and model validation [12]. Remote Sens. 2019, 11, x FOR PEER REVIEW 2 of 25 models have demonstrated amongst the highest forecast improvement in NWP models of all data types. The data are regularly used in studies of processes affecting weather [7] and climate as well as the validation of weather and climate models [8]. The data are also used in applications including volcano alerts [9], drought prediction [10], and air quality and pollution transport [11]. The AIRS instrument has shown exceptional radiometric stability to date, making it a valuable tool for climate trending and model validation [12]. Remote Sens. 2020, 12, 1338 3 of 25The AIRS is a reference sensor in the Global Space-Based Intercalibration of Sensors (GSICS) [17,18]. AIRS is used operationally by the Japan Meteorological Agency (JMA) for comparison to Himawari 8/9 AHI JM [19], the Korean Meteorological Administration (KMA) for comparison to COMS [20], and NOAA for comparison to CrIS [21] and GOES [22], and EUMETSAT for comparison to IASI [23]. The results show that the AIRS compares with IASI and CrIS to better than 0.2 K (1-sigma) in most cases. The long record of AIRS, com...

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