The Atmospheric Infra-Red Sounder (AIRS) was launched, in conjunction with the Advanced Microwave Sounding Unit-A (AMSU-A) on the NASA polar-orbiting Earth Observing System (EOS) research satellite Aqua in May 2002 as a next-generation atmospheric sounding system. Atmospheric sounders measure outgoing radiation in discrete channels (spectral intervals) that are sensitive to variations in atmospheric temperature and water vapor, and provide information about their vertical distributions. The primary objectives of AIRS/AMSU were to use such information to improve numerical weather prediction and measure climate variability and trends.
AIRS is a multidetector-array grating spectrometer containing 2378 channels covering the spectral range from 650 to 2660cm−1(15–3.6μm) with a resolving power ν / δν≈1200, where ν represents frequency and δν the spectral-channel bandpass. Atmospheric-temperature profiles can be determined from channel observations taken within both the 15 and 4.2μm channels (the long- and short-wave carbon dioxide—CO2—absorption bands, respectively). Radiances in these (and all other) IR spectral intervals are also sensitive to the presence of clouds in the instrument's field of view (FOV), which are present approximately 95% of the time. AIRS was designed to produce accurate quality-controlled atmospheric soundings under most cloud conditions. Therefore, the instrument has extremely low channel-noise values in the short-wave portion of the spectrum and a very flat spatial-response function within a given channel's FOV. The high-spectral-resolution Infrared Atmospheric Sounding Interferometer (IASI) on the European Meteorological Operational Platform (METOP) satellite does not contain either of these important characteristics. The AIRS instrument was also designed to be extremely stable with regard to its spectral radiometric characteristics, which is critical for measuring accurate long-term trends.
AIRS, at nadir viewing, has nine 13.5×13.5km2 FOVs that lie within a single 45×45km2 AMSU field of regard (FOR). The AIRS science team version 5 retrieval algorithm1 generates a single sounding per FOR, using the nine FOVs to derive clear column radiances for each channel i (which represent the radiance channel i would have seen if the FOR were completely cloud free). Geophysical parameters are determined to be consistent with clear column radiances for a select set of channels. Based on theoretical considerations, coefficients needed to obtain clear column radiances for all channels are determined using only observations in the long-wave CO2 band and window region, while atmospheric and surface temperatures are determined using clear column radiances only in the short-wave CO2 band and window region. This optimal approach is practical for AIRS because of the very low noise in the short-wave channels.
Figure 1 shows a sample AIRS cloud-free brightness-temperature spectrum and indicates (using stars of different colors) those channels used in the different steps of our version 5 algorithm. Tropospheric sounding 15μm CO2 channels are used only for cloud clearing, while stratospheric channels (which do not see clouds) are included in the determination of the temperature profile. Clear column radiances for select other channels are used to determine constituent profiles. This new version 5 retrieval methodology also enables generation of error estimates of the retrieved products, which are used for quality control and are critical for weather and climate applications.
Figure 1. AIRS cloud-free brightness-temperature spectrum (in Kelvin, K). CH4: Methane. CO: Carbon monoxide. LW: Long wave. T(p): Atmospheric temperature.
Important results using AIRS version 5 products include the following. First, assimilation of quality-controlled temperature profiles produced superior 5–7 day forecasts compared to those obtained using the National Oceanic and Atmospheric Administration's operational procedure of assimilating observed AIRS radiances. Assimilation of quality-controlled AIRS temperature soundings resulted in a significant improvement in predicting the track of tropical storm Nargis (that devastated parts of Indonesia in 2006), compared to the operational procedure.2
Second, outgoing long-wave radiation (OLR) computed from AIRS products confirms the result observed by the Clouds and the Earth's Radiant Energy System (CERES) EOS instruments that the global mean OLR has been decreasing at a rate of 0.12W/m2/yr over the Aqua operational period from September 2002 to December 2009. The majority of this decrease occurred in the tropics because of a significant redistribution of cloud cover and water vapor in response to a very strong La Niña event starting in late 2007.
Third, twice-daily global fields of carbon monoxide (CO) derived from AIRS accurately depict the space and time propagation of CO originating from fires. Finally, AIRS provided the first accurate global monthly mean fields of CO2 concentration, which showed both local seasonal cycles and CO2 growth over time.
NASA has released a new proposed design employing 2D detector arrays for a new low-earth-orbiting instrument, the Advanced Remote-sensing Imaging Emission Spectrometer (ARIES), which has AIRS-like spectral and radiometric characteristics but a spatial resolution of 1km. Products derived from such an instrument would result in significant improvements in weather prediction, monitoring of climate parameters, and derivation of atmospheric water-vapor and trace-gas distributions.
Lena Iredell, Joel Susskind
NASA Goddard Space Flight Center
Lena Iredell received a BS in meteorology from the State University of New York at Oswego and a MS in forestry (forest micrometeorology) from West Virginia University. Her areas of expertise include development of quality-control methodologies, development of level 3 gridding algorithms, statistical and graphical analysis, and validation of retrieved surface and atmospheric products from the AIRS/AMSU and Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) instruments.
Joel Susskind received his PhD in physical chemistry from the University of California at Berkeley. He is a senior scientist in the Laboratory for Atmospheres, engaged in research on IR and microwave remote sensing. He is the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project (NPP) and Hyperspectral Environmental Suite (HES) sounder scientist and a member of the AIRS science team.
2. O. Reale, W. K. Lau, J. Susskind, E. Brin, E. Liu, L. P. Riishojgaard, M. Fuentes, R. Rosenberg, AIRS impact on the analysis and forecast track of tropical cyclone Nargis in a global data assimilation and forecasting system, Geophys. Res. Lett. 36, pp. L06812, 2009. doi:10.1029/2008GL037122