The Atmospheric Infrared Sounder (AIRS), the hyperspectral infrared sounder on the NASA Aqua mission, both improves operational weather prediction and provides high-quality research data for climate studies. The Atmospheric Infrared Sounder (AIRS), and its two companion microwave instruments, the Advanced Microwave Sounding Unit (AMSU) and the Humidity Sounder for Brazil (HSB), form the integrated atmospheric sounding system flying on the Earth Observing System (EOS) Aqua spacecraft since its launch in May 2002.1 The primary scientific achievement of AIRS has been to improve weather prediction (Le Marshall et al. 2005a,b,c) and to study the water and energy cycle (Tian et al. 2006). AIRS also provides information on several greenhouse gases. The measurement goal of AIRS is the retrieval of temperature and precipitable-water vapor profiles with accuracies approaching those of conventional radiosondes. In the following text we use the terms AIRS and AIRS-AMSU-HSB interchangeably.1 The HSB ceased functioning after 5 February 2003. This did not have an impact on the accuracy, coverage, or resolution of the AIRS core data product, but its loss has had a significant impact on AIRS research products.A comprehensive set of articles on AIRS and AMSU design details, prelaunch calibration, and prelaunch retrieval performance expectations were published in a special issue of IEEE Transactions on Geoscience and Remote Sensing (2003, vol. 41, no. 2). This paper discusses the performance of AIRS and examines how it is meeting its operational and research objectives based on the experience of more than 2 yr with AIRS data. We describe the science background and the performance of AIRS in terms of the accuracy and stability of its observed spectral radiances. We examine the validation of the retrieved temperature and water vapor profiles against collocated operational radiosondes, and then we assess the impact thereof on numerical weather forecasting of the assimilation of the AIRS spectra and the retrieved temperature. We close the paper with a discussion on the retrieval of several minor tropospheric constituents from AIRS spectra.

Human activity has increased the concentration of the earth's atmospheric carbon dioxide, which plays a direct role in contributing to global warming. Mid‐tropospheric CO2 retrieved by the Atmospheric Infrared Sounder shows a substantial spatiotemporal variability that is supported by in situ aircraft measurements. The distribution of middle tropospheric CO2 is strongly influenced by surface sources and large‐scale circulations such as the mid‐latitude jet streams and by synoptic weather systems, most notably in the summer hemisphere. In addition, the effects of stratosphere‐troposphere exchange are observed during a final stratospheric warming event. The results provide the means to understand the sources and sinks and the lifting of CO2 from surface layers into the free troposphere and its subsequent transport around the globe. These processes are not adequately represented in three chemistry‐transport models that have been used to study carbon budgets.

[1] We present a general method for the determination of minor gases in the troposphere from high spectral resolution observations. In this method, we make use of a general property of the total differential of multi-variable functions to separate the contributions of each individual minor gas. We have applied this method to derive the mixing ratio of carbon dioxide in the mid-troposphere using data from the Atmospheric Infrared Sounder (AIRS) currently flying on the NASA Aqua Mission. We compare our results to the aircraft flask CO 2 measurements obtained by H. Matsueda et al. over the western Pacific and demonstrate skill in tracking the measured 5 ppmv seasonal variation with an accuracy of 0.43 ± 1.20 ppmv. Citation: Chahine, M., C. Barnet, E. T. Olsen, L. Chen, and E. Maddy (2005), On the determination of atmospheric minor gases by the method of vanishing partial derivatives with application to CO 2 , Geophys. Res. Lett., 32, L22803,

The Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit/Humidity Sounder for Brazil (AIRS/AMSU/HSB) instrument suite onboard Aqua observes infrared and microwave radiances twice daily over most of the planet. AIRS offers unprecedented radiometric accuracy and signal to noise throughout the thermal infrared. Observations from the combined suite of AIRS, AMSU, and HSB are processed into retrievals of atmospheric parameters such as temperature, water vapor, and trace gases under all but the cloudiest conditions. A more limited retrieval set based on the microwave radiances is obtained under heavy cloud cover. Before measurements and retrievals from AIRS/AMSU/HSB instruments can be fully utilized they must be compared with the best possible in situ and other ancillary "truth" observations. Validation is the process of estimating the measurement and retrieval uncertainties through comparison with a set of correlative data of known uncertainties. The ultimate goal of the validation effort is retrieved product uncertainties constrained to those of radiosondes: tropospheric rms uncertainties of 1.0 C over a 1-km layer for temperature, and 10% over 2-km layers for water vapor. This paper describes the data sources and approaches to be used for validation of the AIRS/AMSU/HSB instrument suite, including validation of the forward models necessary for calculating observed radiances, validation of the observed radiances themselves, and validation of products retrieved from the observed radiances. Constraint of the AIRS product uncertainties to within the claimed specification of 1 K/1 km over well-instrumented regions is feasible within 12 months of launch, but global validation of all AIRS/AMSU/HSB products may require considerably more time due to the novelty and complexity of this dataset and the sparsity of some types of correlative observations.

[1] This study is our first step toward the generation of 6 hourly 3-D CO 2 fields that can be used to validate CO 2 forecast models by combining CO 2 observations from multiple sources using ensemble Kalman filtering. We discuss a procedure to assimilate Atmospheric Infrared Sounder (AIRS) column-averaged dry-air mole fraction of CO 2 (Xco 2 ) in conjunction with meteorological observations with the coupled Local Ensemble Transform Kalman Filter (LETKF)-Community Atmospheric Model version 3.5. We examine the impact of assimilating AIRS Xco 2 observations on CO 2 fields by comparing the results from the AIRS-run, which assimilates both AIRS Xco 2 and meteorological observations, to those from the meteor-run, which only assimilates meteorological observations. We find that assimilating AIRS Xco 2 results in a surface CO 2 seasonal cycle and the N-S surface gradient closer to the observations. When taking account of the CO 2 uncertainty estimation from the LETKF, the CO 2 analysis brackets the observed seasonal cycle. Verification against independent aircraft observations shows that assimilating AIRS Xco 2 improves the accuracy of the CO 2 vertical profiles by about 0.5-2 ppm depending on location and altitude. The results show that the CO 2 analysis ensemble spread at AIRS Xco 2 space is between 0.5 and 2 ppm, and the CO 2 analysis ensemble spread around the peak level of the averaging kernels is between 1 and 2 ppm. This uncertainty estimation is consistent with the magnitude of the CO 2 analysis error verified against AIRS Xco 2 observations and the independent aircraft CO 2 vertical profiles.