Scott E. Hannon

AIRS: Improving Weather Forecasting and Providing New Data on Greenhouse Gases

Abstract The Atmospheric Infrared Sounder (AIRS) and its two companion microwave sounders, AMSU and HSB were launched into polar orbit onboard the NASA Aqua Satellite in May 2002. NASA required the sounding system to provide high-quality research data for climate studies and to meet NOAAs requirements for improving operational weather forecasting. The NOAA requirement translated into global retrieval of temperature and humidity profiles with accuracies approaching those of radiosondes. AIRS also provides new measurements of several greenhouse gases, such as CO2, CO, CH4, O3, SO2, and aerosols. The assimilation of AIRS data into operational weather forecasting has already demonstrated significant improvements in global forecast skill. At NOAA/NCEP, the improvement in the forecast skill achieved at 6 days is equivalent to gaining an extension of forecast capability of six hours. This improvement is quite significant when compared to other forecast improvements over the last decade. In addition to NCEP, ECM...

An overview of the AIRS radiative transfer model

The two main elements of the Atmospheric Infrared Sounder radiative transfer algorithm (AIRS-RTA) are described in this paper: 1) the fast parameterization of the atmospheric transmittances that are used to perform the AIRS physical retrievals and 2) the spectroscopy used to generate the parameterized transmittances. We concentrate on those aspects of the spectroscopy that are especially relevant for temperature and water vapor retrievals. The AIRS-RTA is a hybrid model in that it parameterizes most gases on a fixed grid of pressures, while the water optical depths are parameterized on a fixed grid of water amounts. Water vapor, ozone, carbon monoxide, and methane profiles can be varied, in addition to the column abundance of carbon dioxide.

A compilation of first-order line-mixing coefficients for CO2Q-branches

Abstract A parametrization of first-order line-mixing in CO2Q-branches for use in atmospheric radiative transfer codes. Regression coefficients for the temperature dependence of the first-order line-mixing parameters for each J in 11 of the strongest Q-branches between 600 and 3000 cm-1 are given. These coefficients may only be used with the line strengths and widths reported in the HITRAN92 line compilation; otherwise large errors will be introduced into the calculated line shape. Other potential problems in the implementation of first-order line-mixing in radiative transfer codes are also discussed.

In-flight spectral calibration of the Atmospheric Infrared Sounder

Preflight testing of AIRS determined that the shapes of the detector spectral response functions (SRFs) do not vary under different instrument conditions. This reduces in-flight spectral calibration to the determination of detector spectral centroids. A spectrometer grating model has been developed to calculate detector centroids. Only two parameters of this model need to be determined in orbit. An algorithm is presented for determining these two parameters in orbit by correlating observed upwelling radiance spectra with modeled spectra. The method of selecting spectral regions against which to correlate is detailed. The in-orbit spectral calibration algorithm was tested on one day of simulated global AIRS radiances, showing that the uncertainty in the frequencies of the SRF centroids is between 0.006 /spl Delta//spl nu/ and 0.01 /spl Delta//spl nu/, compared to the spectral calibration requirement of 0.01 /spl Delta//spl nu/, where /spl Delta//spl nu/ is the SRF full width at half maximum. The simulation also indicates that the stability of the spectral calibration can be monitored at the 0.001-/spl Delta//spl nu/ level on a daily basis.

Formulation and validation of simulated data for the Atmospheric Infrared Sounder (AIRS)

Models for synthesizing radiance measurements by the Atmospheric Infrared Sounder (AIRS) are described. Synthetic radiances have been generated for developing and testing data processing algorithms. The radiances are calculated from geophysical states derived from weather forecasts and climatology using the AIRS rapid transmission algorithm. The data contain horizontal variability at the spatial resolution of AIRS from the surface and cloud fields. This is needed to test retrieval algorithms under partially cloudy conditions. The surface variability is added using vegetation and International Geosphere Biosphere Programme surface type maps, while cloud variability is added randomly. The radiances are spectrally averaged to create High Resolution Infrared Sounder (HIRS) data, and this is compared with actual HIRS2 data on the NOAA 14 satellite. The simulated data under-represent high-altitude equatorial cirrus clouds and have too much local variability. They agree in the mean to within 1-4 K, and global standard deviation agrees to better than 2 K. Simulated data have been a valuable tool for developing retrieval algorithms and studying error characteristics and will continue to be so after launch.

Impact of a new water vapor continuum and line shape model on observed high resolution infrared radiances

Abstract Line-by-line calculations of up-welling atmospheric radiances are compared to clear-sky radiances observed with the University of Wisconsin High-resolution Interferometer Sounder (HIS). We concentrate on water emission in the 1400–1750 cm −1 region which is sensitive to the N 2 -broadened water continuum. This work applies our recently developed water continuum derived from new laboratory data to atmospheric emission measurements. The atmospheric state under clear-sky conditions was characterized with radiosonde and LIDAR observations. Radiances in-between absorption lines probe the lower troposphere, where the water vapor profiles are best characterized, allowing comparison of several different water continuum models. Closer to the centers of the absorption lines all models yield relatively large errors, most likely due to inaccuracies in the high altitude water vapor radiosonde measurements.

Improved atmospheric radiance calculations using CO2 P/R-branch line mixing

New high-spectral resolution satellite sounders will use channels located between CO2 lines for temperature retrievals. Transmittances for these channels are dominated by spectral line wings that are strongly influenced by line-mixing and duration-of-collision effects. Previous studies demonstrated the importance of Q-branch line mixing for atmospheric sounding in the 15 micrometer region. This work presents an improved model of P/R-branch line mixing and duration-of- collision effects on CO2 transmittances in the 4.3 micrometer and 15 micrometer regions, based on laboratory and spectroscopy data. Most line-by-line codes model non- Lorentzian behavior by using the Cousin chi-function. This empirical function incorporates both P/R line-mixing and duration-of-collision effects by using many parameters. It is common to use the Cousin model parameters obtained from the 4 micrometer band in the 15 micrometer region, overestimating the amount of line-mixing. Comparisons to radiance data taken with high resolution interferometers that fly on NASAs ER-2 partially validates our model. The biggest improvements are at 4.3 micrometer where the differences are reduced by more than 2K, compared to using the Cousin model.

kCompressed atmospheric radiative transfer algorithm (kCARTA)

A new monochromatic radiative transfer algorithm based on compressed lookup tables of pre-computed atmospheric molecular absorption coefficients has been developed. These compressed look-up tables are called the kCompressed Database. Our motivation is to compute monochromatic absorption coefficients for any realistic Earth atmospheric situation (pressure, temperature, gas amount) at the same accuracy as a line-by- line code, but faster. In addition, the procedure for producing atmospheric transmittances is extremely simple, and easy to code. Although the kCompressed Database was originally developed to compute layer-to-space transmittances that are needed to produce fast transmittance models for high spectral resolution infrared temperature and humidity sounders, we have now developed a complete (non-scattering) atmospheric radiative transfer code around the kCompressed Database, called kCARTA (for kCompressed Atmospheric Radiative Transfer Algorithm). In addition, Jacobians with respect to gas amount and temperature can be rapidly performed, providing the user insight to the regions to which the measured radiance is most sensitive.

Two-year comparison of radiances from the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI)

The radiometric intercomparison of instruments is a key element in developing climate-quality data records. In this study we compare data from the first two years of the Infrared Atmospheric Sounding Interferometer (IASI) with the matching data from the Atmospheric Infrared Sounder (AIRS). We compare observed spectra in cloud-free areas of the tropical oceans at night to spectra calculated using data from the European Centre for Medium-Range Weather Forecasts (ECMWF). We use five frequencies-three window channels, one mid-tropospheric sounding channel, and one lower stratospheric sounding channel. The use of ECMWF data as a transfer standard permits comparisons of many more points distributed more widely over the globe than is possible with the traditional simultaneous nadir overpass (SNO) technique. The analysis shows that AIRS and IASI daily mean brightness temperatures track each other within 100 mK, in spite of the fact that the instruments are in different orbits. AIRS was launched into polar orbit on the EOS Aqua spacecraft on May 4, 2002. It is a grating spectrometer with 2378 channels in the range 3.7 to 15.4 microns. IASI was launched into polar orbit in October 2006 on the METOP-A spacecraft. IASI is a Fourier transform spectrometer covering 3.7 to 15.5 microns in three bands with a total of 8461 channels.

Validation of the AIRS Radiative Transfer Algorithm

This paper presents comparisons between observed AQUA-AIRS spectra and spectra computed from the European Center for Medium Range Forecasting (ECMWF) numerical weather prediction model fields.