Retrieval of stratospheric temperatures from Atmospheric Infrared Sounder radiance measurements for gravity wave studies

The Atmospheric Infrared Sounder (AIRS) on board the National Aeronautics and Space Administration’s (NASA’s) Aqua satellite has been continuously measuring mid-infrared nadir and sub-limb radiance spectra since summer of 2002. These measurements are utilized to retrieve three-dimensional stratospheric temperature distributions by applying a new fast forward model for AIRS and an accompanying optimal estimation retrieval processor. The retrieval scheme presented in this article does not require simultaneous observations of microwave instruments like the AIRS operational analyses. Instead, independent retrievals are carried out at the full horizontal sampling capacity of the instrument. Horizontal resolution is enhanced by a factor 3 in along- and across-track directions compared with the AIRS operational data. The total retrieval error of the individual temperature measurements is 1.6 to 3.0 K in the altitude range from 20 to 60 km. Retrieval noise is 1.4 to 2.1 K in the same vertical range. Contribution of a priori information to the retrieval results is less than 1% to 2% and the vertical resolution of the observations is about 7 to 15 km. The temperature measurements are successfully compared with ECMWF operational analyses and AIRS operational Level 2 data. The new temperature data set is well suited for studies of stratospheric gravity waves. We present AIRS observations of small-scale gravity waves induced by deep convection near Darwin, Australia, in January 2003. A strong mountain wave event over the Andes in June 2005 is analyzed in detail. Temperature perturbations derived from the new data set are compared with results from the AIRS operational Level 2 data and coincident measurements of the High Resolution Dynamics Limb Sounder (HIRDLS). The new retrieval does not show response to wave perturbations if the vertical wavelength is below 10 km. For 15 km vertical wavelength, the amplitudes are damped by a factor of two. For vertical wavelengths of greater than 20 km, AIRS shows very similar wave structure to HIRDLS and also has the advantage of providing horizontal phase front information. Data from the new full-resolution retrieval are far more suitable for gravity wave studies than results from the AIRS operational analysis.