QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 133: (S3) 217–232 (2007) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.162 Demonstration and validation of the ϕ -IASI inversion scheme with NAST-I data G. Grieco, a G. Masiello, a M. Matricardi, b C. Serio, a * D. Summa a and V. Cuomo c a DIFA, University of Basilicata, Italy b ECMWF, Reading, UK c IMAA, National Research Council, Italy ABSTRACT: The physical forward/inverse scheme ϕ-IASI has been applied to the retrieval of atmospheric vertical profiles of temperature, water vapour and ozone by inverting high spectral infrared observations recorded by the NAST-I Fourier Transform Spectrometer. For the retrieval exercise we have used clear-sky spectra recorded over the sea surface on the night of 9–10 September 2004, during the Italian phase of the EAQUATE field campaign. During the campaign NAST-I flew on board the NASA Proteus aircraft at an altitude of 15 km. Retrievals have been compared to radiosonde observations, profiles from the ECMWF analysis and AIRS level 2 products. While providing a basis for the validation of the ϕ-IASI inversion scheme, the retrieval exercise has also demonstrated (i) the NAST-I ability to accurately detect H 2 O fine-scale vertical features, and (ii) the good quality of the temperature and water vapour profiles from the ECMWF analysis. Copyright  2007 Royal Meteorological Society KEY WORDS EAQUATE; hyperspectral infrared data; vertical profiles; radiative transfer Received 3 November 2006; Revised 1 August 2007; Accepted 7 August 2007 1. Introduction New-generation meteorological satellites carry infrared sensors able to sense the Earth emission spectrum at very high spectral resolution. High-resolution infrared sen- sors on operational meteorological polar orbiters include the Infrared Atmospheric Sounding Interferometer (IASI) on the European Meteorological Operational Satellite (METOP) launched in October 2006 and the Cross-track Infrared Sounder (CrIS) on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) preparatory project (NPP) satellite. High-resolution radi- ance data are also available for instruments that are part of a National Aeronautics and Space Administration (NASA) research mission, i.e. the Atmospheric Infrared Sounder (AIRS) on the second Earth Observing System (EOS) polar-orbiting platform, EOS-Aqua (launched in April 2002), and the Tropospheric Emission Spectrome- ter (TES) on the AURA satellite (launched in 2004). All these sensors are characterized by a wide-band spectral coverage (3.7 to 15.5 μm) and a spectral sampling rate in the range 0.25 to 2 cm −1 . The high spectral resolution of new advanced infrared sensors should provide temperature and constituent pro- files at a higher accuracy and with more vertical resolu- tion than the existing filter wheel radiometers. However, * Correspondence to: C. Serio, Department of Environmental Engineer- ing and Physics, DIFA, University of Basilicata, Via Ateneo Lucano 10, Potenza 85100, Italy. E-mail: carmine.serio@unibas.it the full exploitation of these new sensors requires more accurate radiative transfer calculations and, in general, new data processing algorithms which, in turn, need to be validated. The need to create a database that could be used to check and validate new retrieval algorithms and data products has led to the European Aqua Thermodynamic Experiment (EAQUATE) campaign. Data collected dur- ing the EAQUATE campaign have been used to check the retrieval product quality and the accuracy of the ϕ- IASI package (e.g. Amato et al., 2002; Carissimo et al., 2005), a physically based methodology for the retrieval of geophysical parameters (temperature, water vapour and ozone) from highly spectrally resolved infrared radi- ance. The ϕ-IASI package is mainly intended for IASI. Nevertheless, with minor modifications, the code can be adapted to any satellite-borne and aircraft-borne nadir- viewing infrared sensor with a sampling rate in the range 0.1–2 cm −1 . Given the characteristics of the two instruments, the ϕ-IASI package has been applied to NAST-I data con- sidered as a proxy for IASI data. NAST-I is the NPOESS Aircraft Sounder Testbed Interferometer (Cousins and Gazarick, 1999). The EAQUATE NAST-I data were obtained during the NASA Proteus flight on the night of 9–10 September 2004 with Proteus flying along the Aqua satellite track at a cruise altitude of 15–16 km. To date, the capability of ϕ-IASI of producing improved retrievals has been assessed mainly for tropical air masses using data from the Japanese Interferometric Copyright  2007 Royal Meteorological Society