The physical retrieval methodology for IASI: the δ-IASI code

This paper describes a physical-based methodology for the retrieval of geophysical parameters (temperature, water vapor and ozone) from highly resolved infrared radiance, and presents the algorithm which implements the procedure. The algorithm we have implemented is mostly intended for the Infrared Atmospheric Sounding Interferometer which is planned to be flown on the first European Meteorological Operational Satellite (Metop/1) in 2006. Nevertheless, with minor modifications, the code is well suited for any nadir viewing satellite and airborne infrared sensor with a sampling rate in the range of 0.1-2 cm−1. Basically, the implementation of the inverse scheme follows Rodgers' Statistical Regularization method. However, an additional regularization parameter is introduced in the inverse scheme which gains to the algorithm the capability of improving the retrieval accuracy and to constraint the step size of Newton updates in such a way to lead iterates toward the feasible region of the inverse solution. Although, the paper mostly focuses on documenting and discussing the mathematical details of the inverse method, retrieval exercises have been provided, which exemplify the use and potential performance of the method. These retrieval exercises have been performed for the Infrared Atmospheric Sounding Interferometer. In addition, examples of application to real observations have been discussed based on the Interferometric Monitoring Greenhouse (IMG) gases Fourier Transform Spectrometer which has flown on the Japanese Advanced Earth Observation Satellite.