Non-linear profile retrievals for observations in the lower stratosphere with the Odin sub-mm radiometer

The planned Odin satellite mission will include limb sounding observations of the atmosphere in the range 480-580 GHz. In the present study an introductory study of the possibilities to apply non-linear methods for the profile retrievals is presented. As a first step the limitations of linear retrievals are examined. For some species an iterative solution gives clear improvements while for other tested cases, the limitation appears to be of more fundamental nature. Among the implemented iterative schemes, the Marquardt-Levenberg method shows the best performance. Non-linear solutions could also be needed if spectra from different frequency bands are inverted simultaneously. For this, spectroscopic uncertainties become very relevant and they have to be handled satisfactorily to get best possible results.