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Derivation of temperature and pressure from submillimetric limb observations.

We demonstrate the feasibility of retrieval of temperature and pressure profiles from spaceborne limb-sounding spectroscopic measurements in the submillimetric region. Whether these profiles can be retrieved determines whether a submillimetric instrument can be self-sufficient as far as the atmospheric model is concerned or whether complementary sensors in other spectral regions, dedicated to temperature and pressure measurements, are needed. Molecular-oxygen transitions are selected for the purpose of temperature and pressure retrieval on the basis of realistic observational parameters. We use a mathematical model of the retrieval process to evaluate the information content of the spectral features and to study the trade-off between the uncertainty of the retrieved profiles and their vertical resolution. It is shown that, using only one oxygen transition and without any constraint, one can achieve uncertainties of 5% for both temperature and pressure from 10 to approximately 40 km of altitude with a vertical resolution of 3 km; above 40 km the vertical resolution needs to be degraded to limit the uncertainties. The possibility of exploiting a priori information is discussed, as well as the effects of external constraints that can be used to improve the quality of the retrieved profiles. The sources of systematic error that need to be considered for the compilation of the total error budget are also evaluated.