Sensitivity of trace gas abundances retrievals from infrared limb emissionspectra to simplifying approximations in radiative transfer modelling

The accuracy of trace gas abundances retrieved from spectrally resolved inf rared limb emission measurements relies, among other things. on the appropr iate modelling of radiative transfer through the actual atmosphere. We quan tify the mapping of several commonly applied simplifications in radiative t ransfer modelling on the trace gas abundances retrieval error at the exampl e of the Michelson interferometer for passive atmospheric sounding (MIPAS)/ environmental satellite (ENVISAT) space borne Fourier transform infra-red l imb emission experiment. The Karlsruhe optimized and precise radiative tran sfer algorithm (KOPRA) which was used as a tool for this study will be intr oduced. KOPRA supports accurate modelling of the particular instrument requ irements of MIPAS and the observation scenarios during the ENVISAT mission, in particular with respect to its viewing direction and its altitude cover age of the atmosphere. We show that disregarding of horizontal temperature inhomogeneities and non-local thermodynamic equilibrium effects, insufficie nt accuracy in modelling of field-of-view and apodisation effects, and disr egarding individual profiles of isotopic species play the key roles in radi ative transfer modelling and lead to systematic retrieval errors which can by far exceed the expected random retrieval error caused by measurement noi se. (C) 2001 Elsevier Science Ltd. All rights reserved.