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

Abstract

The accuracy of trace gas abundances retrieved from spectrally resolved infrated limb emission measurements relies, among other things, on the appropriate modelling of radiative transfer through the actual atmospher. We quantify the mapping of several commonly applied simplifications in radiative transfer modelling on the trace gas abundances retrieval error at the example of the Michelson interferometer for passive atmosphere sounding (MIPAS)/environment satellite (ENVISAT) space borne Fourier transform infra-red limb emission experiment. The Karlsruhe optimized and precise radiative transfer algorithm (KOPRA) which was used as a tool for this study will be introduced. KOPRA supports accurate modelling of the particular intrument requirements of MIPAS and the observation scenarios during the ENVISAT mission, in particualr with reapect to its viewing direction and iots altitude coverage of the atmosphere. We show that disregarding of horizontal temperature inhomogeneities and non-local thermodynamic equilibrium effects, insufficient accuracy in modelling of field-of-view and apodisation effects, and disregarding individual profiles of isotopic species play the key roles in radiative transfer modelling and lead to systematic retrieval errors which can by far exceed the expected random retrieval error caused by measurement noise. © 2001 Elsevier Science Ltd. All rights reserved.