A fast radiative-transfer model for the assimilation of infrared limb radiances from MIPAS

A new fast radiative-transfer model to compute emitted infrared limb radiances for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) has been developed and validated. The model, referred to as RTMIPAS, can simulate apodized radiances for all channels of the high spectral resolution MIPAS instrument in the 685-2000 cm(-1) wave-number region. RTMIPAS is part of a wider effort to develop the capability to assimilate infrared limb radiances into the ECMWF model. The model uses a linear regression scheme to parametrize the effective layer optical depths, and it can simulate the effect of variable water vapour and ozone; for other gases included in the model, a fixed climatological profile is assumed. The development of the model has involved the calculation of an accurate line-by-line transmittance database, the selection of suitable predictors for the gases and the viewing geometry modelled, and the generation of regression coefficients for 43205 MIPAS channels. RTMIPAS can reproduce line-by-line radiances to an accuracy that is below the noise level of the instrument for most spectral points and tangent heights, while offering significantly more rapid radiance calculations compared with currently available radiative-transfer models. The comparison of RTMIPAS transmittances with line-by-line model equivalents indicates that the accuracy of the RTMIPAS transmittance model is comparable to that of similar regression-based radiative-transfer models for nadir-viewing geometry.