Implementation of a model for the horizontal gradients in the ESA retrieval algorithm for MIPAS

Neglecting the horizontal variability of the atmosphere in the forward model for the simulation of limb emission radiances causes a systematic error in MIPAS retrieved profiles. The horizontal gradient model will be introduced into the Optimized Retrieval Model (ORM) v8, which will be used for the final ESA reprocessing of the whole mission. Several optimizations exploiting the spherical symmetry of the atmosphere can no longer be used. Therefore, both the ray tracing and the radiative transfer integration algorithms have been completely rewritten. We illustrate the choices adopted for the implementation of the horizontal gradient model. We show its performances versus the previous algorithm that assumes the horizontal homogeneity of the atmosphere. Finally we compare our results to those of other retrieval models that take into account the horizontal variability of the atmosphere.