Mars Climate Sounder Retrievals with Two-dimensional Radiative Transfer: Implications for the Temperature Structure in the Winter Polar Region

The Mars Climate Sounder (MCS) is a mid- and far-infrared thermal emission radiometer on board the Mars Reconnaissance Orbiter (MRO). It measures radiances in limb and on-planet viewing geometries. From these radiance measurements, profiles of atmospheric temperature, dust and water ice are operationally retrieved from the surface to ~80 km altitude with a vertical resolution of ~5 km. While limb geometry provides superior vertical resolution over sounding in nadir geometry, it leads to significant horizontal averaging along the line-of-sight. This can lead to misrepresentations in the retrieved quantities in the presence of significant horizontal gradients of these quantities, if the retrieval assumes spherical symmetry. In MCS retrievals, this effect is particularly important in the polar winter regions due to the strong latitudinal gradients in atmospheric temperature. It leads to an apparent shift of the cold pole of the polar vortex away from the viewing direction of the instrument. The assumption of spherical symmetry can lead to misrepresentations of several K in temperature at a given latitude. Here we present an approach to consider these horizontal gradients by applying a two-dimensional radiative transfer scheme to the MCS retrieval. In a first pass a retrieval with the assumption of spherical symmetry is performed. From these retrieval results, horizontal gradients in temperature, pressure, dust and water ice are determined for all measurements along an MRO orbit. These gradient fields are then imposed on a second pass of the retrieval using a two-dimensional radiative transfer scheme. We show that the approach reduces misrepresentations in the retrieved temperature to typically less than 1-2 K in the wall of the polar vortex. Application of this approach to the operational MCS retrieval will lead to a significant improvement in the quality of the retrieved parameters, in particular of temperature in the winter polar regions, which have emerged as a focus of recent scientific interest.