The Conical-scanning Microwave Imager/Sounder (CMIS) is required to produce temperature profiles from the surface to the mesopause, operating on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) platforms. For sounding within the mesosphere, the skill at retrieval of temperature profiles depends on the polarization at which measurements are made. The polarization dependence arises from the Zeeman effect in microwave radiative transfer, which is a function of the magnitude of the geomagnetic field and its orientation with respect to the satellite view path. A forerunner to CMIS (SSMIS)will measure in linear polarization. It has been suggested by Rosenkranz and others that circular polarization is preferable for temperature sounding.

This paper examines the polarization issue from the perspective of global climatology. A conical-scanning orbit simulator has been used to document the frequency of occurrence for each geomagnetic condition (field strength and view orientation). For conditions that occur with significant frequency, temperature retrievals were simulated for measurements at circular, vertical, and horizontal polarization. Within this global range of conditions, there are conditions where circular polarization performs better than vertical or horizontal polarization, but there are also conditions where vertical or horizontal polarizations perform best. The relative performances are evaluated with regard to changes in the locations and shapes of weighting functions and the physics that gives rise to those changes. With respect to overall global performance, circular polarization was found to be the best choice in the lower and middle mesosphere. Circular and vertical polarizations performed about the same in the upper mesosphere. Horizontal polarization performed worse than either circular or vertical polarization throughout the mesosphere, in the global average.