Hemispheric differences in Polar Mesospheric Cloud morphology observed by the Student Nitric Oxide Explorer

The limb-scanning ultraviolet spectrometer on the Student Nitric Oxide Explorer (SNOE) observed Polar Mesospheric Clouds (PMCs) at 215 and 237 nm from 1998 to 2003. The altitude and brightness relative to the background atmosphere were determined for each detected cloud. In the nominal mode of operations, SNOE observed forward scattered radiance in the southern hemisphere and backward scattered radiance in the northern hemisphere. This geometry and the strong asymmetry in the Mie scattering phase function makes southern clouds appear to be much brighter in the south relative to the north. During the northern 2000 PMC season and the southern 2000/2001 season, SNOE was operated in a special mode that permitted observation of both forward and backward scattered radiance over the entire polar region throughout the season. Observing the two hemispheres in identical geometries allows for a comparison of the spatial and temporal morphology of PMCs during those two seasons that is not limited by observational differences, either in scattering angle or local time. SNOE observed more than 32,000 clouds in the Northern Hemisphere (NH) in 2000 and more than 15,800 clouds in the Southern Hemisphere (SH) in 2000/2001. This data set offers the following conclusions. There were significantly fewer PMCs observed in the south relative to the north, by nearly a factor of two, for the seasons observed. The frequency of occurrence of PMC was larger in the north with greater equatorward extent. Using a statistical description of the distribution of cloud brightness, we show that a larger population of bright clouds exist in the north relative to the south. In agreement with previous studies, we show that the northern clouds are on average 1.1 km lower in altitude than those of the south. The southern hemisphere clouds exhibit larger variability in frequency of occurrence than the northern hemisphere clouds.