Seasonal–latitudinal variations of PMC particle size from SME measurements for the northern 1983 season and SNOE measurements for the northern 2000 and southern 2000/2001 seasons

The ultraviolet spectrometers (UVS) on the solar mesosphere explorer (SME) and student nitric oxide explorer (SNOE) measured scattered limb radiance at small and large scattering angles from polar mesospheric clouds (PMCs). The SME data are for the northern summer hemisphere (NH) in 1983 and 1984. The SNOE measurements are for the NH in 2000 and for the southern hemisphere (SH) in the 2000/2001 season. From this database, we deduce the modal particle size from the measured scattering angle asymmetry in radiance. This quantity is determined as a function of time within the PMC season, and latitude, assuming several scattering models depending on the adopted size distribution and particle shape. For assumed spherical particles with a Gaussian distribution of width of 14 nm, the results for SME show mode particle sizes that vary from about 35 to 60 nm throughout the season. The results for SNOE under the same assumption show that for high latitudes in the NH the particle size grows systematically from the seasonal onset, from about 25 nm to a maximum of about 45 nm at 30 days after solstice. Lower latitudes show a similar time dependence, but with smaller particle sizes. SH PMC particle sizes display a more complicated seasonal variability. Generally, variability in measured cloud height is anti-correlated with particle size for the seasons analyzed here. Particle sizes in the SH are generally smaller than those in the NH, consistent with the northern bias in PMC brightness, and with previous satellite studies. These results are interpreted in terms of our understanding of PMC microphysics and inter-hemispheric differences in temperature and dynamics. Our quantitative results for mode radius depend on the assumption of a constant distribution width. If the width varies with latitude or time, our calculated gradients of mode radius would be different.