Polar mesospheric cloud properties, variability, and microphysics determined from satellite observations

Data from the Halogen Occultation Experiment (HALOE) have been used to study Polar Mesospheric Clouds (PMCs), their spatial and temporal variability, and their relationship to the temperature and water vapor in the mesosphere. HALOE observed the polar summer regions between the fall of 1991 and summer of 2005 providing observations of 14 northern hemisphere (NH) and southern hemisphere (SH) seasons. This research has assisted in the development of a modified data retrieval algorithm (VPMC) that removes the PMC signal from the temperature and water vapor channels, resulting in significantly improved temperature and water vapor measurements. A computer algorithm has been created which identifies all HALOE PMCs. The PMC season is found to start roughly 1 month before, and end ∼2 months after, each summer solstice and correlates with the cold polar summer mesospheric temperatures and the seasonal distribution of water vapor. PMC occurrence frequency and extinction seasonal distributions are similar, but occurrence frequencies peak near the summer solstices while extinction peaks about a month after the solstices. PMC occurrence frequency is twice as high, PMC extinction is ∼30% higher, and PMC altitudes are ∼1 km lower on average, in the NH. Analysis of all temperature profiles (PMC and non-PMC) measured during the high altitude polar summer reveal a colder NH than SH at all altitudes suggesting this as one possible cause for the observed interhemispheric differences in PMC properties. Interhemispheric PMC altitude differences are supported by a lower altitude region of saturation in the NH, and by a lower H2O peak altitude in the NH. HALOE data show a positive correlation between PMC extinction and water vapor at, and slightly below, the peak cloud altitude. PMC extinction appears to be modulated by the cloud temperature with colder temperature supporting larger extinctions, on average. These relationships are confirmed with microphysical simulations using the Community Aerosol and Radiation Model for Atmospheres (CARMA) model. Finally, HALOE PMC extinction data are used to derive PMC median radii values. The distribution of median radii peaks near 50 run and is comprised primarily of values between 30 and 100 nm.