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Seasonal and latitudinal changes in atmospheric conditions favouring the formation of polar mesospheric clouds

A zonally-averaged dynamical model of the middle atmosphere is used to investigate the seasonal and latitudinal changes of atmospheric conditions at mesospheric heights during polar summer with respect to the possibility of mesospheric cloud formation. It can be shown that supersaturation occurs poleward of 65 degrees latitude at the cold summer mesopause from the end of May till the beginning of August. This is essentially consistent with the temporal behaviour seen in ground-based observations of Noctilucent clouds and the polar scattering layer observed with the uv-spectrometer onboard the Solar Mesosphere Explorer (SME). The rapid changes of atmospheric background conditions during the onset and the end of the observed cloud season may explain the sudden appearance/disappearance of the clouds, which is observed to be similar from year to year. The growth rates of an idealized spherical ice crystal are calculated using the model's temperature and water vapor mixing ratio. Growth rates are of the order of some nm/h, decreasing from pole to lower latitudes. A simple calculation of an ice crystal growing due to sublimation of water vapor in the supersaturated air at the mesopause gives a radius of about 100 nm at the pole.