FREEZING OF POLAR STRATOSPHERIC CLOUDS IN OROGRAPHICALLY INDUCED STRONG WARMING EVENTS

Results from laboratory experiments and microphysical modeling are pre sented that suggest a potential freezing nucleation mechanism for pola r stratospheric cloud (PSC) particles above the water ice frost point (T-ice). The mechanism requires very high HNO3 concentrations of about 58 wt% in the droplets, and leads to the freezing of nitric acid dihy drate (NAD) in a highly selective manner in the smallest droplets of a n ensemble. In the stratosphere such liquid compositions are predicted to occur in aerosol droplets which are warmed adiabatically with rate s of about +150 K/h from below 190 K to 194 K. Such rapid temperature changes have been observed in mountain leewaves that occur frequently in the stratosphere, clearly demonstrating the need for a stratospheri c gravity wave climatology.