ICE NUCLEATION PROCESSES IN UPPER-TROPOSPHERIC WAVE-CLOUDS OBSERVED DURING SUCCESS

We have compared in situ measurements near the leading-edges of wave-c louds observed during the SUCCESS experiment with numerical simulation s. Observations of high supersaturations with respect to ice (> 50%) n ear the leading edge of a very cold wave cloud (T < -60 degrees C) are approximately consistent with recent theoretical and laboratory studi es suggesting that large supersaturations are required to homogeneousl y freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4 cm(-3)) are consistent with the numbe r densities calculated in our model. In the warmer wave-cloud (T simil ar or equal to -37 degrees C) relatively large ice number densities we re observed (20-40 cm(-3)). Our model calculations suggest that these large number densities are probably caused by activation of sulfate ae rosols into liquid droplets followed by subsequent homogeneous freezin g. If moderate numbers of effective heterogeneous freezing nuclei (0.5 -1 cm(-3)) had been present in either of these clouds, then the number densities of ice crystals and the peak relative humidities should hav e been lower than the observed values.