Ice crystal concentration in midlatitude cirrus clouds: In situ measurements with the balloonborne hydrometeor videosonde (HYVIS)

Median ice crystal concentrations were typically several tens per liter regardless of temperature or their vertical location. While the concentrations were sometimes the highest near the cloud top, some clouds had their maximum concentration near the cloud base. As ice particles near the cloud base were usually in sublimation zones, it is suggested that crystal breakup through the sublimation process enhanced the concentrations in some cases.  There was a large difference between the measured concentrations and simulated ones in earlier modeling studies of cirrus cloud formation that treated the ice crystal generation process through homogeneous ice nucleation of aqueous solution droplets, although the measured ones are probably affected by other physical processes such as secondary ice formation and gravitational sedimentation and turbulent mixing of ice particles after the initial cloud formation.

Furthermore, a strong temperature dependence expected from heterogeneous ice nucleation formulas at relatively warm temperatures (> −25°C) was not found over all temperature ranges. Some implications for ice nucleation mechanisms in cirrus clouds in comparison with recent modeling studies involving heterogeneous ice nucleation at temperatures below −40°C are briefly discussed.