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

This study reports on the concentration of ice crystals measured in midlatitude cirrus clouds by a balloonborne hydrometeor videosonde (HYVIS), which has the advantage of measuring small ice crystals in the size range of 10-100 μm more reliably. The cirrus clouds were generally associated with warm or stationary fronts of synoptic-scale lows. The microphysical dataset consisted of 37 launches from Tsukuba, Japan, during the observation period 1994-2007. On the basis of the comparison with concurrent data by other airborne instruments in the laboratory, the ice crystal concentrations can be measured by the HYVIS with an uncertainty factor of 2-3, although significant uncertainties are still included in the size range 10-30 μm. The reliability of the measured concentrations is supported by the observed size spectra of the dataset in this study and the simulated total concentrations of ice particles with a parcel model.  Vertical profiles of size distributions of cirrus cloud ice crystals were obtained for clouds with top temperatures ranging from −33° to −72°C and base temperatures from −3° to −49°C. Ice crystal concentrations varied approximately from the order of 10−1 to 102 L−1. 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.