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

This studyreports on the concentration of ice crystals measured in midlatitude cirrus clouds bya balloonborne hydrometeor videosonde (HYVIS), which has the advantage ofmeasuring small ice crystals in the size range of 10-100 μm more reliably. The cirrus clouds were generallyassociated with warm or stationaryfronts 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 bythe HYVIS with an uncertaintyfactor of 2-3, although significant uncertainties are still included in the size range 10-30 μm. The reliabilityof the measured concentrations is supported bythe observed size spectra of the dataset in this studyand 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 usuallyin 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 probablyaffected byother physical processes such as secondaryice 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 brieflydiscussed.