Observational Evidence of Deposition Ice Nucleation in Wave Clouds

Airborne observations were obtained on eighteen flights in two wave cloud projects Wave-ICE (2000) and ICE-L (2007). Measurements of cloud particles were made with optical array probes, forward-scattering probes, a Cloud Particle Imager (CPI) and a new Small Ice Detector (SID). Measurements were also made of thermodynamics and kinematics, aerosols size and concentration, CN, CCN and ice nuclei.

The target clouds were mid-troposphere, alto-cumulus standing lenticular clouds in Wyoming and Colorado. The clouds selected for this study had dry air above them that reduced the chance of ice crystals falling from higher altitude cirrus and contaminating measurements in the lower wave clouds. Sampling temperatures in the clouds and in the updraft regions near cloud base ranged from -4 to -40°C. Typical horizontal wind speeds were sufficient (>~20 m/s) for complete exchange of the cloud air between passes of the aircraft. Thus, any contamination of the cloud by the aircraft would be flushed out before the next pass.

This paper focuses on detecting ice particles in the updraft regions below water cloud base, where deposition is the primary ice nucleation mode. The concentration and size of ice particles in this region will be compared with in-cloud measurements, where other ice nucleation mechanisms are active. Also, measurements of the aerosol deposition ice nuclei activity will be compared with the observed cloud particles.