Ice generation in wave clouds: Observation, analysis and parameterization evaluation

Heterogeneous ice nucleation affects cloud radiative forcing and precipitation generation. It is a major source of uncertainties in models. In order to understand the formation and growth of ice particles, this study utilizes measurements of wave clouds obtained during the Wyoming Airborne Integrated Cloud Observation (WAICO) experiments in 2008 and 2009. Eighty eight aircraft penetrations of clouds at temperatures between -34 °C and -17 °C are analyzed. An aerosol-droplet closure study has been carried out to link aerosol measurements with ice crystal concentration. It demonstrates good agreement between activated aerosol concentration and droplet concentration. The simple dynamical structure of wave clouds allows streamlines to be derived and combined with flight level in situ measurements to provide parcel history and related properties. Ice crystal concentrations are observed to continually increase along the streamlines until RHi<1 is reached on the downwind terminus of the wave cloud. Droplet concentrations remain constant within the mixed-phase region. Both single and multiple wave cloud penetrations show that ice crystal concentrations increase with increasing mixed-phase growth time. The data set demonstrates that in wave clouds there is a statistically strong time dependency of ice generation, which is also characterized empirically with parcel mixed-phase growth time. With the addition of parcel mixed-phase growth time, it is further demonstrated that variations of ice crystal concentrations in wave clouds correlate well with cloud temperature and large size aerosol concentrations. This result has an important implication for ice concentration parameterization in numerical models.