The effects of surface kinetics on crystal growth and homogeneous freezing in parcel simulations of cirrus

Abstract

The uptake of water vapor excess by ice crystals is a key process regulating the supersaturation in cold clouds. Both the ice crystal number concentration and depositional growth rate control the vapor uptake rate and are sensitive to the deposition coefficient αd. The deposition coefficient depends on temperature and supersaturation; however, cloud models either ignore or assume a constant αd. In this study, the effects of αd on crystal growth and homogeneous freezing of haze solution drops in simulated cirrus are examined. A Lagrangian parcel model is used with a new ice growth model that predicts the deposition coefficients along two crystal growth axes. Parcel model results indicate that predicting αd can be critical for predicting ice nucleation and supersaturation at different stages of cloud development. At cloud base, model results show that surface kinetics constrain the homogeneous freezing rate primarily through the growth impact of small particle sizes in comparison to the mean free path. The deposition coefficient has little effect on homogeneous freezing rates, because the high cloud-base supersaturation produces αd near unity. Above the cloud-base nucleation zone, decreasing supersaturation causes αd to decrease to values as low as 0.001. These low values of αd lead to higher steady-state supersaturation. Also, the low values of αd produce substantial impacts on particle shape evolution and particle size, both of which are dependent on updraft strength.