Effects of Parameterizations of Water Activity and Saturation Vapor Pressure on Homogeneous Ice Nucleation

Abstract:

Homogeneous nucleation of ice crystals, i.e. the freezing of small supercooled solution particles, represents a major pathway in the formation of cirrus clouds with high ice water content at low temperatures. A reasonable physical explanation of this type of freezing is provided by Koop's nucleation theory. This theory relates the homogeneous nucleation rate to the water activity of the solution particles. While the homogeneous nucleation rate encodes the probability of freezing of a solution particles, the water activity represents the ratio of saturation vapor pressures of water over the solution to pure water. By using the detailed ice microphysics models "CLaMS-Ice" and "MAID", we investigate the effect of various formulations of parameterizations of the water activity and the water saturation vapor pressure on the resulting cirrus cloud. Both models implement a comparatively detailed ice microphysics, although the MAID model may be considered as the more detailed and accurate model since it tracks the partitioning of the chemical species while the cloud forms. The CLaMS-Ice model implements the two-moment ice microphysics scheme by Spichtinger and Gierens, which is more suitable to be implemented in full three dimensional atmospheric models. Instead of exclusively representing the water activity by its definition as a ratio of saturation water vapor pressures, we additionally consider a direct parameterization of water activity based on a thermodynamic model. A combination of this parameterization with a more recent saturation water vapor pressure formulation allows observations of increased freezing thresholds compared with Koop's freezing lines.