A parameterization of cirrus cloud formation: Heterogeneous freezing

TLDR

The freezing threshold of ice nuclei in mixed particles is largely independent of their size, and near‑saturation freezing would occur for perfect nuclei even in slow updrafts at cirrus altitudes. The study develops a physically based parameterization of cirrus cloud formation by heterogeneous freezing and introduces a novel method to compute associated nucleation rates. The parameterization focuses on immersion freezing, considered the dominant pathway for heterogeneous cirrus formation below 235 K. When only a single, less potent ice nucleus type (threshold >1.3–1.4) initiates cirrus, cloud properties are relatively insensitive to IN characteristics, whereas competition between multiple nucleus types with distinct thresholds can markedly suppress ice crystal concentrations and amplify indirect aerosol effects.

Abstract

A physically based parameterization of cirrus cloud formation by heterogeneous freezing is developed along with a novel method to compute associated nucleation rates. The analysis is restricted to immersion freezing, possibly the dominant pathway for heterogeneous cirrus formation under cold (<235 K) conditions. The size of ice nuclei (IN) immersed in a liquid particle does not significantly affect the heterogeneous freezing threshold (the saturation ratio over ice where ice formation is initiated) of the mixed particle. If perfect IN were present at cirrus altitudes, almost all of them would freeze near ice saturation, even in slow updrafts. If only one type of less potent IN with freezing thresholds >1.3–1.4 triggers cirrus formation, cloud properties are not very susceptible to changes of IN properties, as in the case of homogeneous freezing. In contrast, much stronger indirect aerosol effects on cirrus clouds are possible if at least two types of IN with distinct freezing thresholds compete during the freezing process, most likely leading to a suppression of ice crystal concentrations.