A Parameterization of Cirrus Cloud Formation: Revisiting Competing Ice Nucleation

• Published in: Journal of geophysical research. Atmospheres 2022-09, Vol.127 (18), p.n/a
• Main Author: Kärcher, B.
• Format: Article
• Language: English
• Subjects: Approximation; Atmospheric aerosols; Atmospheric models; Atmospheric particulates; Aviation; cirrus; Cirrus clouds; Cloud formation; Clouds; Computational efficiency; Computer applications; Crystal growth; Crystals; Droplets; Dust; Freezing; Geophysics; Ice; Ice crystal formation; Ice crystals; Ice formation; Ice nucleation; Kinetics; Mathematical models; Nucleation; Numerical integration; Numerical simulations; Parameterization; Soot; Soot particles; Statistical methods; Statistics; Stochasticity; Supersaturation; Updraft; Uptake; Vertical wind velocities; Water vapor; Water vapour; Wind; Wind speed
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2022JD036907

This study develops an advanced physically‐based parameterization of heterogeneous ice nucleation in cirrus clouds that includes an updated parameterization of stochastic homogeneous freezing of supercooled solution droplets. Both components are formulated based on the same methodology and level of approximation, without numerical integration of the underlying ice supersaturation equation. The new scheme includes measured ice nucleation spectra describing deterministic ice activation from an arbitrary number of types of ice‐nucleating particles (INPs), tracks the competition for available water vapor between the different ice nucleation modes, and allows for new ice formation and growth within pre‐existing cirrus clouds. The computationally efficient scheme works with a minimal set of physical input parameters and predicts total nucleated ice crystal number concentrations (ICNCs) along with the maximum ice supersaturation attained during cirrus formation events. Aspects of its implementation into host models are discussed, including the provision of suitably parameterized vertical wind speeds. The parameterization is validated by comparisons to numerical simulations. First off‐line applications to mineral dust and aviation soot particles are presented, including ICNC ensemble statistics resulting from the coupling with statistics of updraft speed variability. Plain Language Summary Two decades after introduction of the first parameterization of cirrus cloud formation by freezing of ubiquitous liquid solution droplets, an improved version is developed based on the latest experimental findings regarding solid ice‐nucleating particles, a small subset of the atmospheric aerosol. The new scheme allows to predict ice crystal formation in cirrus from competing homogeneous freezing and heterogeneous ice activation more realistically and with greater computational efficiency. It considers new developments regarding the properties of vertical wind speeds (triggering ice formation) and the molecular kinetics of water vapor uptake onto ice crystals (controlling ice growth). This study explains the foundation of cirrus ice formation and growth based on cloud physical theory, derives and explains the parameterization, discusses its use in host models to facilitate applications, checks its performance by comparison to comprehensive numerical simulations, and presents first results involving mineral dust and aircraft‐emitted soot particles as examples for good and poor at