Abstract
Abstract. Several different types of parameterization of heterogeneous ice nucleation for cloud and climate models have been developed over the past decades, ranging from empirically-derived expressions to parameterizations of ice crystal nucleation rates derived from theory, including the parameterization developed by the authors that includes simultaneous dependence on the temperature and saturation ratio, hereafter referred to as KC. Parameterizations schemes that address the deliquescence-heterogeneous-freezing (DHetF), which combines the modes of condensation freezing and immersion freezing, are assessed here in the context of thermodynamic constraints, laboratory measurements, and recent field measurements. It is shown that empirical schemes depending only on the ice saturation ratio or only on temperature can produce reasonable crystal concentrations, but ice crystal nucleation is thermodynamically prohibited in certain regions of the temperature-saturation ratio phase space. Some recent empirical parameterizations yield clouds that are almost entire liquid at temperatures as low as −35 °C in contrast to cloud climatology. Reasonable performance of the KC ice nucleation scheme is demonstrated by comparison with numerous data from several recent field campaigns, laboratory data, climatology of cloud phase-state. Several mis-applications of the KC parameterization that appeared recently in the literature are described and corrected. It is emphasized here that a correct application of the KC scheme requires integration of the individual nucleation rates over the measured size spectrum of ice nuclei that represent a fraction or several fractions of the environmental aerosol with specific ice nucleation properties. The concentration in these fractions can be substantially smaller than that of the total aerosol, but greater than the crystal concentration measured by an experimental device. Simulations with temperature-dependent active site area or with several IN fractions having different properties show that ice nucleation in the KC scheme occurs in a wide temperature range of 10–20 °C, which depends on IN properties. Simulation with a spectral bin model and correct application of KC scheme adequately describes ice nucleation via the DHetF mode and yields crystal concentrations and phase state close to those measured in the single-layer stratocumulus cloud observed in the Mixed Phase Arctic Cloud Experiment (MPACE). An assessment of some deficiencies in current parcel modeling methods and cloud chamber observations and their impact on parameterization development and evaluation is provided.
Highlights
Ice formation in atmospheric clouds influences the cloud life cycle, precipitation processes, and cloud radiative properties
The authors of this paper have developed a theory of heterogeneous ice nucleation by deliquescence-heterogeneousfreezing, DHetF (Khvorostyanov and Curry, 2000, 2004a, b, 2005, 2009, hereafter referred to as the KC scheme)
Some empirical parameterizations of heterogeneous ice nucleation for cloud and climate models were analyzed and compared with the theoretical scheme developed by the authors (KC scheme) and based on a modification of the classical nucleation theory for freezing
Summary
Ice formation in atmospheric clouds influences the cloud life cycle, precipitation processes, and cloud radiative properties. The utility of classical nucleation theory for parameterization of heterogeneous ice nucleation via solution freezing was limited until recently by the lack of any dependence on supersaturation of the critical radius rcr and energy Fcr of ice germs and nucleation rates of freezing process as formulated by Thomson (1888), with temperature dependence only (PK97, eq 9– 38). Khvorostyanov and Curry (2000, 2004a, b, 2005, 2009, hereafter KC00, KC04a, b, KC05, KC09, respectively) extended classical nucleation theory for heterogeneous freezing and derived equations for the critical radius and energy that included dependencies on both T and water saturation ratio Sw = ρv/ρsw simultaneously (with ρsw being the vapor density saturated over water), or on water supersaturation sw = (ρv − ρsw)/ρsw = Sw − 1, generalizing the previous expressions derived for homogeneous ice nucleation theory by Khvorostyanov and Sassen (1998).
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