Classical nucleation theory of immersion freezing: sensitivity of contact angle schemes to thermodynamic and kinetic parameters

Abstract

Abstract. Heterogeneous ice formation by immersion freezing in mixed-phase clouds can be parameterized in general circulation models (GCMs) by classical nucleation theory (CNT). CNT parameterization schemes describe immersion freezing as a stochastic process, including the properties of insoluble aerosol particles in the droplets. There are different ways to parameterize the properties of aerosol particles (i.e., contact angle schemes), which are compiled and tested in this paper. The goal of this study is to find a parameterization scheme for GCMs to describe immersion freezing with the ability to shift and adjust the slope of the freezing curve compared to homogeneous freezing to match experimental data.We showed in a previous publication that the resulting freezing curves from CNT are very sensitive to unconstrained kinetic and thermodynamic parameters in the case of homogeneous freezing. Here we investigate how sensitive the outcome of a parameter estimation for contact angle schemes from experimental data is to unconstrained kinetic and thermodynamic parameters. We demonstrate that the parameters describing the contact angle schemes can mask the uncertainty in thermodynamic and kinetic parameters.Different CNT formulations are fitted to an extensive immersion freezing dataset consisting of size-selected measurements as a function of temperature and time for different mineral dust types, namely kaolinite, illite, montmorillonite, microcline (K-feldspar), and Arizona test dust. We investigated how accurate different CNT formulations (with estimated fit parameters for different contact angle schemes) reproduce the measured freezing data, especially the time and particle size dependence of the freezing process. The results are compared to a simplified deterministic freezing scheme. In this context, we evaluated which CNT-based parameterization scheme able to represent particle properties is the best choice to describe immersion freezing in a GCM.

Prognostic parameterization of cloud ice with a single category in the aerosol-climate model ECHAM(v6.3.0)-HAM(v2.3)

“…In this study we focus on the pathways subsequent to ice initiation by a more physically-based description of cloud ice but acknowledge the importance of ongoing research to understand freezing mechanisms (Welti et al, 2014;Ickes et al, 2015;Marcolli, 2017) and resulting parameterization development (Phillips et al, 2013;Ickes et al, 2017). We will use the microphysical properties of ice described in Morrison and Milbrandt (2015) (hereafter MM15) and embed them in the ECHAM6-HAM2 microphysics scheme.…”

Global relevance of marine organic aerosols as ice nucleating particles

“…5), a similar difference in freezing occurrence frequency can also be noted between results from the two dust freezing schemes, which point to the parametrisation as the main reason behind the difference. The sharp decrease in freezing occurrence at warmer temperatures following Ickes et al (2017) when compared to the results using the Niemand et al (2012) dust parametrisation indicates a faster decrease of the dust ice activity with increasing temperature for the former set-up, which is shown in Fig. 6.…”

“…6. Indeed, the FF of 0.5 µm radius particles following Ickes et al (2017) quickly drops below that following Niemand et al (2012) at around -31…”

“…Also, anthropogenic emissions are fixed at year 2000 levels in the following simulations and the minimum cloud droplet number concentration (CDNC) is 10 cm . In the base version used in the current study, aerosol species considered include sulphate, dust, black carbon, organic carbon, and sea salt, among which dust is allowed to nucleate ice through immersion freezing, following either Ickes et al (2017) or Niemand et al (2012) as opposed to Lohmann and Diehl (2006) in the default model set-up. No other types of heterogeneous ice nucleation are considered.…”

“…Without sufficiently large dust particles and/or sufficiently high number concentrations, the Ickes et al (2017) CNT parametrisation will thus not lead to much ice nucleation occurrence in the warmer mixed-phase temperatures. Interestingly, a surface active site density approach for montmorillonite (consistent in dust type with the CNT parametrisation), which is also compared in Fig.…”