Abstract
Abstract. Based on the outcome of laboratory results, new particle-dependent parameterizations of heterogeneous freezing were derived and used to improve and extend a two-dimensional spectral microphysics scheme. They include (1) a particle-type-dependent parameterization of immersion freezing using the numbers of active sites per mass, (2) a particle-type and size-resolved parameterization of contact freezing, and (3) a particle-type-dependent description of deposition freezing. The modified microphysical scheme was embedded in an adiabatic air parcel model with entrainment. Sensitivity studies were performed to simulate convective situations and to investigate the impact of ice nuclei concentrations and types on ice formation. As a central diagnostic parameter, the ice water fraction (IWF) was selected, which is the relation of the ice water content to the total amount of water in the condensed form. The following parameters were varied: initial aerosol particle number size distributions, types of ice nucleating particles, final temperature, and the fractions of potential ice nucleating particles. Single and coupled freezing processes were investigated. The results show that immersion freezing seems to be the most efficient process. Contact freezing is constrained by the collision kernel between supercooled drops and potential ice nucleating particles. The importance of deposition freezing lies in secondary ice formation; i.e., small ice particles produced by deposition nucleation trigger the freezing of supercooled drops by collisions. Thus, a broader ice particle spectrum is generated than that by immersion and contact freezing. During coupled immersion–contact and contact–deposition freezing no competition was observed, and both processes contribute to cloud ice formation but do not impede each other. As already suggested in the literature, mineral dust particles seem to be the most important ice nucleating particles. Biological particles are probably not involved in significant ice formation. The sensitive parameters affecting cloud properties are temperature, aerosol particle composition and concentration, and particle size distribution.
Highlights
The importance of the ice phase in mixed-phase convective clouds is indisputable
To evaluate the efficiency of the different freezing processes and ice nucleating particle types, as a central diagnostic parameter the ice water fraction IWF was selected, which is calculated from the ice water content IWC and the liquid water content LWC: IWC
In this paper improvements and modifications of the spectralbin microphysics embedded in an adiabatic air parcel model with entrainment as described in Diehl et al (2006) are presented
Summary
The importance of the ice phase in mixed-phase convective clouds is indisputable. The additional release of the latent heat of freezing enforces the strength of the convection, and the presence of ice particles in the cloud substantially modifies the dynamical structure and the amount of precipitation (e.g., Gilmore et al, 2004). Other parameterizations were related directly to different particle types, and model studies showed that certain aerosol types significantly alter cloud microphysics (Diehl et al, 2006; Lohmann and Diehl, 2006; Phillips et al, 2008; Hoose et al, 2008; Storelvmo et al, 2008; Lee et al, 2009) They allow one to simulate the effects of particular aerosols such as biomass burning particles (Diehl et al, 2007), biological particles (Phillips et al, 2009), bacteria (Diehl and Wurzler, 2010), or mineral dust (DeMott et al, 2015; Hande et al, 2015). One of the models employed during INUIT is an adiabatic air parcel model with entrainment and a detailed sectional description of the cloud microphysics It describes immersion and contact freezing for various ice nuclei types such as mineral dust, soot, and biological particles (Diehl and Wurzler, 2004; Diehl et al, 2006). The effects of various ice nucleating particles were compared to each other, considering the different freezing modes, to estimate their importance
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