Partitioning the primary ice formation modes in large eddy simulations of mixed-phase clouds

Luke B Hande,Corinna Hoose

doi:10.5194/acp-17-14105-2017

Luke B Hande, Corinna Hoose

Open Access

https://doi.org/10.5194/acp-17-14105-2017

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Abstract

Abstract. State-of-the-art aerosol-dependent parameterisations describing each heterogeneous ice nucleation mode (contact, immersion, and deposition ice nucleation), as well as homogeneous nucleation, were incorporated into a large eddy simulation model. Several cases representing commonly occurring cloud types were simulated in an effort to understand which ice nucleation modes contribute the most to total concentrations of ice crystals. The cases include a completely idealised warm bubble, semi-idealised deep convection, an orographic cloud, and a stratiform case. Despite clear differences in thermodynamic conditions between the cases, the results are remarkably consistent between the different cloud types. In all the investigated cloud types and under normal aerosol conditions, immersion freezing dominates and contact freezing also contributes significantly. At colder temperatures, deposition nucleation plays only a small role, and homogeneous freezing is important. To some extent, the temporal evolution of the cloud determines the dominant freezing mechanism and hence the subsequent microphysical processes. Precipitation is not correlated with any one ice nucleation mode, instead occurring simultaneously when several nucleation modes are active. Furthermore, large variations in the aerosol concentration do affect the dominant ice nucleation mode; however, they have only a minor influence on the precipitation amount.

Highlights

Ice crystals in the atmosphere can form spontaneously through homogeneous nucleation, which becomes increasingly probable at temperatures lower than −35 ◦C (Koop and Murray, 2016)
This paper aims to help clarify, in a systematic way, which ice nucleation modes dominate for various cloud types found over continental regions
A number of high-resolution modelling case studies are presented in order to systematically investigate which ice nucleation modes dominate for a number of typical cloud types

Summary

Introduction

Ice crystals in the atmosphere can form spontaneously through homogeneous nucleation, which becomes increasingly probable at temperatures lower than −35 ◦C (Koop and Murray, 2016). At warmer temperatures an ice nucleating particle (INP) is required to initiate freezing. INPs represent a small fraction of all atmospheric aerosols (Rogers et al, 1998), they have a disproportionately large influence on mixed-phase cloud microphysics (DeMott et al, 2010). Deposition nucleation occurs at cold temperatures, where water vapour is deposited as ice directly onto an aerosol particle. Immersion and condensation freezing require the particle to be immersed in super-cooled liquid water, after which freezing occurs. Contact freezing occurs when an aerosol particle comes into contact with a supercooled droplet, which subsequently initiates freezing. A similar mechanism called inside-out freezing has been identified, where a immersed particle comes into contact with the water–air interface, which initiates freezing (Durant and Shaw, 2005). Recent results from a modelling study support this idea (Hande et al, 2017)

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