Secondary Ice Production in Cold Clouds: Theoretical concept, Observations and Modelling

While clouds are a crucial component of the global climate system, their representation in climate models is subject to considerable uncertainty. Predicting cloud phase, the amount of liquid and ice in the cloud, is critical for the correct representation of radiation processes and precipitation patterns. Yet, much of the uncertainty stems from the cloud ice phase, as ice formation processes are poorly understood and largely not described in the case of secondary ice production (SIP). In this session, we invite contributions from several fields (theoretical physics, laboratory experiments, field observations, remote sensing and multiscale modeling) that aim to: (i) understand secondary ice mechanisms, (ii) identify the conditions favorable for SIP, (iii) quantify SIP in relation to primary ice production, (iv) explore the impact of SIP on cloud life-cycle, radiation and/or precipitation, (v) investigate the interactions between SIP and atmospheric electricity, (vi) evaluate SIP representation in models and (vii) improve relevant parameterizations. Studies that link the importance of SIP within the context of deep convective systems, ground-cloud interactions (e.g. blowing snow), mountain, polar and marine systems are strongly encouraged.

Convener: Georgia SotiropoulouECSECS | Co-conveners: Paul DeMott, Corinna Hoose, Athanasios Nenes, Vaughan Phillips
vPICO presentations
| Thu, 29 Apr, 11:45–12:30 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Georgia Sotiropoulou, Vaughan Phillips, Corinna Hoose
Tom Choularton, Gary Lloyd, Keith Bower, and Martin Gallagher

Ground based and airborne observations of ice crystal concentrations are often found to exceed the concentration of ice nucleating particles by many orders of magnitude. This discrepancy between the expected ice particle concentrations formed through primary ice nucleation and observed ice particle concentration has led to the search for missing physical processes capable of creating new ice crystals. Secondary ice production (SIP) is a mechanism that produces new ice crystals without requiring the action of an ice nucleating particle. Evidence has now been found for several of these

Increasingly sophisticated cloud microphysical representations are being used in Numerical Weather Prediction and climate models to provide more realistic simulations of clouds. This drive towards greater complexity is motivated by the recognition of the importance of microphysical processes to the evolution of clouds, precipitation and the atmospheric environment.  

One important challenge for the successful implementation of cloud microphysics is the prediction of ice crystal concentrations, these influence the water budget of the cloud s through precipitation processes and the radiative properties of clouds especially when the ice crystals are in the majority over water droplets. The understanding and quantification of primary ice nucleation has grown in recent years, secondary ice production processes have received relatively little attention but are potentially very important for controlling the ice concentrations found in some types of clouds. 

In this stalk a number of SIP mechanisms will be discussed: The Hallett-Mossop process, by far the most powerful mechanism when conditions are right; the fracture on freezing of supercooled raindrops, the fragmentation of falling snow flakes; the detachment of frost crystals from a surface.

How to cite: Choularton, T., Lloyd, G., Bower, K., and Gallagher, M.: An Overview of Secondary ice Processes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7278,, 2021.

Tim Lüttmer and Peter Spichtinger

Several physical mechanisms of secondary ice production are proposed and studied in laboratory experiments and observational measurements. We implemented a selection of empirical parameterisations for rime splintering, frozen droplet fragmentation and ice-ice collisional break-up in the two-moment microphysics ice modes scheme within the atmosphere model ICON.

The newly developed ice modes scheme distinguishes between different ice modes of origin including homogeneous nucleation, deposition freezing, immersion freezing, homogeneous freezing of water droplets and secondary ice production respectively. Each ice mode is described by its own size distribution, prognostic moments and unique formation mechanism while still interacting with all other ice modes and microphysical classes
like cloud droplets, rain and rimed cloud particles. This allows to evaluate the contribution of each ice formation mechanism, especially
secondary ice, to the total ice content.

Using this set-up we investigated the sensitivity and behavior of rime splintering, frozen droplet fragmentation and ice-ice collisional break-up for various parameterisations, coefficients and environmental conditions. We will present findings from idealized convection simulations as well as synoptic simulations of Europe and the North Atlantic.

How to cite: Lüttmer, T. and Spichtinger, P.: A microphysical scheme for secondary ice in ICON – evaluation and case studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2934,, 2021.

Paul Connolly, Rachel James, and Vaughan Phillips

This work presents new laboratory data investigating collisions between supercooled drops and ice particles as a source of secondary ice particles in natural clouds. Furthermore we present numerical model simulations to put the laboratory measurements into context.

Secondary ice particles form during the breakup of freezing drops due to so-called “spherical freezing” (or Mode 1), where an ice shell forms around the freezing drop. This process has been studied and observed for drops in free-fall in laboratory experiments since the 1960s, and also more recently by Lauber et al. (2018) with a high-speed camera. Aircraft field measurements (Lawson et al. 2015) and lab data (Kolomeychuk et al. 1975) suggest that such a process is dependent on the size of drops, with larger drops being more effective at producing secondary ice.  Collision induced break-up of rain drops has been well studied with pioneering investigations in the mid-1980s, and numerous modelling studies showing that it is responsible for observed trimodal rain drop size distributions in the atmosphere, which can be well approximated by an exponential distribution.


In mixed-phase clouds we know that rain-drops can collide with more massive ice particles. This, depending on the type of collision, may lead to the break-up of the supercooled drop (e.g. as hinted by Latham and Warwicker, 1980), potentially stimulating secondary ice formation (Phillips et al. 2018 - non-spherical, Mode 2).  There is a dearth of laboratory data investigating this mechanism.  This mechanism is the focus of the presentation.

Here we present the results of recent experiments where we make use of the University of Manchester (UoM) cold room facility. The UoM cold room facility consists of 3 stacked cold rooms that can be cooled to temperatures below -55 degC. A new facility has been built to study secondary ice production via Mode 2 fragmentation. We generate supercooled drops at the top of the cold rooms and allow them to interact with different ice surfaces near the bottom. This interaction is filmed with a new camera setup.

Our latest results will be presented at the conference.


Kolomeychuk, R. J., D. C. McKay, and J. V. Iribarne. 1975. “The Fragmentation and Electrification of Freezing Drops.” Journal of the Atmospheric Sciences 32 (5): 974–79.<0974>2.0.CO;2.

Latham, J., and R. Warwicker. 1980. “Charge Transfer Accompanying the Splashing of Supercooled Raindrops on Hailstones.” Quarterly Journal of the Royal Meteorological Society 106 (449): 559–68.

Lauber, Annika, Alexei Kiselev, Thomas Pander, Patricia Handmann, and Thomas Leisner. 2018. “Secondary Ice Formation during Freezing of Levitated Droplets.” Journal of the Atmospheric Sciences 75 (8): 2815–26.

Lawson, R. Paul, Sarah Woods, and Hugh Morrison. 2015. “The Microphysics of Ice and Precipitation Development in Tropical Cumulus Clouds.” Journal of the Atmospheric Sciences 72 (6): 2429–45.



How to cite: Connolly, P., James, R., and Phillips, V.: A laboratory and model investigation of secondary ice production during to supercooled drop collisions with ice surfaces , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15696,, 2021.

Michail Karalis, Georgia Sotiropoulou, Steven J. Abel, Elissavet Bossioli, Paraskevi Georgakaki, Georgia Methymaki, Athanasios Nenes, and Maria Tombrou

The representation of boundary layer clouds during marine Cold-Air Outbreaks (CAO) remains a great challenge for weather prediction models. Recent studies have shown that the representation of the transition from stratocumulus clouds to convective cumulus open cells largely depends on microphysical and precipitation processes, while Abel et al. (2017) further suggested that Secondary Ice Processes (SIP) may play a crucial role in the evolution of the cloud fields. In this study we use the Weather Research Forecasting model to investigate the impact of the most well-known SIP mechanisms (rime-splintering or Hallet-Mossop, mechanical break-up upon collisions between ice particles and drop-shattering) on a CAO case observed north of UK in 2013. While Hallet-Mossop is the only SIP process extensively implemented in atmospheric models, our results indicate that collisional break-up is also important in these conditions.


Abel, S. J., Boutle, I. A., Waite, K., Fox, S., Brown, P. R. A., Cotton, R., Lloyd, G., Choularton, T. W., & Bower, K. N. (2017). The Role of Precipitation in Controlling the Transition from Stratocumulus to Cumulus Clouds in a Northern Hemisphere Cold-Air Outbreak, Journal of the Atmospheric Sciences, 74(7), 2293-2314. Retrieved Jan 9, 2021, from

How to cite: Karalis, M., Sotiropoulou, G., Abel, S. J., Bossioli, E., Georgakaki, P., Methymaki, G., Nenes, A., and Tombrou, M.: The impact of Secondary Ice Processes on a stratocumulus-to-cumulus transition during a Cold-Air Outbreak, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3369,, 2021.

Alice Keinert, Judith Kleinheins, Alexei Kiselev, and Thomas Leisner

During the freezing of supercooled drizzle droplets, the ice shell forms at the droplet surface and propagates inwards, causing a pressure rise in the droplet core. If the pressure exceeds the mechanical stability of the ice shell, the shell can crack open and eject secondary ice particles or cause the full disintegration of the ice shell leading to droplet shattering. Recent in-cloud observations and modeling studies have suggested the importance of secondary ice production upon shattering of freezing drizzle droplets. The details of this process are poorly understood and the number of secondary ice particles produced during freezing remains to be quantified.

Here we present insight into experiments with freezing drizzle droplets levitated in electrodynamic balance under controlled conditions with respect to temperature, humidity and airflow velocity. Individual droplets are exposed to a flow of cold air from below, simulating free fall conditions. The freezing process is observed with high-speed video microscopy and a high-resolution infrared thermal measuring system. We show the observed frequencies for various events associated with the production of secondary ice particles during freezing for pure water droplets and aqueous solution of analogue sea salt droplets (300 µm in diameter) and report a strong enhancement of the shattering probability as compared to our previous study (Lauber et al., 2018) conducted in stagnant air. Analysis of pressure release events recorded by high-resolution infrared thermography suggest that pressure release events associated with the possible ejection of secondary ice particles occur far more frequent than previously quantified with observations by high speed video microscopy only.


Lauber, A., A. Kiselev, T. Pander, P. Handmann, and T Leisner (2018). “Secondary Ice Formation during Freezing of Levitated Droplets”, Journal of the Atmospheric Sciences 75, pp. 2815–2826.

How to cite: Keinert, A., Kleinheins, J., Kiselev, A., and Leisner, T.: Laboratory experiments on secondary ice production upon free falling drizzle droplets observed by high speed video and thermal imaging, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5831,, 2021.

Paraskevi Georgakaki, Georgia Sotiropoulou, Etienne Vignon, Alexis Berne, and Athanasios Nenes

In-situ observations of mixed-phase clouds (MPCs) forming over mountain tops regularly reveal that ice crystal number concentrations (ICNCs) are orders of magnitude higher than ice-nucleating particle concentrations. This discrepancy has often been attributed to the influence of surface processes such as blowing snow and airborne hoar frost. Ιn-cloud secondary ice production (SIP) processes may also explain this discrepancy, but their contribution has received less attention.
Here we explore the potential role of SIP processes on orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the mountain-top site of Jungfraujoch in the Swiss Alps using the Weather Research and Forecasting model (WRF). The Hallett-Mossop (H-M) mechanism, included in the default version of the Morrison scheme in WRF, is ruled out since the simulated clouds were outside the active temperature range for this process. This study investigates if the implementation of two additional SIP mechanisms in WRF, namely collisional break-up (BR) between ice hydrometeors and frozen droplet shattering (DS), can bridge the gap between observed and modeled ICNCs. DS is inefficient in the examined conditions due to a lack of sufficiently large raindrops to trigger this process. The BR mechanism is likely important in Alpine MPCs, but the process is activated only within seeder-feeder situations, when precipitation particles are seeding the low-level MPCs inducing their glaciation. At times when a cloud exists near the ground, blowing snow ice particles may be mixed among supercooled liquid droplets and thus contribute significantly to ice growth, but they cannot account for the observed ICNCs. Our findings indicate that outside the H-M temperature range, ice-seeding and blowing snow can initiate ice multiplication in the Alps through the BR mechanism, which is found to elevate the modeled ICNCs up to 3 orders of magnitude, providing a better agreement with in-situ measurements. This highlights the importance of considering both SIP and surface-based processes in weather-prediction and climate models.

How to cite: Georgakaki, P., Sotiropoulou, G., Vignon, E., Berne, A., and Nenes, A.: The relative contribution of secondary ice processes in Alpine mixed-phase clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14659,, 2021.

Susan Hartmann, Alice Keinert, Alexei Kiselev, and Frank Stratmann

Mixed-phase clouds are essential elements in Earth’s weather and climate system. Aircraft measurements of mixed-phase clouds demonstrated a strong discrepancy between the observed ice particle and ice nucleating particle number concentration of one to four orders of magnitude [1-4]. Different secondary ice production (SIP) mechanisms have been hypothesized which can increase the total ice particle number concentration by multiplication of primary ice particles and hence might explain the observed discrepancy [5-7].

In a joint project of KIT and Tropos, we focus on the investigation two SIP processes: shattering of large freezing droplets (KIT) and SIP as a result of droplet-ice collisions (Tropos), commonly known as Hallett-Mossop [9] or rime-splintering process. Thereby, we aim at a quantitative understanding of the SIP underlying physical mechanisms, utilizing a newly developed experimental set-up (Ice Droplets splintEring on FreezIng eXperiment, IDEFIX).  

IDEFIX is based on a modular concept and consists of three modules, i.e., the SIP chamber, the growth section, and the ice particle detector. We developed two different versions of the SIP chamber: in the KIT-SIP chamber a freezing drizzle droplet is levitated in electrodynamic balance; and in the TROPOS-SIP chamber quasi-monodisperse droplets collide with an ice particle which is fixed on thin carbon fibers. IDEFIX is designed to match realistic fall or impact velocities and collision rates of the droplets with the ice particle. The SIP process will be observed with high-speed video microscopy and an infrared measuring system. In the growth section, which features supersaturated conditions with respect to ice, the presumably small secondarily produced ice particles will be grown to detectable sizes. Finally, to count the number of secondarily produced ice particles either an optical particle spectrometer will be used for distinguishing between droplets and ice particles, or the ice particles will be impacted on a metastable sugar solution. Currently, we characterize velocity, temperature and humidity fields of the TROPOS-collision chamber and determine droplet-ice particle collision rates.

How to cite: Hartmann, S., Keinert, A., Kiselev, A., and Stratmann, F.: Secondary Ice Production – development of a new experimental set-up, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11327,, 2021.

Georgia Sotiropoulou, Anna Lewinschal, Annica Ekman, and Athanasios Nenes

Arctic clouds are among the largest sources of uncertainty in predictions of Arctic weather and climate. This is mainly due to errors in the representation of the cloud thermodynamic phase and the associated radiative impacts, which largely depends on the parameterization of cloud microphysical processes. Secondary ice processes (SIP) are among the microphysical processes that are poorly represented, or completely absent, in climate models. In most models, including the Norwegian Earth System Model -version 2 (NorESM2), Hallet-Mossop (H-M) is the only SIP mechanism available. In this study we further improve the description of H-M and include two additional SIP mechanisms (collisional break-up and drop-shattering) in NorESM2. Our results indicate that these additions improve the agreement between observed and modeled ice crystal number concentrations and liquid water path in mixed-phase clouds observed at Ny-Alesund in 2016-2017. We then conclude by quantifying the impact of these overlooked SIP mechanisms for cloud microphysical characteristics, properties and the radiative balance throughout the Arctic.


How to cite: Sotiropoulou, G., Lewinschal, A., Ekman, A., and Nenes, A.: Secondary ice production in NorESM2 climate model: quantifying the impact on Arctic clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10241,, 2021.