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Atmos. Chem. Phys., 21, 18519–18530, 2021
https://doi.org/10.5194/acp-21-18519-2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
Secondary ice production during the break-up of freezing
water drops on impact with ice particles
Rachel L. James1, Vaughan T. J. Phillips2, and Paul J. Connolly1
1Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK
2Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden
Correspondence: Rachel L. James (rachel.james@manchester.ac.uk)
Received: 1 July 2021 – Discussion started: 15 July 2021
Revised: 15 November 2021 – Accepted: 18 November 2021 – Published: 21 December 2021
Abstract. We provide the first dedicated laboratory study of collisions of supercooled water drops with ice par-
ticles as a secondary ice production mechanism. We experimentally investigated collisions of supercooled water
drops (∼5 mm in diameter) with ice particles of a similar size (∼6 mm in diameter) placed on a glass slide at
temperatures >−12 ◦C. Our results showed that secondary drops were generated during both the spreading and
retraction phase of the supercooled water drop impact. The secondary drops generated during the spreading phase
were emitted too fast to quantify. However, quantification of the secondary drops generated during the retraction
phase with diameters >0.1 mm showed that 5–10 secondary drops formed per collision, with approximately
30 % of the secondary drops freezing over a temperature range between −4 and −12 ◦C. Our results suggest that
this secondary ice production mechanism may be significant for ice formation in atmospheric clouds containing
large supercooled drops and ice particles.
1 Introduction
Most surface rainfall events that occur across the globe
are associated with the ice phase within clouds in Earth’s
atmosphere (Field and Heymsfield, 2015), as are severe
weather events such as freezing rain, hail and thunderstorms
(Changnon, 2003; Púˇ
cik et al., 2019; Elsom, 2001). There-
fore, understanding the processes which govern ice forma-
tion in clouds is crucial for determining their effects on both
climate and weather.
Where subzero temperatures are warmer than the homo-
geneous freezing point of −35 ◦C, supercooled water drops
can heterogeneously freeze via a subset of aerosol particles
present in the atmosphere. This subset of aerosol particles,
called ice-nucleating particles (INPs), are relatively rare, and
while number concentrations of INPs vary in time and space,
they typically fall between 1 ×10−5to 1 L−1at ∼ −10 ◦C
(Kanji et al., 2017). Yet, observed ice particle concentrations
in mixed-phase clouds can be several orders of magnitude
higher than concentrations predicted from ice particles form-
ing due to INPs (e.g. Crawford et al., 2012; Lloyd et al.,
2015; Lasher-Trapp et al., 2016; Ladino et al., 2017). Ice can
also form at temperatures >−35 ◦C via secondary ice pro-
duction (SIP), where new ice particles are formed from pre-
existing ice particles. However, our understanding of ice for-
mation from SIP mechanisms is incomplete (e.g. see reviews
by Field et al., 2017; Korolev and Leisner, 2020), resulting in
poor representation of SIP mechanisms in numerical weather
prediction (NWP) models.
Observations within mixed-phase clouds often show ice
crystal number concentrations higher than the numbers of
INPs present in the atmosphere. For instance, ice particle
number concentrations exceeding 100 L−1, in shallow con-
vection with a cloud top temperature no lower than −12 ◦C,
have been observed over the UK (Crawford et al., 2012).
Furthermore, thin mixed-phase layer clouds have been ob-
served to continually generate snow (Westbrook and Illing-
worth, 2013). Conventional thinking would suggest that the
ice in mixed-phase layer clouds should fall out, leaving the
layer “depleted” of INPs; however, the observations clearly
show that ice continues to form in these clouds over time.
The rime splintering SIP mechanism has been successful
in predicting the glaciation of mixed-phase clouds in many
Published by Copernicus Publications on behalf of the European Geosciences Union.
18520 R. L. James et al.: Secondary ice formation during drop–ice collisions
cases, especially those involving a warm cloud base creating
sufficiently large cloud drops in the rime splintering temper-
ature region between −3 and −8◦C (e.g. Harris-Hobbs and
Cooper, 1987; Blyth and Latham, 1993, 1997; Phillips et al.,
2001, 2005; Crosier et al., 2011; Crawford et al., 2012; Tay-
lor et al., 2016; Huang et al., 2017). However, there are also
numerous cases where significant concentrations of ice ob-
served in clouds cannot be explained by the rime splintering
SIP mechanism. Hobbs and Rangno (1985) compiled tables
of aircraft observations from a wide range of cloud environ-
ments. They found that the maximum ice particle concentra-
tions were independent of the cloud top temperature but were
strongly dependent on the broadness of the supercooled drop
spectrum near the cloud top, with approximately half of the
clouds exhibiting ice enhancement.
Several SIP mechanisms have been identified and studied
both in the laboratory and theoretically, but only the rime
splintering SIP mechanism is widely implemented in NWP
models. Active between −3 and −8◦C, rime splintering oc-
curs when supercooled water drop diameters are <13 and
>24 µm (Hallett and Mossop, 1974; Mossop and Hallett,
1974; Mossop, 1978). Another SIP mechanism, the fragmen-
tation of freezing drops, has received a significant proportion
of laboratory-based SIP investigations. Fragmentation due to
freezing drizzle drops or raindrops can occur over a wider
temperature range between 0 and −32 ◦C, but quantification
of ice fragment generation rates and temperature dependence
within these rates between laboratory studies varies signifi-
cantly (see Table 1 of Korolev and Leisner, 2020, for a sum-
mary). A range in diameters of freezing supercooled water
drops has also been investigated between laboratory studies
from 4 to 1000 µm (see Table 1 of Korolev and Leisner, 2020,
for a summary). While other SIP mechanisms exist (e.g. ice–
ice collisions and sublimation fragmentation), the attention
of laboratory studies has overwhelmingly focussed on the
SIP mechanisms of rime splintering and fragmentation due
to freezing drops. Furthermore, unidentified SIP mechanisms
may also exist.
In this paper, we present a SIP mechanism involving the
formation of secondary drops from the collision of a super-
cooled water drop with a larger ice particle. This SIP mecha-
nism has been investigated via a theoretical study by Phillips
et al. (2018), who referred to it as “mode 2”, as it involves
collisions of supercooled water drops with more massive ice
particles resulting in fragmentation of the supercooled water
drop. Ice contained in some of the secondary drops was as-
sumed to initiate freezing, yielding secondary ice fragments.
By contrast, “mode 1” involved either collisions of super-
cooled water drops with less massive ice particles resulting
in spherical freezing of the supercooled water drop or activa-
tion of immersed INPs, with a quasi-spherical outer ice shell
that fragments.
While there are no dedicated laboratory studies of this SIP
mechanism involving collisions of supercooled water drops
with more massive ice particles or the activation of INPs im-
mersed in them, there are laboratory studies that have indi-
rectly studied aspects of this process. For example, a sim-
ilar mechanism was alluded to by Latham and Warwicker
(1980) in their experimental investigation of charge transfer
during interactions between hailstones and supercooled wa-
ter drops. They observed that frost could occasionally be bro-
ken during impact, thus forming new ice particles. Although
this was an unwanted outcome of their experiments, it pro-
vided some hints of a potential SIP mechanism during the
interactions between ice particles and supercooled raindrops.
Later, Schremb et al. (2018) studied the fluid flow and solid-
ification of supercooled water drops on elevated ice targets,
briefly observing the formation of secondary drops from the
rim of the supercooled water drop during impact. However,
for both of these studies no quantification of the secondary
drops was made.
In this paper, we describe a set of experiments performed
at the University of Manchester to determine the freezing
fraction of secondary drops (8) formed in the splash dur-
ing the collision of a 5 mm diameter supercooled raindrop on
a 6 mm diameter ice particle, providing the first laboratory
quantification of this SIP mechanism. This freezing fraction
(8) is the ratio of secondary drops that freeze to all such
drops emitted. The experimental setup is described in Sect. 2.
The results are presented in Sect. 3, and the discussion is in
Sect. 4. Finally, the conclusions are given in Sect. 5.
2 Experimental setup
A schematic of the experimental setup is shown in Fig. 1. The
setup was purpose-built to study the impact of a supercooled
water drop on an ice particle. For this study, we used two con-
figurations of the experimental setup. The first configuration
was used to study the drop impact with a high-speed camera
(Chronos 1.4, Kron Technologies Inc.) equipped with a mi-
croscopic lens (Kron Technologies Inc.) and a 0.5×Barlow
lens (Kron Technologies Inc.) in a side-on view. The second
configuration was used to quantify the fraction of secondary
drops that froze after impact with the ice particle using two
Raspberry Pi Camera modules (Raspberry Pi Camera Mod-
ule V2), referred to as RPicams, with a polarising filter (stan-
dard 55 mm circular polariser) attached to one camera. At
present, the two configurations are not compatible to work
concurrently. Recordings using the high-speed camera were
recorded at 1069 frames per second (fps), and recordings us-
ing the RPicams were recorded at 24 fps.
The experimental setup is operated in a cold room which
can achieve a base temperature as low as −50 ◦C and pro-
vided the means of achieving a supercooled environment.
The experimental setup was housed in a Bosch strut and Per-
spex panel frame to prevent the accidental introduction of
frost particles during the experiments. A glass slide was sup-
ported on 3D-printed plastic stilts approximately 10 cm in
height which had a fan attached to dissipate the heat emitted
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R. L. James et al.: Secondary ice formation during drop–ice collisions 18521
from the polarised light source (LCD – liquid-crystal display
– monitor). The temperature of the glass slide was monitored
using a K-type thermocouple attached to the glass slide with
aluminium tape. The relative humidity was not measured but
will be below ice saturation, and possibly very small ice
fragments were not observed due to sublimation preventing
growth to visible sizes. The ice particles were prefabricated
by freezing ultrapure water drops (endotoxin-free UP H2O,
Merck) of approximately 6 mm in diameter on a glass slide
coated in a water repellent (Rain-X) using a Peltier cool-
ing system. The typical freezing shape of the ice particle is
shown in Fig. 1. A pipette was modified to allow an ultrapure
water drop (endotoxin-free UP H2O, Merck) at room temper-
ature with a diameter of approximately 5 mm to be placed on
the pipette using a disposable needle (22 gauge, sterile) and
syringe. The modified pipette was held in a 3D-printed tipper
mechanism parallel to the glass slide, and the water drop was
allowed to reach thermal equilibrium with the cold room for
90 s. The supercooled drop was released from the modified
pipette perpendicular to the glass slide and was controlled by
an Arduino and servomotor.
As the drop height and initial supercooled water drop di-
ameter before impact (D) were kept constant at 1.36 m and
5 mm, respectively, the normal impact velocity (V0) for all
experiments was 5.2 m s−1. The terminal velocity of a 5 mm
diameter drop is approximately 9 m s−1(Gunn and Kinzer,
1949). Initially, the impact velocity may seem unrealistic.
However, the ice particle in these experiments was held sta-
tionary on a glass slide, but in the atmosphere the ice particle
would also be falling. The terminal velocity will depend on
the ice particle shape, but for aggregates of a similar size it is
typically around 1 ms−1(Locatelli and Hobbs, 1974). More-
over, turbulence, especially in deep convective clouds, may
also affect the impact velocity (Pinsky and Khain, 1998).
While such large droplets are rare in the atmosphere, the pur-
pose here is to demonstrate that the process is a potential SIP
mechanism. The supercooled water drop and the ice particle
on the glass slide were in thermal equilibrium for all experi-
ments.
The temperature range investigated was between −4 and
−12 ◦C. As the temperature of water decreases, the surface
tension (σ) and viscosity (µ) of water increases (Hrubý et al.,
2014; Dehaoui et al., 2015). In fluid dynamics, the We-
ber number, W e =ρDV 2
0/σ , and Reynolds number, Re =
ρDV0/µ, are used to relate the inertial forces of the fluid
to its interfacial and viscous forces, respectively. In this
case, the fluid is the supercooled water drop, and the di-
ameter of the supercooled water drop, D, refers to the di-
ameter before impact. The inertial force is from the ini-
tial impact velocity of the supercooled water drop, and the
interfacial (surface tension) and viscous forces are proper-
ties of the supercooled water drop. Taking into account the
temperature-dependent values of surface tension and viscos-
ity of the supercooled water between −4 and −12 ◦C, the W e
Figure 1. Schematic diagram of the experimental setup. Compo-
nents labelled (i) were used in the high-speed configuration and (ii)
were used in the RPicam configuration. The setup was operated in
a cold room to achieve a supercooled environment.
and Re number ranges obtained were 1747 ≤W e ≤1772 and
8781 ≤Re ≤12 240, respectively.
We conducted 32 experiments using the RPicam configu-
ration during quantification of the freezing fraction of sec-
ondary drops, and the data are given in Table A1.
3 Results
From our high-speed and RPicam recordings we present a
schematic diagram of the formation of secondary drops from
a supercooled water drop impact on an ice particle on a glass
slide in Fig. 2. The W e and R e numbers used were suffi-
ciently large, i.e. W e 2.5 and Re 25, such that inertia
dominated the spreading of the thin film (Roisman, 2009).
Surface tension and viscosity forces were therefore consid-
ered negligible during the spreading phase of the drop (Ro-
isman, 2009), as was the wettability of the surface (Antonini
et al., 2012). Figure 2a depicts the filament-like structures
which were ejected during the spreading phase of the drop
impact. We were unable to track the positions of these sec-
ondary drops or quantify them with our current high-speed
camera or RPicam configurations. As the kinetic energy is
transferred from that of a vertical to horizontal motion at im-
pact, the water drop spread out radially, and instabilities at
the rim were also observed. Figure 2b depicts the retraction
of the drop, which caused the instabilities to “pinch off” or
rupture, followed by a partial rebound. On superhydrophobic
surfaces, rupturing of the instabilities has been attributed to
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18522 R. L. James et al.: Secondary ice formation during drop–ice collisions
Figure 2. A schematic diagram of a supercooled water drop im-
pact on an ice particle on a glass slide and subsequent secondary
drop formation during (a) the spreading phase and (b) the retraction
phase.
surface tension (Zhang et al., 2020). Our glass slide, coated in
a water repellent, is probably superhydrophobic, and surface
tension is likely the cause of the rupture of the rim instabili-
ties.
3.1 Drop impact: high-speed recordings
We performed control experiments at room temperature
(23 ◦C) and several supercooled temperatures using the high-
speed camera configuration to characterise the water drop
(diameter of 5 mm) impacting the glass slide. Figure 3 shows
the frames from a high-speed recording of (a) a water drop
impact on the glass slide at room temperature and (b) a su-
percooled water drop impact at −5◦C.
On impact with the glass slide, the water drop deformed
and spread radially outwards as a thin film bordered by a
thicker rim. Instabilities at the rim were observed for both the
room temperature drop and the supercooled drop at −5◦C.
The supercooled drop shown in Fig. 3b ejected straight
filament-like structures at an angle to the glass surface close
to the impact, and these filament-like structures disintegrated
into secondary drops. This was in contrast to the impact of
the water drop at room temperature where no ejection of
filament-like structures was observed, perhaps due to higher
viscosity and surface tension of water at supercooled temper-
atures. During the retraction phase, some of the rim instabili-
ties pinched off from the thin film in the experiments with the
water drop at room temperature forming secondary drops, in
a process called “receding break-up”. In contrast, no reced-
ing break-up was observed for the supercooled drop.
Figure 4 shows the frames of a supercooled water drop
impacting the side of an ice particle at −5◦C. Similar to the
supercooled water drop on a glass slide, filament-like struc-
tures, which dissipated into secondary drops, formed at or
close to impact with the ice particle glass slide. Unlike the
impact of a supercooled water drop on a bare glass slide, sec-
ondary drops formed via receding break-up. These secondary
drops were observed around the parts of the rim of the thin
film which contacted the ice particle.
3.2 Determining the freezing fraction of the secondary
drops: RPicams
We performed supercooled water drop impacts on ice parti-
cles between −4 and −12 ◦C. To unambiguously identify if
a secondary drop had frozen, we used a polarising filter with
a polarised light source, exploiting the birefringent proper-
ties of ice. Figure 5 shows selected frames of a supercooled
water drop impact at −4◦C using the RPicam configuration.
The top row of Fig. 5 shows frames from the camera with no
polarising filter (a) before, (b) at and (c) ∼10 s after impact.
The number of secondary drops observed is indicated by red
arrows in Fig. 5c. The difference between Fig. 5a and c is
presented in Fig. 5d, clearly indicating the secondary drops
formed. The bottom row shows frames from the camera with
a polarising filter (e) before, (f) at and (g) ∼10 s after impact.
The frozen secondary drop is indicated by a white arrow in
Fig. 5g. The difference between Fig. 5e and g is presented in
Fig. 5h, clearly indicating the frozen secondary drop formed.
For this particular experiment, five secondary drops
formed, of which one froze, giving a freezing fraction, 8=
0.2. During these experiments, two types of supercooled wa-
ter drop impacts occurred: direct impact on the ice particle
and partial impact on the ice particle. These different impacts
arose due to practical difficulties with consistently impact-
ing the ice particle with the supercooled water drop due to
changes in viscosity of water at different temperatures. For
the experiment shown in Fig. 5, the impact was a side im-
pact towards the top left of the ice particle as indicated in
Fig. 5b. The RPicam configuration only observed the larger
>0.1 mm diameter drops formed during retraction of the thin
film. The smaller secondary drops (<0.1 mm diameter) ob-
served at impact from the high-speed configuration were not
observed using this configuration, as the minimum drop di-
ameter the RPicams could detect was 0.1 mm.
Figure 6a shows the average freezing fraction of secondary
drops formed when a supercooled water drop with a diame-
ter of 5 mm collided with an ice particle, 8, as a function
of temperature. The raw data can be found in Table A1, and
the averaged data of the freezing fraction of secondary drops
are in Table A2. The average number of secondary drops, Ns,
liquid or solid, shown in Fig. 6b as a function of temperature,
reached a maximum at approximately −7.5 ◦C. The averaged
data of the number of secondary drops can be found in Ta-
ble A3.
4 Discussion
We discuss some aspects of the experimental setup that may
affect the occurrence and rate of secondary drop production
and freezing here.
As the ice particles were placed on a flat glass slide, dur-
ing impact, the supercooled water drop spread across the ice
particle and on to the glass slide where the larger >0.1 mm
diameter secondary drops formed. We acknowledge that the
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R. L. James et al.: Secondary ice formation during drop–ice collisions 18523
Figure 3. Frames from the high-speed camera configuration of a water drop impact on a glass slide when both the water drop and glass slide
are at (a) room temperature (23 ◦C) and (b) −5◦C. The impact phase (I), spreading phase (S), secondary drop formation/ejection during the
spreading phase (E), retraction phase (R), secondary drop formation due to the receding break-up (B) and partial rebound (PR) of the water
drop are indicated in the frames. Arrows indicate secondary drop formation during the retraction phase of the water drop.
glass slide presents an artificially flat surface compared to at-
mospheric conditions. However, a study by Schremb et al.
(2018) showed that, on an elevated ice surface, the thin film
of a supercooled water drop with a diameter of ∼4 mm and
similar W e and Re numbers at −14 ◦C was ejected and sub-
sequently ruptured, forming secondary drops. While quan-
tification was not the focus of their study, it was observed that
the rim of the supercooled water drop was largely frozen, and
only some of the secondary drops were observed as ice. The
size of the secondary drops formed in the study by Schremb
et al. (2018) is comparable to our secondary drops despite
the different generation mechanism, and their supplementary
videos indicate that 10 s of secondary drops were formed.
Furthermore, water drops with diameters of ∼3–4 mm col-
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18524 R. L. James et al.: Secondary ice formation during drop–ice collisions
Figure 4. Frames from the high-speed camera configuration of a supercooled water drop impact on an ice particle when both the drop and ice
particle are at −5◦C. The impact phase (I), spreading phase (S), secondary drop formation/ejection during the spreading phase (E), retraction
phase (R), secondary drop formation due to the receding break-up (B) and partial rebound (PR) of the water drop are indicated in the frames.
Arrows indicate secondary drop formation during the retraction phase of the supercooled water drop.
Figure 5. Selected frames from the impact of a supercooled water drop on an ice particle at −4◦C using the RPicam configuration. Frames
(a)–(c) before, at and ∼10 s after impact using the camera with no polarising filter. Red arrows in (c) indicate the number of secondary
drops formed. Frame (d) shows the difference between (a) and (c). Frames (e)–(g) before, at and ∼10 s after impact using the camera with a
polarising filter. The white arrow in (h) indicates the frozen secondary drop. Frame (h) shows the difference between (e) and (g).
liding with a steel disk of ∼4 mm in diameter (Rozhkov
et al., 2002) and water drops with diameters of 6 mm col-
liding with an iron cylinder of the same diameter (Viller-
maux and Bossa, 2011) produced 100 s of secondary drops.
Clearly, secondary drops still form, emitted from the rim of
the thin film during impact, when there is no supporting flat
surface, such as the glass slide used in this study. However,
there is much uncertainty about the number of secondary
drops formed.
In addition, the ice particle in our experiments is in a fixed
position on the glass slide, whereas, in the atmosphere, the
ice particle is in free fall. When the faster-moving super-
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R. L. James et al.: Secondary ice formation during drop–ice collisions 18525
Figure 6. (a) The average freezing fraction of the secondary drops
(8, black triangles) and (b) the average number of secondary drops
(Ns, blue circles) as a function of temperature. Average data in-
cluded both direct and partial collisions. The error bars represent
the standard error in the freezing fraction or secondary drop num-
ber for the temperature intervals which are listed in Tables A2 and
A3.
cooled water drop collides with the ice particle, the ice parti-
cle will move in response to the collision, likely affecting the
formation of the secondary drops and their subsequent freez-
ing. However, currently, it is difficult to ascertain how this
will influence secondary drop formation and freezing with-
out further investigations into the mechanisms of secondary
drop formation on an elevated ice particle.
Another factor that will influence the generation of sec-
ondary drops is the ice particle shape. Our ice particles have
a pointed tip, as shown in Fig. 2, which is a typical shape
formed when a liquid water drop is frozen on a cold sub-
strate (Snoeijer and Brunet, 2012) but not representative of
atmospheric ice particles. According to Phillips et al. (2018),
who refer to this SIP mechanism as mode 2, for it to occur,
the supercooled water drops must have a diameter larger than
150 µm, and the ice particle must be more massive still. In the
atmosphere, ice particles which are larger than 150 µm are
typically irregular in shape (Korolev and Sussman, 2000). A
study by Zhang et al. (2020) shows that at room temperature,
water drop impact on curved surfaces induce additional frag-
mentation mechanisms compared to flat surfaces. Therefore,
we expect the irregular shape of an ice particle to introduce
additional fragmentation mechanisms of the supercooled wa-
ter drop, which may enhance secondary drop formation.
We observed a decrease in the number of secondary drops
formed during receding break-up as the temperature de-
creased below −8◦C. Figure 7 shows the frames after a su-
percooled water drop impact with an ice particle for the ex-
periments between −11 and −12 ◦C, which was the range
where the smallest number of secondary drops formed. At
these temperatures, the supercooled water drop froze either
during the spreading phase or in the early stages of the re-
traction phase. The growth velocity of ice in supercooled wa-
ter increases with decreasing temperature (e.g. at −2◦C it is
around 0.2 cm s−1, whereas at −10 ◦C it is around 5 cm s−1;
Pruppacher, 1967), which may explain why a decrease in sec-
ondary drops was observed. We believe the decrease in sec-
ondary drop formation at temperatures below −8◦C may be
due to the artificially flat geometry presented by the glass
slide and to the large size of the incident drop, both fac-
tors which prolonged the interaction time between the su-
percooled water drop and ice. For example, the supplemen-
tary videos from Schremb et al. (2018) showed 10 s of sec-
ondary drops forming at −14 ◦C after impact on an elevated
ice target, more than we observed at our lowest temperature
of −12 ◦C.
Whilst the freezing mechanism of the secondary drops was
not specifically studied in this work, we consider the follow-
ing mechanisms. The freezing of supercooled water drops
occurs in two stages. The first stage is characterised by the
formation of ice dendrites throughout the supercooled water
drop. The latent heat from the formation of the ice dendrites
is released during this stage, warming the temperature of the
supercooled water drop to ∼0◦C. The second stage is char-
acterised by the freezing of the remaining supercooled water
drop and is controlled by the loss of latent heat due to the sur-
roundings of the supercooled water drop. Stage 1 of freezing
is fast, and the time taken for this stage to complete (ti) can be
estimated from the following equation (Macklin and Payne,
1967):
ti≈δR
G,(1)
where δRis the thickness of the layer of supercooled water
on the ice particle and Gis the growth velocity of ice, which
is temperature dependent.
From Fig. 4, we can estimate that the rim of the super-
cooled water drop, which is also the thickest part of the su-
percooled water drop, is approximately 0.78 mm. Taking this
value for δRand given that the growth velocity of ice at
−5◦C is approximately 1 cm s−1(Pruppacher, 1967), then
ti≈0.078 s. Figure 4 shows the timescale for the retraction
phase is of the order of 0.1 s. It is plausible that the initial ice
dendrites can propagate through the supercooled water drop
and that water containing these ice dendrites may then break
off during the retraction phase and initiate freezing. The sec-
ond phase of freezing will take longer, but as long as the drop
contains ice dendrites it will eventually freeze. This explana-
tion is also proposed by Schremb et al. (2018) and Phillips
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18526 R. L. James et al.: Secondary ice formation during drop–ice collisions
Figure 7. Frames from the RPicam configuration approximately 10 s after a supercooled water drop impact for experiments at T > −11 ◦C.
The top panel shows frames from the RPicam with no polarising filter, and the bottom panel shows frames from the RPicam with a polarising
filter.
et al. (2018), who suggest that seeding ice crystals are trans-
ported during the initial spreading phase. Alternative freez-
ing mechanisms include the formation of a thin, unobserved
film of liquid water present on the glass slide after the retrac-
tion phase. The contact between the thin film of water and
the ice particle could induce freezing in the thin film, which
could then trigger freezing in the seemingly detached sec-
ondary drop. Mechanical agitation or shock may also play
a role in the freezing of the secondary drops (Alkezweeny,
1969; Czys, 1989). Regardless of the freezing mechanism,
the glass slide will likely have some influence, and it will be
pertinent to remove this in future investigations.
As a proof-of-concept investigation, we studied super-
cooled water drops with diameters of 5 mm and ice parti-
cles with diameters of 6 mm, as larger sizes of supercooled
water drops were easier to work with experimentally. While
these sizes are not necessarily representative of cloud con-
ditions, theoretically, this new SIP mechanism should occur
where supercooled water drop diameters are >150 µm and
the ice particles are more massive still. Supercooled water
drops and ice particles are present within a variety of dif-
ferent clouds. For example, Hobbs and Rangno (1990) pre-
sented aircraft observations in small polar–maritime cumuli
that displayed ice enhancement with cloud bases too cold
for large cloud droplets (>24 µm) between −3 and −8◦C
as required for rime splintering. Their discussion highlighted
that ice enhancement proceeded in two stages. The first stage
consisted of the formation of frozen drops, <400 µm diam-
eter, and small graupel particles, <1 mm diameter. The sec-
ond stage was characterised by the appearance of high con-
centrations of vapour-grown ice crystals in the upper regions
of the cloud. A key finding of this series of papers was that
high concentrations of small ice particles appeared simulta-
neously with frozen drizzle drops. Furthermore, Rangno and
Hobbs (2001) showed that large supercooled drops were of-
ten a requirement for ice enhancement in moderately cooled
Arctic stratiform clouds, and ice enhancement was often co-
incident with observations of large supercooled raindrops.
Supercooled drizzle drops and raindrops are common in con-
vective clouds (e.g. Crawford et al., 2012; Taylor et al.,
2016), as are large ice particles. Hence, the broad continuum
of drizzle and raindrop sizes, where the larger drops freeze
first, followed by accretion of the smaller unfrozen drops in-
dicates that collisions of supercooled water drops with ice
particles that are more massive may be of importance in a
wide range of clouds.
5 Conclusions and future work
In this study, we confirmed that during collisions of super-
cooled water drops with ice particles, frozen secondary drops
formed over the temperature range between −4 and −12 ◦C.
Our main findings are the following:
1. Approximately 5 to 10 secondary drops are formed by
receding break-up during the retraction phase of a su-
percooled water drop (D=5 mm) after collision with
an ice particle (D=6 mm) placed on a glass slide.
2. An average of 30 % of these secondary drops formed
froze between −4 and −12 ◦C.
3. Experiments with a high-speed camera highlighted that
secondary drops formed as a jet of smaller droplets
produced separately from the receding break-up of the
drop. No quantification of the freezing fraction of the
secondary drops can currently be made.
One of the main experimental challenges of this work was
dropping the supercooled water drop consistently onto the
ice particle, which limited the number of experiments we
could perform. As shown in Table A1, the majority of the
successful impacts were classified as partial hits despite the
intention for them to be direct hits. While partial hits are ex-
pected in clouds, as well as direct hits, we also conducted
Atmos. Chem. Phys., 21, 18519–18530, 2021 https://doi.org/10.5194/acp-21-18519-2021
R. L. James et al.: Secondary ice formation during drop–ice collisions 18527
many experiments where the supercooled water drop missed
the ice particle. One method of achieving better control of
the supercooled water drop impact could be via growth and
supercooling of a water drop at the end of a needle similar
to the system shown in Schremb et al. (2018). Compared to
our current mechanism, which involved tilting a pipette to
allow the supercooled water drop to roll off, the supercooled
water drop would remain fixed to a certain point before de-
taching under gravity, making it easier to drop consistently
in the same position.
Another experimental challenge we would like to address
is quantifying the secondary drops formed during the spread-
ing phase of the supercooled water drop during impact.
Thoroddsen et al. (2012) quantified secondary drops ejected
with velocities of up to 100 m s−1using an ultra-high-speed
camera capable of recording at 1 000 000 fps, and we could
use a similar setup. We could then exploit the birefringent
properties of ice to determine whether these ejected sec-
ondary drops froze.
The number of secondary drops per collision is sensitive to
geometry and the material of collision, even for drops of the
same size. We quantify about 10 secondary drops per col-
lision. Schremb et al. (2018) observe of the order of tens
of drops per collision for impacts on elevated ice surface.
Finally, Rozhkov et al. (2002) observe 100 s for drop im-
pacts on steel disks at room temperature, as do Villermaux
and Bossa (2011) for drop impacts on iron cylinders at room
temperatures. Consequently, after addressing the above chal-
lenges and elevating the ice particle off the glass surface,
which may be achieved simply by fixing the ice particle on a
wire, further work is needed to investigate, more systemati-
cally, this new SIP mechanism over a range of experimental
parameters, not limited to the following: supercooled drop
sizes, size ratios of supercooled water drops to ice particles,
ice particle shapes, temperatures, drop height (and hence
impact velocity), airflow, relative humidity conditions and
chemical compositions of the supercooled water drop.
Appendix A
Table A1. Total number of secondary drops and the number of
frozen secondary drops for each experiment.
Temperature Total number of Frozen secondary
(◦C) secondary drops drops
Direct −4.2 14 1
−4.2 0 0
−5.3 7 6
−5.5 10 2
−7.8 12 2
−9.9 7 0
Partial −3.8 16 5
−4.0 5 1
−4.0 8 5
−4.3 9 6
−5.6 5 0
−5.6 5 1
−5.8 9 5
−6.0 4 1
−6.0 8 2
−6.1 2 1
−6.1 12 3
−7.7 17 7
−8.0 5 0
−8.0 11 7
−8.1 8 1
−8.5 16 0
−9.4 0 0
−9.4 21 6
−9.8 11 4
−10.0 2 2
−10.1 10 6
−11.3 0 0
−11.5 4 1
−11.8 4 1
−11.9 0 0
−11.9 0 0
https://doi.org/10.5194/acp-21-18519-2021 Atmos. Chem. Phys., 21, 18519–18530, 2021
18528 R. L. James et al.: Secondary ice formation during drop–ice collisions
Table A2. The mean (8) and standard deviation (SD) of the fraction
of frozen secondary drops within a specified temperature interval
(Tinterval) along with the number of experiments (n) within the T
interval, the average degree of supercooling within the temperature
interval (T) and the error in the sample mean (σ8).
Tinterval (◦C) T(◦C) 8SD n σ8
−3.8 −4.3 −4.1 0.38 0.26 5 0.12
−5.3 −5.8 −5.6 0.36 0.34 5 0.15
−6.0 −6.1 −6.1 0.44 0.38 4 0.19
−7.7 −7.8 −7.8 0.29 0.17 2 0.12
−8.0 −8.5 −8.2 0.19 0.30 4 0.15
−9.4 −9.9 −9.7 0.22 0.19 3 0.11
−10.0 −10.1 −10.1 0.80 0.28 2 0.20
−11.3 −11.9 −11.7 0.25 0.00 2 0.00
Table A3. The mean (Ns) and standard deviation (SD) of the num-
ber of secondary drops within a specified temperature interval (T
interval) along with the number of experiments (n) within the Tin-
terval, the average degree of supercooling within the temperature
interval (T) and the error in the sample mean (σNs).
Tinterval (◦C) T(◦C) NsSD n σNs
−3.8 −4.3 −4.1 8.7 5.9 6 2.4
−5.3 −5.8 −5.6 7.2 2.3 5 1.0
−6.0 −6.1 −6.1 6.5 4.4 4 2.2
−7.7 −7.8 −7.8 14.5 3.5 2 2.5
−8.0 −8.5 −8.2 10.0 4.7 4 2.3
−9.4 −9.9 −9.6 9.8 8.8 4 4.4
−10.0 −10.1 −10.1 6.0 5.7 2 4.0
−11.3 −11.9 −11.7 1.6 2.2 5 1.0
Data availability. All datasets are provided in Appendix A.
Video supplement. All video recordings from the high-
speed configuration and RPicam configuration are deposited
in Figshare, a FAIR-aligned (findable, accessible, interoper-
able and re-usable) data repository, and can be accessed at
https://doi.org/10.48420/c.5476557 (James et al., 2021).
Author contributions. VTJP and PJC conceived the original
study. RLJ and PJC designed the new experimental setup with ad-
vice from VTJP. RLJ and PJC performed the experiments. RLJ anal-
ysed the data and wrote the paper. VTJP and PJC provided com-
ments on the paper.
Competing interests. The contact author has declared that nei-
ther they nor their co-authors have any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. We thank Sylvia Sullivan; the two anony-
mous reviewers; and the editor, Timothy Garrett, whose comments
helped significantly improve the quality of this paper.
Financial support. This research has been supported by the U.S.
Department of Energy (grant no. DE-SC0018932) and the Natural
Environment Research Council (grant no. NE/T001496/1).
Review statement. This paper was edited by Timothy Garrett and
reviewed by Sylvia Sullivan and two anonymous referees.
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