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  • View in gallery

    A schematic diagram of the experimental apparatus (not scaled). Holes open only during droplet introduction (1), ice crystal introduction (2), and ice crystal and droplet measurements (3).

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    Average size distribution of the droplets in the column.

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    Replica of droplets on slides covered with formvar.

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    Average size distribution of the ice crystals in the column (ICc) and in the syringe (ICs).

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    Mean, maximum, and minimum values of ICc and ICs.

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    Replica of the ice crystals captured on glass slides.

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The Mystery of Ice Crystal Multiplication in a Laboratory Experiment

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  • 1 Institute of Atmospheric Sciences and Climate (ISAC), National Research Council, Bologna, Italy
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Abstract

This paper addresses the problem of the large discrepancies between ice crystal concentrations in clouds and the number of ice nuclei in nearby clear air reported in published papers. Such discrepancies cannot always be explained, even by taking into account both primary and secondary ice formation processes. A laboratory experiment was performed in a cylindrical column placed in a cold room at atmospheric pressure and temperature in the −12° to −14°C range. Supercooled droplets were nucleated in the column, in the absence of aerosol ice nuclei, by injecting ice crystals generated outside in a small syringe. A rapid increase in the ice crystal concentration was observed in the absence of any known ice multiplication. The ratio between the mean number of ice crystals in the column, after complete droplet vaporization, and the number of ice crystals introduced in the column was about 10:1. The presence of small ice crystals (introduced at the top of the column) in the unstable system (supercooled droplets) appears to trigger the transformation in the whole supercooled liquid cloud. A possible explanation could be that the rapidly evaporating droplets cool sufficiently to determine a homogeneous nucleation.

Corresponding author address: Gianni Santachiara, Institute of Atmospheric Sciences and Climate (ISAC), National Research Council, Via Gobetti, 101, Bologna 40129, Italy. E-mail: g.santachiara@isac.cnr.it

Abstract

This paper addresses the problem of the large discrepancies between ice crystal concentrations in clouds and the number of ice nuclei in nearby clear air reported in published papers. Such discrepancies cannot always be explained, even by taking into account both primary and secondary ice formation processes. A laboratory experiment was performed in a cylindrical column placed in a cold room at atmospheric pressure and temperature in the −12° to −14°C range. Supercooled droplets were nucleated in the column, in the absence of aerosol ice nuclei, by injecting ice crystals generated outside in a small syringe. A rapid increase in the ice crystal concentration was observed in the absence of any known ice multiplication. The ratio between the mean number of ice crystals in the column, after complete droplet vaporization, and the number of ice crystals introduced in the column was about 10:1. The presence of small ice crystals (introduced at the top of the column) in the unstable system (supercooled droplets) appears to trigger the transformation in the whole supercooled liquid cloud. A possible explanation could be that the rapidly evaporating droplets cool sufficiently to determine a homogeneous nucleation.

Corresponding author address: Gianni Santachiara, Institute of Atmospheric Sciences and Climate (ISAC), National Research Council, Via Gobetti, 101, Bologna 40129, Italy. E-mail: g.santachiara@isac.cnr.it

1. Introduction

Clouds in Earth’s atmosphere modify climate, as they interact with both incoming shortwave and outgoing longwave radiation (Stephens 2005; Gettelman et al. 2012). More than 50% of Earth’s precipitation originates in the ice phase (Heymsfield et al. 2006). Clouds are usually classified as “warm” (liquid water droplets), “cold” (ice clouds), or “mixed phase” (the latter consisting of supercooled liquid droplets and ice particles). Ice nucleation in clouds is one of the key processes in initiating precipitation (Heymsfield et al. 2006).

The formation of ice in clouds can occur in two ways: (i) primary processes (nucleation of ice from the liquid or water vapor phases), either homogeneously or heterogeneously triggered by aerosol particles called ice nuclei (IN), and (ii) secondary processes. The homogeneous nucleation process involves only pure water or solution droplets. Micrometer-sized pure water droplets should nucleate homogeneously in stationary conditions at about −38°C (Pruppacher and Klett 1997). Homogeneous nucleation from nearly pure water does occur in the atmosphere—for example, in the updrafts of cumulus clouds (Rosenfeld and Woodley 2000; Heymsfield et al. 2005). The probability of a given droplet freezing spontaneously increases with its volume, so the largest droplets tend to freeze first and the smallest tend to freeze last.

Four heterogeneous nucleation mechanisms are distinguished for atmospheric ice formation: deposition, condensation freezing, contact freezing, and immersion freezing. In addition to these standard model nucleation mechanisms, Durant and Shaw (2005) generalized the notion of contact nucleation to include the crystallization from particles contacting a supercooled droplet from the inside out, as well as from the outside in. It has been proposed that this may occur while a drop evaporates. However, such an association has not been found in the Ice in Clouds Experiment–Layer Clouds (ICE-L) field program (Heymsfield et al. 2011). Heterogeneous freezing occurs at lower supersaturation and higher temperatures T than homogeneous freezing.

Secondary ice formation processes can occur through

  1. fracture of ice crystals exposed to dry air layers or collision of preexisting ice crystal (Vardiman 1978; Oraltay and Hallett 1989);
  2. fragmentation of large individual cloud drops during freezing in free fall, thus playing a role in multiplying the number of ice particles in clouds (Hobbs and Alkezweeny 1968); and
  3. fragmentation of freezing droplets following their collision with ice particles in the cloud (riming), which is known as the Hallett–Mossop process (Hallett and Mossop 1974).

For the past four decades, heterogeneous ice nucleation has been parameterized as two independent functions of temperature (Cooper 1986; Fletcher 1962) and of supersaturation over ice or water (e.g., Huffman 1973; Meyers et al. 1992). There have also been several experimental attempts to represent concentrations of IN as a function of temperature and supersaturation (e.g., Berezinsky and Stepanov 1986; Cotton et al. 1986). Recently, several different heterogeneous ice nucleation parameterizations have been suggested that take into account that different types of aerosols can act as IN with different nucleating efficiency (Diehl and Wurzler 2004; Khvorostyanov and Curry 2004; Phillips et al. 2008; Connolly et al. 2009; DeMott et al. 2010).

A few published papers report ice crystal number concentrations in good agreement with the number concentrations of ice nuclei in orographic wave clouds, where secondary ice formation processes are avoided (Cooper and Vali 1981; Eidhammer et al. 2010). Some authors have even found a rapid formation of exceptionally high ice particle concentrations at cloud temperatures between −4° and −10°C (Koenig 1963; Mossop et al. 1968; Hallett et al. 1978; Hobbs and Rangno 1990; Rangno and Hobbs 1991; Choularton et al. 2008). Many published papers show large discrepancies between ice crystal concentrations in clouds and the number of IN in nearby clear air (Koenig 1963; Mossop et al. 1968; Hobbs 1969; Mason 1973; Hobbs 1974; Hallett et al. 1978; Langer et al. 1979; Keller and Sax 1981). Such large discrepancies can sometimes be explained by considering either primary (homogeneous and heterogeneous) or secondary ice formation processes (Bower et al. 1996), but in other cases the cause of the disagreement is unclear (e.g., Prenni et al. 2007).

Rosinski and Morgan (1991) suggested that, in convective clouds, evaporating droplets leave behind “dry” residues, which may act as IN activated either by condensation followed by freezing or by deposition, depending on relative humidity. Field et al. (2001) and Cotton and Field (2002) also found a greatly enhanced ice nucleation in the wave-cloud evaporation zone. The observation of ice nucleation in the downdraft phase of the wave cloud led Cooper (1995) to suggest that evaporating droplets would cool rapidly—perhaps enough to enhance the homogeneous freezing rate.

In most previous measurements the strong increase in IN concentrations with increasing supersaturation with respect to ice has not been considered sufficiently, along with the time lag for ice germs to grow to ice crystals (Isaac and Douglas 1972).

In natural clouds, however, there are circumstances that can lead to supersaturation with respect to water Sw considerably in excess of zero. It has been suggested (Gagin 1972; Nix and Fukuta 1974; Gagin and Nozyce 1984; Fukuta and Lee 1986; Rangno and Hobbs 1991) that the increased ice-nucleating ability of particles at high Sw might explain ice enhancement in some clouds.

Additional explanations for the observed discrepancy between IN and ice particles in clouds may certainly hinge on some deficiencies in counting the number of IN. In fact, IN may represent less than 1 in 106 of the aerosol population, presenting a difficult measurement challenge. It is now recognized that many circumstances result in erroneous measurements of small ice crystals because ice crystals larger than a few hundred microns can impact the probe’s upstream tips or inlet of the measuring instruments and shatter into small fragments (Field et al. 2006; Jensen et al. 2009; Korolev et al. 2011). Artifact crystal production can be extreme in some circumstances, reaching an enhancement factor of 100 (DeMott et al. 2011).

The occurrence of a thin layer of supercooled liquid water drops at the top of both stratiform and convective ice-phase clouds has been frequently observed (Cunningham 1951; Cooper and Vali 1981; Hobbs and Rangno 1985; Rauber and Grant 1986; Rauber and Tokay 1991).

Westbrook and Illingworth (2011), using 4 years of radar and lidar observations of layer clouds, showed that supercooled liquid water occurs at the top of the majority of ice-cloud layers warmer than −27°C, and is almost always present when the cloud-top temperature is greater than −20°C. The ice nucleation proceeds via the liquid phase at temperatures greater than −20°C, and deposition nucleation plays a minor role in the formation of ice at these temperatures. Ansmann et al. (2009) studied the formation of the ice phase in tropical altocumulus and found that the liquid droplets were always observed first, before the appearance of ice falling beneath, and concluded that ice nucleation mainly starts via the freezing of supercooled droplets. The process of droplet and ice formation was observed at cloud-top temperature lower than −10°C. Similarly, de Boer et al. (2011), from ground-based lidar, radar, and microwave radiometer observations of low- and midlevel Arctic stratiform clouds (<4-km altitude), observed that clouds composed entirely of ice occur less frequently than liquid-topped mixed-phase clouds at temperatures warmer than −25° to −30°C. These results indicate that ice formation generally occurs in conjunction with liquid at these temperatures and suggest the importance of the liquid-dependent ice-nucleation mechanism at T > −25°C.

Hobbs and Rangno (1985, 1990) and Rangno and Hobbs (1991) documented numerous instances of ice concentrations exceeding those that could be explained by primary nucleation (homogeneous and heterogeneous), even when the Hallett–Mossop riming–splintering process could not have produced ice concentrations of the magnitude that they observed in the time available. They gave a concise outline of their rationale in Hobbs and Rangno (1998). Their most salient conclusions were that the clouds glaciated much faster than the Hallett–Mossop theory could explain, the crystal habits were often not compatible with the temperature range in which the Hallett–Mossop mechanism operates, and high concentrations of small ice particles appeared concurrently with frozen drizzle drops, and not afterward, as would be expected if the smaller crystals were a product of riming–splintering. The authors concluded the final paper in the series with a summary of their findings (Rangno and Hobbs 1994). They admitted that the mechanism giving rise to the high concentrations of ice that they report was perplexing, closing with the statement “In summary, the origin of ice in cumuliform clouds remains a mystery.” This can be considered a conclusion similar to that of Vali (2004), who recognized that “there is ample evidence for unexpectedly high ice concentrations in many situations where the Hallett–Mossop mechanism is not active.”

Heymsfield and Miloshevich (1993) and Baker and Lawson (2006) found in wave clouds, from either all-water or mixed phase, a sudden reduction in the number concentration of small water particles, accompanied by an increase in the mean particle size and ice concentration. They also concluded that “most aspects of the observations are consistent with basic cloud physics, but some aspects remain difficult to interpret.” Similarly, Cantrell and Heymsfield (2005) stated “The Hallett–Mossop process does not explain some documented cases of secondary production. Is there another process operating, or is the Hallett–Mossop process not sufficiently understood?” In addition, Fridlind et al. (2007) wrote “The lack of correlation between ice particle number concentration and temperature points to the likelihood of a ‘universal mechanism’ for which no explanation currently exists, although evaporation nuclei and evaporation freezing are candidates.”

The aim of the performed experiments is to provide a contribution toward explaining the discrepancies observed between in-cloud ice crystal concentrations and the number of IN.

2. Experimental

The experimental instrumentation consisted of a Plexiglas cylinder (diameter of 12 cm and height of 140 cm) placed in a cold room (Fig. 1). It was operated at atmospheric pressure and at temperatures in the −12° to −14°C range, as this is the interval showing the highest difference between the vapor pressure of the water and of the ice. Liquid droplets of Milli-Q water (Millipore Corporation) were produced using an ultrasonic nebulizer (Aerosol Project–Artsana). The droplets were injected at the top of the column for a time of 90 s. During this process an opening at the bottom of the column allowed the gas and the droplets to exit. Therefore, the column was completely filled and droplets became supercooled in a few seconds.

Fig. 1.
Fig. 1.

A schematic diagram of the experimental apparatus (not scaled). Holes open only during droplet introduction (1), ice crystal introduction (2), and ice crystal and droplet measurements (3).

Citation: Journal of the Atmospheric Sciences 71, 1; 10.1175/JAS-D-13-0117.1

A similar ultrasonic nebulizer injected droplets into a syringe (volume of 120 cm3). The droplets were subsequently frozen by introducing for a few seconds a small wire cooled with liquid nitrogen, generating small ice crystals in the syringe. In a second step, the crystals were injected into the column full of supercooled droplets. In a time of the order of 25 s the droplets disappeared, and only crystals remained in the column. Crystals grew at the expense of water vapor from evaporating supercooled droplets and fell to the bottom of the column. The total number of ice crystal concentrations in the column and syringe were obtained by measuring the ice crystal concentrations with an optical particle counter (OPC, Grimm Model 1.108, Grimm Aerosol Technik, GmbH). The OPC measures the light elastically scattered from a single particle illuminated by a well-defined light source while it is passing through the sensing volume of the instrument. The scattered light intensity is utilized as a measure of the particle’s size. The sampling flow rate is 1.2 L min−1 with a size separation range of 0.3–20 μm (15 channels). The instrument is widely used in environmental aerosol monitoring (Grimm and Eatough 2009). The ice crystal counting was considered from 0.5 μm since the first size bin channel might overcount the particles, as shown in a recent comparison work (Belosi et al. 2013). The upper limit of size bin counting is given by the sampler inlet efficiency. Following the Agarwal and Liu criteria for sampling from still air (Agarwal and Liu 1980), the efficiency can be considered unitary until about 35 μm.

After nucleation, ice crystals generated in the column were sampled with the replica technique described by Schaefer (1956). Pieces of microscope slides (~1 cm2) fixed on stubs and covered with a thin layer of 2% formvar solution in chloroform were exposed to the falling crystals at the bottom of the column. The slides were then placed inside a desiccator, where crystals and chloroform evaporated. Subsequently, the stubs were examined by SEM.

In additional runs, maintaining the same experimental conditions (time of droplet injection, etc.) the size distribution of the droplets in the column was measured with the same OPC (Fig. 2). Sampling of droplets was also performed using the same procedure followed for the crystals (Fig. 3).

Fig. 2.
Fig. 2.

Average size distribution of the droplets in the column.

Citation: Journal of the Atmospheric Sciences 71, 1; 10.1175/JAS-D-13-0117.1

Fig. 3.
Fig. 3.

Replica of droplets on slides covered with formvar.

Citation: Journal of the Atmospheric Sciences 71, 1; 10.1175/JAS-D-13-0117.1

The relative humidity before droplet nucleation was measured by a dewpoint hygrometer (Hygro-M2, General Eastern Instrument, United States). The gas phase was found to be saturated with respect to water before the nucleation process. The liquid water content (LWC) of the cloud in the column was evaluated in two ways—that is, from the size distribution measured with the OPC, and by aspirating all the supercooled droplets from the column onto an inertial impactor located at the bottom of the column. This device has a small container at the bottom that is filled with liquid nitrogen. Therefore, the supercooled droplets that impact the surface of the impactor freeze instantaneously. The formed ice deposit was then transferred into a small plastic cylinder and weighed. The LWC turned out to be in the range of 1.2–2.5 g m−3. Both methods agree in the arithmetic droplet diameter and the LWC values.

To test the possible influence of the walls on the nucleation process, experiments were performed by filling the column with droplets, but without introducing crystals. As we did not observe nucleation of the droplets, the nucleation due to column walls can be excluded.

3. Results and discussion

Figure 4 shows the average size distribution of the ice crystals generated in a syringe outside the column (ICs) and the ice crystals in the column (ICc) after the complete droplet vaporization. The total number of ice crystals in the column and in the syringe are reported starting from 0.5 μm in size. Although the size distributions have to be considered qualitative, owing to the nonspherical shape of the ice particles, the counts should be considered accurate.

Fig. 4.
Fig. 4.

Average size distribution of the ice crystals in the column (ICc) and in the syringe (ICs).

Citation: Journal of the Atmospheric Sciences 71, 1; 10.1175/JAS-D-13-0117.1

The number of the crystals generated in the syringe and introduced at the top of the column turns out to be lower than the number of crystals measured at the base of the cloud chamber in all of the size ranges considered, the difference increasing with increasing crystal diameter. This means that the crystals inside the column grow at the expense of water vapor supplied by the evaporating droplets, linked to the fact that vapor pressure over ice eice is less than vapor pressure over liquid water (Wegener–Bergeron–Findeisen process). When ice crystals were not introduced in the column by the syringe, the supercooled droplets remained liquid and settled.

Figure 5 shows the averaged total number of ice crystals in the column, after the complete droplet evaporation, and the number of small ice crystals introduced at the top of the column. The figure shows also the minimum and maximum values obtained in all the experiments. Considering all the experiments, the mean number of ice crystals in the column, after complete droplet vaporization, is found to be higher than the mean number of ice crystals introduced into the column. The ratio is about 10:1. The increase in the number of ice crystals measured in the column, compared to those introduced through the syringe, is not due to the presence of ice nuclei particles. In fact, preliminary measurements inside the column with a condensation particle counter (CPC, TSI Model 3775) gave a very low aerosol particle number concentration. Therefore, aerosol ice nuclei were absent in the column and heterogeneous nucleation processes due to aerosol particles should be excluded. In addition, the measured time necessary to obtain a column completely filled by crystals (about 25 s), after the introduction of ice crystals from the syringe, is lower than the sedimentation time of the crystals from the top of the column, where nucleating ice crystals were introduced. It should be noted that even the largest crystals sampled at the bottom of the column (about 50 μm in diameter) should have a terminal velocity of about 4 cm s−1 and a falling time of about 35 s (Heymsfield 1972). Ice crystal fallout was complete within 2–4 min. The nucleation process appears to be much faster than the growth and subsequent fallout of the crystals.

Fig. 5.
Fig. 5.

Mean, maximum, and minimum values of ICc and ICs.

Citation: Journal of the Atmospheric Sciences 71, 1; 10.1175/JAS-D-13-0117.1

In our experimental runs, the secondary mechanism of ice generation should be excluded, since the riming process is absent, owing to the lack of turbulence and the fact that droplets are prevalently less than 20 μm (mean diameters of 9.9 and 7.8 μm, respectively, counting the droplets on microscope slides or considering the size distribution measured with OPC). The image of crystals sampled at the bottom of the column, after complete droplet evaporation, confirms this statement. Ice crystals were prevalently hexagonal plates and did not show any breaking on tips (Fig. 6). We performed in addition experiments at T = −5°C (i.e., at a temperature where the Hallett–Mossop effect could be effective), but no increase in the crystal concentration was observed in the column compared to values in the syringe.

Fig. 6.
Fig. 6.

Replica of the ice crystals captured on glass slides.

Citation: Journal of the Atmospheric Sciences 71, 1; 10.1175/JAS-D-13-0117.1

The experiments undertaken seem to show that the thermodynamically unstable system considered (supercooled droplets) promptly passed to a more stable state (Murray et al. 2008). The presence of small ice crystals (introduced at the top of the column) in the unstable system appears to trigger the nucleation process in the whole supercooled liquid cloud. Despite the wide variety of techniques used in the study of homogeneous freezing, relatively few consider the freezing of evaporating water droplets (Kuhns and Mason 1968; Krämer et al. 1996; Shaw and Lamb 1999).

A possible explanation of our laboratory results could be that evaporating droplets rapidly cool sufficiently to freeze, even at higher air temperatures (about −13°C) than those usually reported for homogeneous nucleation in wave clouds (e.g., Heymsfield and Miloshevich 1993; Field et al. 2001). The great decrease in droplet temperature during evaporation could depend on the fact that, in the interaction mechanism between droplets and ice crystals, a stationary field is not expected to form. Fuchs (1959) showed that the thermal relaxation time of the droplet in evaporation (τth = r2/α, where r is the droplet radius and α is the thermal diffusivity) is much longer than the relaxation time of the water vapor diffusion field (τvd = r2/D, where D is the water vapor diffusion coefficient). Therefore, a marked decrease in droplet temperature can occur during a fast evaporation. This conclusion agrees with the results of Satoh et al. (2002), who studied the cooling–freezing phenomena of a water droplet of pure distilled water due to evaporation in an evacuated chamber. The results showed that the water droplet is cooled by the evaporation of water itself and that freezing starts from its surface. The droplet froze within a few seconds through a remarkable supercooling state, and the cooling rate of the water droplets was dominated by heat transfer within the droplet. Similar conclusions were shown by Murty and Murty (1972) and Jung et al. (2012), who obtained a homogeneous nucleation of evaporating droplets at the free surface (instead of heterogeneous nucleation due to the presence of the substrate) at the initial temperature of −15°C.

The hypothesis that, even in a simple system like pure water or nitric acid solution, the ice nucleation may occur at the air–droplet interface, and that the homogeneous crystallization may indeed be a surface- and not a volume-related rate process, was theoretically and experimentally suggested by Djikaev et al. (2002) and Tabazadeh et al. (2002). This conclusion goes against the classical theory of homogeneous crystallization, where freezing is assumed to initiate inside a droplet and to propagate afterward to the surface.

A second mode of ice formation suggested by D. L. Mitchell (Desert Research Institute, 2013, personal communication) is as follows. The very high liquid water content implies a high cloud droplet concentration. As terminal velocity of droplets and crystals are similar, ice crystals introduced in the column and droplets can be in close proximity for a relatively long period of time, where a strong vapor flux from the droplet to the crystals is manifested. During such an event, the most energetic water molecules in the droplet are likely to evaporate first, leaving lower-energy molecules behind. The lower-energy molecules are more likely to be hydrogen bonded in a hexagonal geometry similar to that of hexagonal ice. This hexagonal orientation in the liquid phase is likely to trigger a freezing event at these initial temperatures (−12° to −14°C), thus freezing the droplet, which then grows into an ice crystal.

In conclusion, it is our opinion that the performed laboratory experiment provides useful information on the problem of the large discrepancies between ice crystal concentrations in clouds and the number of ice nuclei in nearby clear air reported in published papers. Additionally, it offers support to field campaigns, where a thin layer of supercooled liquid water drops at the top of both stratiform and convective ice-phase clouds is reported, and ice nucleation mainly starts via the freezing of supercooled droplets, even at cloud-top temperatures of up to −10°C (Cooper and Vali 1981; Ansmann et al. 2009; de Boer et al. 2011; Westbrook and Illingworth 2011).

Acknowledgments

We are grateful to David L. Mitchell (Desert Research Institute, Reno, Nevada) for helpful suggestions and two anonymous reviewers.

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