Graupel Pellets Research Articles

Tropical Oceanic Hot Towers: Need They Be Undilute to Transport Energy from the Boundary Layer to the Upper Troposphere Effectively? An Answer Based on Trajectory Analysis of a Simulation of a TOGA COARE Convective System

Abstract This paper addresses questions resulting from the authors’ earlier simulation of the 9 February 1993 Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Research Experiment (TOGA COARE) squall line, which used updraft trajectories to illustrate how updrafts deposit significant moist static energy (in terms of equivalent potential temperature θe) in the upper troposphere, despite dilution and a θe minimum in the midtroposphere. The major conclusion drawn from this earlier work was that the “hot towers” that Riehl and Malkus showed as necessary to maintain the Hadley circulation need not be undilute. It was not possible, however, to document how the energy (or θe) increased above the midtroposphere. To address this relevant scientific question, a high-resolution (300 m) simulation was carried out using a standard 3-ICE microphysics scheme (Lin–Farley–Orville). Detailed along-trajectory information also allows more accurate examination of the forces affecting each parcel’s vertical velocity W, their displacement, and the processes impacting θe, with focus on parcels reaching the upper troposphere. Below 1 km, pressure gradient acceleration forces parcels upward against negative buoyancy acceleration associated with the sum of (positive) virtual temperature excess and (negative) condensate loading. Above 1 km, the situation reverses, with the buoyancy (and thermal buoyancy) acceleration becoming positive and nearly balancing a negative pressure gradient acceleration, slightly larger in magnitude, leading to a W minimum at midlevels. The W maximum above 8 km and concomitant θe increase between 6 and 8 km are both due to release of latent heat resulting from the enthalpy of freezing of raindrops and riming onto graupel from 5 to 6.5 km and water vapor deposition onto small ice crystals and graupel pellets above that, between 7 and 10 km.

Charge separation in low-temperature ice cloud regions

[1] New laboratory measurements of graupel pellet charging due to collision with ice particles at low temperature are presented. The experiments were carried out in the temperature range −37°C to −47°C, with an impact velocity of 7 m s−1 and in the absence of supercooled liquid water. The graupel pellet was simulated with a previously rimed brass cylinder of 4 mm diameter, and the small ice particles were formed by natural freezing of supercooled water droplets at low temperature without seeding the cloud. The effective ice water content used in the experiments was between 0.25 and 0.33 g m−3. Cloud particle samples show small ice particles with diameters up to 24 μm. The results show that the sign of the charging current acquired by the graupel is predominantly negative, and its magnitude ranges from 0 to −150 pA; although there is a significant dispersion of data, a marked dependence on temperature is not observed. It is estimated that the magnitude of the charge transfer per collision is between 0.01 and 0.1 fC; the charging rate of a graupel pellet of 4 mm diameter then would be about 300 pC min−1. Based on this experimental evidence, we suggest that the charging mechanism associated with graupel–frozen droplet collision and separation may be relevant in clouds whose internal temperatures are substantially lower than −37°C and could be the main generator for high-altitude lightning.

Quantifying Snowfall Rates using Underwater Sound

It is well known that rainfall rate can be estimated by analyzing the spectral character and level of the underwater sound that it generates. There have been a few field reports of increased sound levels associated with the occurrence of snow; these reports suggest an increase in sound levels above 20 kHz. It has previously been demonstrated that snowflakes can generate both a small impact sound and a separate high frequency pulse that is consistent with the sound generated by a resonant bubble. One aspect that these earlier studies have not explored is the dependence of this sound generation on the type or intensity of snowfall. We report on observations of sound generated by snow falling into a tank of water, quantifying snowfall rate using an Optical Scientific precipitation gauge. Recorded signals allowed analysis of frequencies between 1 and 50 kHz. Using the classification scheme of the International Association of Hydrological Sciences for snow, seven distinct snow types, as well as rain and freezing rain, were observed with a range of precipitation rates. Snow types that produced a signal included column, needle, irregular crystal, graupel and ice pellet. Snow types for which no signal was detected were plate, stellar crystal and spatial dendrite. The previously reported rise in high frequency sound could not be distinguished unambiguously in the present data. A small peak at around 12 kHz was seen in spectra of some snow types similar to the characteristic 14 kHz peak seen in some rain-generated sounds. There was a clear correlation between sound level and snowfall rate at frequencies above 10 kHz.

Measurements of electric charge separated during the formation of rime by the accretion of supercooled droplets

Abstract. In these experiments, the electric charge carried by single particles ejected from the surface of a graupel particle growing by riming was measured. Simulated graupel pellets were grown by accretion of supercooled water drops, at temperatures ranging from −2 to −10 °C in a wind tunnel at air velocities between 5 and 10 m s−1, with the goal of studying the charging of graupel pellets under conditions of secondary ice crystal production (Hallett-Mossop mechanism). The graupel, and induction rings upstream and downstream of the graupel, were connected to electrometers and analyzing circuits of sufficient sensitivity and speed to measure, correlate and display individual charging events. The results suggest that fewer than 1% of the ejected particles carry a measurable electric charge (>2 fC). Further, it was observed that the graupel pellets acquire a positive charge and the average charge of a single splinter ejected is −14 fC. This mechanism of ejection of charged particles seems adequate to account for a positive charge of around 1 pC that individual precipitation particles of mm-size could acquire in the lower part of the cloud, which in turn could contribute to the lower positive charge region of thunderstorms.

The development of ice in a cumulus cloud over southwest England

An experiment involving the FAAM BAe 146 aircraft, called the ICE and Precipitation Initiation in Cumulus (ICEPIC) project, was conducted in order to measure the microphysical properties of UK summertime cumulus clouds. A line of clouds was penetrated near the ascending tops. Higher concentrations of ice particles than expected from activation on typical ice nuclei using the Meyers formula were observed at relatively high temperatures (T>-10 °C). The observations of numerous ice particles and the coexistence of both small and large cloud droplets, pristine ice columns and graupel pellets within the temperature zone of -3 to -9 °C strongly suggested the Hallett–Mossop (HM) process of splintering during riming. Agreement between the calculated and observed rates of splinter production supported this suggestion. The Model of Aerosols and Chemistry in Convective Clouds (MAC3) was utilized to establish a quantitative understanding of the observed development of glaciation of this cloud. The results of the model confirmed the important role of the HM process. They also showed that the warm rain process was fundamental to the production of graupel in the cloud studied, and hence the HM ice particles. A sensitivity test with double the concentration of aerosol particles showed that the concentration of supercooled raindrops decreased as expected, which resulted in fewer graupel particles and a smaller quantity of precipitation, which was delayed by about 5 min. However, the production rate of secondary ice particles generated by the HM process increased due to the increased concentration of small cloud droplets.

Field identification of a unique globally dominant mechanism of thunderstorm electrification

AbstractTwo wholly distinct studies involving TRMM‐satellite global data were conducted. One involved the relationship between lightning frequency f and brightness temperature, the other between f and ice‐water‐path. Both studies demonstrate that globally valid relationships exist between f and thundercloud ice‐precipitation content, from which it follows that graupel pellets play a crucial role in thundercloud charging. Ground‐based field studies provide further support for this conclusion and show that f is also strongly dependent upon the ice crystal content. All these findings are consistent with the non‐inductive charging mechanism, but not with any other proposed mechanism of thunderstorm electrification. We conclude that the non‐inductive mechanism dominates electric field growth and lightning production in all seasons–for both oceanic and terrestrial thunderstorms–on a global scale. Copyright © 2007 Royal Meteorological Society

The Subaquatic Water Layer

There is now ample evidence for the existence of a crystalline organization of nanoscopic water layers at the interface of differently polar materials. Nanoscopic water layers were first observed in air. Surprisingly, they persist in subaquatic milieus, even under nonstationary conditions, e.g., at the interface between hydrophobic substrates and evaporating water drops known to be subject to massive convective flows, capable of driving micron-sized particles to the periphery of drops. The practical importance of organized water layers is enormous. They have been identified as transmitters of first-contact information in biosystems and are a key determinant for the design of advanced biomaterials. They play a central role in the transfer of charge between ice crystals and graupel pellets, and provide an explanation to the (otherwise unexplainable) capacity of hydrophobic aerosols to act as cloud condensation nuclei and ice crystal nucleators. However, direct observation of the organization of nanoscopic w...

Laboratory studies of the effect of cloud conditions on graupel/crystal charge transfer in thunderstorm electrification

AbstractCollisions between vapour‐grown ice crystals and a riming target, representing a graupel pellet falling in a thunderstorm, were shown by Reynolds, Brook and Gourley to transfer substantial charge, which they showed to be adequate to account for the development of charge centres leading to lightning in thunderstorms. Related experiments by Takahashi and Jayaratne et al. determined that the sign of charge transferred is dependent on the cloud liquid water content and on cloud temperature. There are marked differences between the results of Takahashi and Jayaratne in the details of the dependence they noted of the sign of graupel charging on cloud water and temperature. More recently, Pereyra et al. have shown that results somewhat similar in form to those of Takahashi are obtained by modifying the experimental technique used to prepare the clouds of ice crystals and supercooled water droplets used in the experiments.In order to help resolve the reason for the differences in charge transfer results in various studies, work has continued in the Manchester laboratory with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently. Results indicate a profound effect on the charge sign of the particle growth conditions in the two clouds involved. For example, by suitable adjustments to the water contents of the two clouds, graupel is charged negatively by rebounding ice crystal collisions at higher cloud water contents than have been noted previously. It is suggested that the most important influence on charge sign is the relative diffusional growth rate of the two ice surfaces at the moment of impact and that this is affected by an increase in cloud supersaturation experienced by the ice crystals during the cloud mixing process just prior to collision. A range of cloud conditions is used in the present work in order to help determine the reasons for the various results reported previously.Examination of some thunderstorm observations in the context of the present results points to the importance of mixing on the sign of the charge transferred during particle collisions when two cloud regions of different histories mix together. Copyright © 2006 Royal Meteorological Society

Electrification vs Crystallization: Principles to Monitor Nanoaerosols in Clouds

Models and laboratory experiments indicate that geometrical asymmetries between ascending ice crystals and falling graupel pellets, as well as the physical conditions under which ice crystals are nucleated and grow, have a strong impact on the intensity and direction of the charge transferred between these ice particles. Aerosols in the nanometer size regime are particularly interesting as ice crystal nuclei: they have a long dwell-time in the atmosphere, maximum number count, and high surface-to-volume ratios and charge fields. Depending on their polarity, they can also act as cloud condensation nuclei. Polarities vary in a wide range from hydrophobic (fresh soot) to hydrophilic (for example, soot coated with sulfate, proteins, spores, and nanobacteria). Attached to ice crystals, they can accentuate preexisting surface curvature differences between colliding ice particles and thereby charge transfer efficiency, predominantly in warm clouds over polluted areas. Despite the large concentrations of nanoaerosols in the terrestrial atmosphere, their direct impact on charging phenomena has not been considered before. Soot aerosols from natural and industrial sources or emitted from aircraft jet engines into the upper troposphere and lower stratosphere affect climate by absorption and scattering of light. Studying in situ their interaction with ice crystals or cloud droplets is practically impossible, and laboratory experiments are very difficult. Here we present an experimental method that permits us to simulate charging effects between differently polar proximal surfaces, aerosol and hydrometeor, via drops of organic liquids (for instance, toluene) rotating in water. Rotational form stability of the drop provides a setting to mimic solid aerosols. The observed effect of charge separation justifies the view that soot, and probably other aerosols, contribute to lightning enhancement in convective systems. Conversely, aerosol concentration in the atmosphere could be estimated by monitoring lightning activity globally.

On the relationship of thunderstorm ice hydrometeor characteristics and total lightning measurements

Satellite-borne and ground-based devices for the detection of lightning offer the opportunity to explore relationships–on all significant scales up to global–between lightning frequency, f, and other thundercloud parameters. Calculations predict that f is proportional to the product of the downflux p of solid precipitation and the upward mass flux, I, of ice crystals. This prediction has received support from limited computational studies. The physical reasons for such a relationship are explained in terms of the paramount role of ice in the electrification of thunderstorms. Herein, this prediction is subjected to further, preliminary examination through analysis of lightning and dual-polarimetric radar data collected during the STERAO experiment conducted in Northern Colorado during the summer of 1996. The analysis has yielded some highly provisional support for this flux hypothesis. Computed trends of radar derived hydrometeor fractions of solid precipitation and small ice show correlation to the total lightning frequency and raise the possibility of determining values of p and/or I from lightning measurements. It is shown that the extent to which the observed correlations between f and both solid precipitation and small ice trends are or are not strong can provide an indication as to whether the lightning activity is limited by the available concentrations of precipitating or non-precipitating ice in the upper regions of the charging zone of the thundercloud, where most of the charge transfer occurs. It is demonstrated that the most accurate determinations of precipitation rate p from measurements of lighting frequency f are likely to be for conditions where the field-growth is limited by the availability of graupel pellets. It is shown that the simultaneous time variations of f and solid precipitation trends of the type obtained in the STERAO experiment could enable us to determine the nature of the dominant glaciation process operative in the thunderclouds studied.

Charge sign reversal in irregular ice particle‐graupel collisions

Laboratory measurements of the sign of electric charge transfer separated in rebounding collisions between large ice crystals and a graupel pellet growing by riming are analyzed. In these experiments, the usual negative rimer charging regime noted at temperatures below about −10°C at moderate liquid water contents is reversed to positive when the crystals have grown larger. The experiments were performed with an impact velocity of 5.5 m s−1, the ambient temperature was varied in the range −15 to −26°C with an effective water content from 0.5 to 3 g m−3. A cloud particle imager shows a clear correlation between the rimer charge sign reversal to positive and the presence of a high proportion of irregular and large ice crystals or aggregates in the cloud. The results suggest that in collisions between graupel and these ice particles the graupel will charge positively while the irregular particles carry away a negative charge. These are the first measurements of this kind of collision and the findings may have important implications in the thunderstorm electrification process.

Controlling Arrangement of 60 nm Nanospheres in Evaporating Sessile Drops with Low Level Laser Light

The interplay between liquid flow and surface interaction can arrange colloidal particles in drops evaporating on substrates into crystalline rings. Recently, it was reported that magnetic fields can affect the crystallinity in rings formed by suspensions containing paramagnetic particles. Here, it is demonstrated that laser light, applied at intensities as low as 1000 W m -2 , has a manifest impact on the attachment and self-organization of 60 nm particles on wet substrates. Results indicate that the physical state of water layers attached to substrates can be modulated by low level laser light even in an aqueous milieu, and imply the subaquatic persistence of an ordered interlayer formed by molecules with less degrees of freedom, as compared to those in the bulk liquid. Low level laser light has already been shown to change the height of nanoscopic water layers on substrates in air. Nanoscopic water layers are not only important in determining the arrangement of nanoparticles into two-dimensional crystals, they also control the lifetime of scanning near-field optical microscopy sensors, adhesion processes on biochips, nonspecific interactions at biomaterial/cell interfaces, charge transfer between ice crystals and graupel pellets in thunderclouds, and presumably the attachment of nanobacteria and proteins to tissues in blood. Modalities by which laser light and nanoscopic water layers cooperatively control the attachment of nanoparticles to substrates are illustrated in representative scenarios. Results could be converted into various practical applications, especially in nanotechnology and biomedicine, and unlock a novel and promising field of research.

A laboratory study of the influence of ice crystal growth conditions on subsequent charge transfer in thunderstorm electrification

AbstractLaboratory studies of a thunderstorm charging mechanism involving rebounding collisions between ice crystals and riming graupel pellets, have shown the importance of the growth conditions of the interacting ice particles on the sign of the charge transferred. The present study shows a new result: if an ice crystal is not in thermal equilibrium with the environment (immediately following the mixing of two clouds at different saturations) the crystal surface may experience an enhanced growth rate that can influence the sign of the charge transfer and promote negative rimer charging. Furthermore, when an ice crystal in ice saturation conditions is introduced to a cloud at water saturation, leading to transient growth and heating, the period of thermal non‐equilibrium is shown to be sufficiently brief that the enhanced negative rimer charging is short lived. These results suggest that the earlier conclusions of Berdeklis and List—that the cloud saturation conditions around a growing ice crystal impart to the crystal surface a property that is carried with it and that influences the sign of subsequent charge transfer—are unfounded. The discrepancy is because in their laboratory simulations of thunderstorm conditions there is adequate time for the growing ice crystal surface to come to equilibrium with its environment. The established concept of the relative diffusional growth rate of the interacting surfaces controlling the sign of charge transfer, such that the faster growing surface charges positively, is consistent with the observations. Copyright © 2004 Royal Meteorological Society

Image processing of snow particles and classification into snowflakes and graupel

AbstractTo analyze the relationship between the radar reflectivity and the intensity of a snowfall, it is important to know the percentages of snowflakes and graupel in snow particles. This paper proposes an automatic method of classification of falling snow particles. The method consists of two CCD cameras which record each particle simultaneously with different shutter speeds and magnifications. The first CCD camera has a slow shutter speed (1/60 s) and a relatively wide view angle, and records the trailing image and diameter of each falling snow particle so that its fall velocity is obtained, and this camera also records the distribution of diameters of the snow particles. The second CCD camera has a higher shutter speed (1/8000 s) and a narrow view field (1/5 the first CCD camera), and records a binary image and a gray‐level image of each snow particle so that the shape parameter of each particle can be obtained. Each snow particle is classified as a snowflake or a graupel pellet by image processing based on the fall velocity and shape parameter. The proposed method has been successfully tested in snowfall (continuously for 2 weeks), obtaining the percentages of snowflakes and graupel once a minute in real time. © 2002 Scripta Technica, Syst Comp Jpn, 33(2): 43–53, 2002

A laboratory study of the effect of velocity on Hallett–Mossop ice crystal multiplication

Laboratory experiments into the effect of ice multiplication in clouds by the Hallett–Mossop (H–M) mechanism have been extended to higher velocities than in earlier studies. The secondary ice particle production rate per milligram of rime accreted is found to be dependent on the velocity of the graupel pellet as it accretes supercooled droplets. In the range 1.5 to 12 m s −1, the maximum secondary ice particle ejection occurs at 6 m s −1 with a rate of one crystal per 70 mg of accreted rime. The relevance of these new results to the particle production mechanism and to behaviour in natural clouds is discussed.

Determination of ice precipitation rates and thunderstorm anvil ice contents from satellite observations of lightning

The continuous satisfactory functioning of satellite-borne devices for the detection of global lightning offers the opportunity to explore relationships between lightning frequency f and other thundercloud parameters, notably, in this paper, the precipitating and non-precipitating contents and fluxes of ice. Simple calculations predict that the lightning frequency f is proportional to the product of the downward flux of solid precipitation through the body of the thundercloud and the upward flux of ice crystals into its anvil. This prediction is reinforced by computations performed using the multiple lightning model of Baker et al. [Q. J. R. Meteorol. Soc. 121 (1995) 1525; Atmos. Res. 51 (1999) 221]. Calculations indicate that the separation of charge and associated field development in thunderclouds are not significantly limited by charge saturation of the interacting hydrometeors: and that the mutual interactions of graupel pellets in the charging zones of thunderstorms can significantly enhance electric field development, culminating in lightning. An examination of data from the satellite-borne Lightning Imaging Sensor (LIS) and TRMM Microwave Imager (TMI) suggests that thunderstorms with the highest frequency of total lightning also possess the most pronounced microwave scattering signatures at 37 and 85 GHz. A total of 292 individual thunderstorms were examined, and a log-linear relationship was shown to exist (one for each frequency) between the number of optical lightning pulses produced by each storm and the corresponding microwave brightness temperatures. These relationships are consistent throughout the seasons in a variety of regimes (12 sites encompassing five continents, as well as oceanic measurements), suggesting that global relationships may be found to exist between lightning activity and cloud ice content.

A laboratory study of graupel charging

Measurements have been made of charge transfer when vapor grown ice crystals rebound from a riming target representing a graupel pellet falling in a thunderstorm. Earlier studies in the laboratory in Córdoba of charge transfer between an individual falling ice sphere and a riming target noted that the sign of the charge transfer was dependent upon temperature and effective liquid water content (EW). The new work uses a similar experimental technique; however, a cloud of ice crystals is grown in order to study multiple interactions with the riming target. The results also show charge sign dependence on temperature and EW; positive rimer charging is observed at high temperatures and for low and high values of EW at low temperature, while negative rimer charging is noted at low temperatures for intermediate values of EW. These results are similar to those obtained by Takahashi (1978) and, as has been reported before, are rather different from those obtained in Manchester by Jayaratne et al. (1983), Saunders et al. (1991), and Saunders and Peck (1998). Significant differences between the two types of data sets are attributed to the experimental techniques used in the various studies. In the present work the ice crystal cloud and the cloud of supercooled droplets responsible for riming the target are grown in separate chambers and then mixed shortly before the crystals and droplets encounter the riming target, so that the droplet cloud is not depleted by the growing ice crystals. In the Manchester experiments, the ice crystals grow in the same supercooled droplet cloud used to rime the target. It is possible that the mixing process provides an undepleted droplet cloud and a transient enhanced vapor supply that affects both the ice crystal and graupel vapor depositional growth rates, leading to the present results.

Dependence of the average surface temperature on cloud droplet spectra for rime ice accreted on fixed spherical collectors

The heat balance of a riming spherical target, representing a graupel pellet or small hailstone in a thunderstorm, has been studied in the laboratory in Manchester as a function of cloud droplet size. With droplets of mean volume diameter between 18 and 33 μm, a riming target of 5 mm diameter, temperatures of −6° and −15°C, and velocities between 4 and 8 m s−1, the effect of the heat rise of the graupel was measured for a range of cloud effective liquid water contents (EW). For a constant rate of rime accretion, the heating effect of the droplets was found to increase with droplet size, the effect being more pronounced at lower temperatures and higher velocities. The value of χ, a numerical term in the ventilation coefficient, was also found to be dependent on the droplet size, which implies a new dependence of the Nusselt number, a measure of ventilation, on droplet size. The results were viewed as being indicative of a droplet collision efficiency effect with smaller droplets producing a rougher, better ventilated surface which is cooler than a rime surface produced with larger droplets. The results show that the required value of EW to achieve wet growth increases with smaller droplets, and they indicate that the collisional charging of riming graupel pellets in thunderstorms, which is influenced by the surface temperature of the rime and its effect on vapor diffusion, will be more negative with larger droplets for which the rime structure is less readily ventilated.

The effect of the cloud‐droplet spectrum on electrical‐charge transfer during individual ice‐ice collisions

AbstractExperiments were conducted with a wind tunnel in a cold room, in order to investigate the influence of the cloud‐droplet spectrum on the charges transferred when individual ice spheres collided with a fixed artificial graupel pellet growing by riming. the experiments were carried out with ice spheres of about 100 μm in diameter, impact velocities around 4 m s−1, temperatures between −10°C and −30°C and effective water contents representative of real clouds. Two different cloud‐droplet spectra were used. One had more than 30% of the droplets with sizes greater than 13 μm, and the other had more than 50% of the droplets greater than that. the new results show that the size distribution of the droplets is very important to the sign of electric charge transferred. the target graupel charged positively over all the temperature range covered when the smaller‐droplet spectrum was used, but negatively at temperatures below −18°C for the larger‐droplet spectrum. These results show the importance of droplet sizes to thunderstorm charging.

A laboratory study of the effects of rime ice accretion and heating on charge transfer during ice crystal/graupel collisions

In a laboratory study of thunderstorm electrification involving charge transfer between ice crystals and a riming graupel pellet, the effect on charge transfer of rimer heating by droplet accretion has been separated from the associated influence of the vapour flux to the rimer surface. This was accomplished by heating internally a riming target rod whose surface conditions represent those of a falling graupel pellet in thunderstorms while keeping the rate of rime accretion constant. The results show that the positive charging of a rimer may be reversed to negative by artificial heating, with increased heat required at higher rates of rime accretion. It is hypothesised that ice crystals rebounding from riming graupel pellets charge the graupel positively or negatively depending on the cloud and rimer conditions, which influence the relative thicknesses of charge carrying layers on the surfaces of the particles. In the natural case, negative charging of graupel is associated with rime surface heating, which reduces the vapour diffusional growth rate below that of the ice crystals, while positive charging of graupel is associated with vapour provision to the rimer surface from the freezing droplets, which overcomes the rime heating effect. This work compares the results of charge transfer to a riming target obtained in UMIST Manchester, involving multi-crystal interactions, with data from Cordoba Argentina involving single ice sphere interactions. The fact that broadly similar charging behaviour was seen in both studies suggests that it is the rate of growth of the ice surfaces, rather than their particular nature, that is the important factor in controlling the charge transfer during ice particle collisions with a riming ice surface.