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C P R Saunders - One of the best experts on this subject based on the ideXlab platform.
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the influence of supersaturation at low rime accretion rates on thunderstorm electrification from field independent Graupel ice crystal collisions
Atmospheric Research, 2020Co-Authors: Christopher Emersic, C P R SaundersAbstract:Abstract Laboratory cloud chamber experiments have revealed the significance of cloud supersaturation on field-independent Graupel-ice crystal charging associated with thunderstorm electrification. At rime accretion rates below approximately 1 g m−2 s−1, altering supersaturation can reverse the polarity of Graupel charging when other conditions are maintained. Explanations are given to reconcile variation between many published observations by considering the factors associated with particle surface diffusional growth that contribute to the resulting polarity of charge. Findings are consistent with the Relative Diffusional Growth Rate hypothesis. Variations between observations can be qualitatively accounted for by considering the effects of ice crystal thermal preconditioning, crystal size, and cloud supersaturation. The varieties of observed charge polarity reversal as a function of cloud temperature and liquid water content are also characterised. Recommendations are given for future laboratory experimental measurements to avoid ambiguity during interpretation.
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a modelling study of the effect of ice particle sizes and relative velocity on ice crystal Graupel collisional charge transfer
Atmospheric Research, 2009Co-Authors: Boryana Tsenova, R Mitzeva, C P R SaundersAbstract:Abstract The effect of ice particle sizes and relative velocity on the sign of charge transfer during Graupel/ice crystal interactions in thunderstorms is considered based on the concept of the Relative Growth Rate Hypothesis that the ice surface growing faster by water vapour diffusion charges positively. Diffusional growth rates of ice crystals and riming Graupel particles are calculated for various particle sizes and velocities, and the sign of charging during crystal/Graupel collisions is determined. The study reveals that larger Graupel charges more negatively, while larger ice crystals, or higher relative velocity, lead to increased positive Graupel charging.
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laboratory studies of the effect of cloud conditions on Graupel crystal charge transfer in thunderstorm electrification
Quarterly Journal of the Royal Meteorological Society, 2006Co-Authors: C P R Saunders, Eldo E Avila, Christopher Emersic, H Baxnorman, Nesvit E CastellanoAbstract:Collisions 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
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charge sign reversal in irregular ice particle Graupel collisions
Geophysical Research Letters, 2005Co-Authors: Eldo E Avila, C P R Saunders, Holly Baxnorman, Nesvit E CastellanoAbstract:[1] 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.
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a laboratory study of the influence of water vapour and mixing on the charge transfer process during collisions between ice crystals and Graupel
Atmospheric Research, 2001Co-Authors: C P R Saunders, Eldo E Avila, S L Peck, Aguirre G Varela, Nesvit E CastellanoAbstract:Abstract Laboratory experiments, in which vapour grown ice crystals interact with riming Graupel targets, simulate charging processes in thunderstorms. The introduction of cooled, moist, laboratory air into a supercooled droplet and ice crystal cloud enhances charge transfer and, when the air-stream is directed at the riming target, can reverse its charge sign. The suggestion is that the extra water vapour introduced increases the supersaturation and influences particle diffusional growth. The results have been considered in terms of the Relative Growth Rate Hypothesis, which states that the interacting ice surface growing fastest by vapour diffusion charges positively. A corollary to this was noted, when dry air is introduced into a cloud of ice crystals so that both the crystals and target surface sublimate, the ice surface that sublimates fastest charges negatively. The experiments are relevant to considerations of the reasons why earlier sets of charge transfer results give different liquid water and temperature boundaries between positive and negative Graupel charge sign. The differences appear to be connected to the techniques used, in particular, to the mixing of separate droplet and ice crystal clouds before riming, which can lead to positive rimer charging in conditions of low-rime accretion rate, as observed in the present study. Further work is needed to resolve questions concerning the most naturally representative manner of performing these laboratory simulations.
Nesvit E Castellano - One of the best experts on this subject based on the ideXlab platform.
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laboratory studies of the effect of cloud conditions on Graupel crystal charge transfer in thunderstorm electrification
Quarterly Journal of the Royal Meteorological Society, 2006Co-Authors: C P R Saunders, Eldo E Avila, Christopher Emersic, H Baxnorman, Nesvit E CastellanoAbstract:Collisions 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
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charge sign reversal in irregular ice particle Graupel collisions
Geophysical Research Letters, 2005Co-Authors: Eldo E Avila, C P R Saunders, Holly Baxnorman, Nesvit E CastellanoAbstract:[1] 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.
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a laboratory study of the influence of water vapour and mixing on the charge transfer process during collisions between ice crystals and Graupel
Atmospheric Research, 2001Co-Authors: C P R Saunders, Eldo E Avila, S L Peck, Aguirre G Varela, Nesvit E CastellanoAbstract:Abstract Laboratory experiments, in which vapour grown ice crystals interact with riming Graupel targets, simulate charging processes in thunderstorms. The introduction of cooled, moist, laboratory air into a supercooled droplet and ice crystal cloud enhances charge transfer and, when the air-stream is directed at the riming target, can reverse its charge sign. The suggestion is that the extra water vapour introduced increases the supersaturation and influences particle diffusional growth. The results have been considered in terms of the Relative Growth Rate Hypothesis, which states that the interacting ice surface growing fastest by vapour diffusion charges positively. A corollary to this was noted, when dry air is introduced into a cloud of ice crystals so that both the crystals and target surface sublimate, the ice surface that sublimates fastest charges negatively. The experiments are relevant to considerations of the reasons why earlier sets of charge transfer results give different liquid water and temperature boundaries between positive and negative Graupel charge sign. The differences appear to be connected to the techniques used, in particular, to the mixing of separate droplet and ice crystal clouds before riming, which can lead to positive rimer charging in conditions of low-rime accretion rate, as observed in the present study. Further work is needed to resolve questions concerning the most naturally representative manner of performing these laboratory simulations.
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a laboratory study of Graupel charging
Journal of Geophysical Research, 2000Co-Authors: Rodolfo G Pereyra, Eldo E Avila, Nesvit E Castellano, C P R SaundersAbstract: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 Cordoba 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.
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dependence of the average surface temperature on cloud droplet spectra for rime ice accreted on fixed spherical collectors
Journal of Geophysical Research, 1999Co-Authors: Nesvit E Castellano, Eldo E Avila, C P R SaundersAbstract: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 tm, a riming target of 5 mm diameter, temperatures of -6 o and -15oC, and velocities between 4 and 8 m s 'l, 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 2; 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.
Hugh Morrison - One of the best experts on this subject based on the ideXlab platform.
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prediction of Graupel density in a bulk microphysics scheme
Journal of the Atmospheric Sciences, 2013Co-Authors: Jason A Milbrandt, Hugh MorrisonAbstract:AbstractA method to predict the bulk density of Graupel ρg has been added to the two-moment Milbrandt–Yau bulk microphysics scheme. The simulation of Graupel using the modified scheme is illustrated through idealized simulations of a mesoscale convective system using a 2D kinematic model with a prescribed flow field and different peak updraft speeds. To examine the relative impact of the various approaches to represent rimed ice, simulations were run for various Graupel-only and Graupel-plus-hail configurations.Because of the direct feedback of ρg to terminal fall speeds, the modified scheme produces a much different spatial distribution of Graupel, with more mass concentrated in the convective region resulting in changes to the surface precipitation at all locations. With a strong updraft, the model can now produce solid precipitation at the surface in the convective region without a separate hail category. It is shown that a single rimed-ice category is capable of representing a realistically wide range...
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sensitivity of idealized squall line simulations to the level of complexity used in two moment bulk microphysics schemes
Monthly Weather Review, 2012Co-Authors: Kwinten Van Weverberg, Hugh Morrison, Andrew M Vogelmann, Jason A MilbrandtAbstract:AbstractThis paper investigates the level of complexity that is needed within bulk microphysics schemes to represent the essential features associated with deep convection. To do so, the sensitivity of surface precipitation is evaluated in two-dimensional idealized squall-line simulations with respect to the level of complexity in the bulk microphysics schemes of H. Morrison et al. and of J. A. Milbrandt and M. K. Yau. Factors examined include the number of predicted moments for each of the precipitating hydrometeors, the number and nature of ice categories, and the conversion term formulations. First, it is shown that simulations of surface precipitation and cold pools are not only a two-moment representation of rain, as suggested by previous research, but also by two-moment representations for all precipitating hydrometeors. Cold pools weakened when both rain and Graupel number concentrations were predicted, because size sorting led to larger Graupel particles that melted into larger raindrops and cause...
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sensitivity of a simulated squall line to horizontal resolution and parameterization of microphysics
Monthly Weather Review, 2012Co-Authors: George H Bryan, Hugh MorrisonAbstract:Idealizedsimulations ofthe 15May2009 squalllinefromthe SecondVerification ofthe OriginsofRotation in Tornadoes Experiment (VORTEX2) are evaluated in this study. Four different microphysical setups are used, with either single-moment (1M) or double-moment (2M) microphysics, and either hail or Graupel as the dense (rimed) ice species. Three different horizontal grid spacings are used: Dx 5 4, 1, or 0.25 km (with identical vertical grids). Overall, results show that simulated squall lines are sensitive to both microphysical setupandhorizontalresolution,althoughsomequantities(i.e.,surfacerainfall)aremoresensitiveto Dxinthis study. Simulations with larger Dx are slower to develop, produce more precipitation, and have higher cloud tops, all of which are attributable to larger convective cells that do not entrain midlevel air. The highestresolution simulations have substantially more cloud water evaporation, which is partly attributable to the development of resolved turbulence. For a given Dx, the 1M simulations produce less rain, more intense cold pools, and do not have trailing stratiform precipitation at the surface, owing to excessive rainwater evaporation. The simulations with Graupel as the dense ice species have unrealistically wide convective regions. Comparison against analyses from VORTEX2 data shows that the 2M setup with hail and Dx 5 0.25 km producesthemostrealisticsimulationbecause(i)thissimulationproducesrealisticdistributionsofreflectivity associatedwithconvective,transition,andtrailingstratiformregions,(ii)thecoldpoolpropertiesarereasonably close to analyses from VORTEX2, and (iii) relative humidity in the cold pool is closest to observations.
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comparison of two moment bulk microphysics schemes in idealized supercell thunderstorm simulations
Monthly Weather Review, 2011Co-Authors: Hugh Morrison, Jason A MilbrandtAbstract:AbstractIdealized three-dimensional supercell simulations were performed using the two-moment bulk microphysics schemes of Morrison and Milbrandt–Yau in the Weather Research and Forecasting (WRF) model. Despite general similarities in these schemes, the simulations were found to produce distinct differences in storm structure, precipitation, and cold pool strength. In particular, the Morrison scheme produced much higher surface precipitation rates and a stronger cold pool, especially in the early stages of storm development. A series of sensitivity experiments was conducted to identify the primary differences between the two schemes that resulted in the large discrepancies in the simulations.Different approaches in treating Graupel and hail were found to be responsible for many of the key differences between the baseline simulations. The inclusion of hail in the baseline simulation using the Milbrant–Yau scheme with two rimed-ice categories (Graupel and hail) had little impact, and therefore resulted in a...
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an improved representation of rimed snow and conversion to Graupel in a multicomponent bin microphysics scheme
Journal of the Atmospheric Sciences, 2010Co-Authors: Hugh Morrison, Wojciech W GrabowskiAbstract:Abstract This paper describes the development of a new multicomponent detailed bin ice microphysics scheme that predicts the number concentration of ice as well as the rime mass mixing ratio in each mass bin. This allows for local prediction of the rime mass fraction. In this approach, the ice particle mass size, projected area size, and terminal velocity–size relationships vary as a function of particle mass and rimed mass fraction, based on a simple conceptual model of rime accumulation in the crystal interstices that leads to an increase in particle mass, but not in its maximum size, until a complete “filling in” with rime and conversion to Graupel occurs. This approach allows a natural representation of the gradual transition from unrimed crystals to rimed crystals and Graupel during riming. The new ice scheme is coupled with a detailed bin representation of the liquid hydrometeors and applied in an idealized 2D kinematic flow model representing the evolution of a mixed-phase precipitating cumulus. Re...
Eldo E Avila - One of the best experts on this subject based on the ideXlab platform.
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laboratory studies of the effect of cloud conditions on Graupel crystal charge transfer in thunderstorm electrification
Quarterly Journal of the Royal Meteorological Society, 2006Co-Authors: C P R Saunders, Eldo E Avila, Christopher Emersic, H Baxnorman, Nesvit E CastellanoAbstract:Collisions 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
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charge sign reversal in irregular ice particle Graupel collisions
Geophysical Research Letters, 2005Co-Authors: Eldo E Avila, C P R Saunders, Holly Baxnorman, Nesvit E CastellanoAbstract:[1] 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.
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a laboratory study of the influence of water vapour and mixing on the charge transfer process during collisions between ice crystals and Graupel
Atmospheric Research, 2001Co-Authors: C P R Saunders, Eldo E Avila, S L Peck, Aguirre G Varela, Nesvit E CastellanoAbstract:Abstract Laboratory experiments, in which vapour grown ice crystals interact with riming Graupel targets, simulate charging processes in thunderstorms. The introduction of cooled, moist, laboratory air into a supercooled droplet and ice crystal cloud enhances charge transfer and, when the air-stream is directed at the riming target, can reverse its charge sign. The suggestion is that the extra water vapour introduced increases the supersaturation and influences particle diffusional growth. The results have been considered in terms of the Relative Growth Rate Hypothesis, which states that the interacting ice surface growing fastest by vapour diffusion charges positively. A corollary to this was noted, when dry air is introduced into a cloud of ice crystals so that both the crystals and target surface sublimate, the ice surface that sublimates fastest charges negatively. The experiments are relevant to considerations of the reasons why earlier sets of charge transfer results give different liquid water and temperature boundaries between positive and negative Graupel charge sign. The differences appear to be connected to the techniques used, in particular, to the mixing of separate droplet and ice crystal clouds before riming, which can lead to positive rimer charging in conditions of low-rime accretion rate, as observed in the present study. Further work is needed to resolve questions concerning the most naturally representative manner of performing these laboratory simulations.
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a laboratory study of Graupel charging
Journal of Geophysical Research, 2000Co-Authors: Rodolfo G Pereyra, Eldo E Avila, Nesvit E Castellano, C P R SaundersAbstract: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 Cordoba 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.
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dependence of the average surface temperature on cloud droplet spectra for rime ice accreted on fixed spherical collectors
Journal of Geophysical Research, 1999Co-Authors: Nesvit E Castellano, Eldo E Avila, C P R SaundersAbstract: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 tm, a riming target of 5 mm diameter, temperatures of -6 o and -15oC, and velocities between 4 and 8 m s 'l, 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 2; 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.
E R Jayaratne - One of the best experts on this subject based on the ideXlab platform.
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density and surface temperature of Graupel and the charge separation during ice crystal interactions
Journal of Geophysical Research, 1998Co-Authors: E R JayaratneAbstract:Significant amounts of charge are separated when vapor-grown ice crystals interact with riming Graupel a mechanism widely believed to be responsible for the generation of electric charge in thunderstorms. This study shows that the density of Graupel, by itself, is not an effective parameter in determining the sign of the separated charge. The well-known charge sign reversal temperature is found to be strongly influenced by the surface temperature of the rime at low cloud water content (CWC). At a cloud temperature of -10°C and a CWC of 0.4 g m -3 the charging current to the Graupel reversed from positive to negative as the surface temperature of the Graupel exceeded -6°C. The maximum CWC at which the sign could thus be reversed increased as the cloud temperature decreased, ranging from less than 0.7 g m -3 at -10°C to less than 0.9 g m -3 at -20°C. At higher cloud water contents the rime surface temperature had no effect on the charging sign. The observations are shown to be broadly explicable in terms of the hypothesis where, during an interaction between two ice particles, the particle that is growing faster from the vapor acquires the positive charge.
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the heat balance of a riming Graupel pellet and the charge separation during ice ice collisions
Journal of the Atmospheric Sciences, 1993Co-Authors: E R JayaratneAbstract:Abstract It has been suggested that the sign of charge acquired by a riming Graupel pellet during ice crystal interactions depends on its surface state being negative when it is evaporating and positive when growing by vapor diffusion. Experiments were conducted to determine the surface states of riming cylinders in two previous laboratory studies of thunderstorm electrification. Calculations of the heat balance of the simulated Graupel pellets are considered in terms of experimental measurements of the charge transferred under similar conditions. The results show that the contribution from in-cloud supercooled droplets is not always adequate to enable the Graupel surface to grow by vapor diffusion in all laboratory cloud conditions where it acquires a net positive charge. It is suggested that factors such as the vapor fields around droplets freezing on its surface and the shape and surface roughness of the Graupel may play important roles in determining the surface state and hence the overall sign of the...
