Nesvit Edit Castellano | Universidad Nacional de Córdoba (original) (raw)

Papers by Nesvit Edit Castellano

Universidad Nacional de Córdoba, Mar 31, 2017

Quarterly Journal of the Royal Meteorological Society, 2019

Experimental evidence has demonstrated that temperature on the surface of an ice particle growing... more Experimental evidence has demonstrated that temperature on the surface of an ice particle growing by accretion is not uniform. This non-uniformity is relevant because the ice particle surface temperature is an important microphysical parameter that influences the sign and magnitude of the charge transfer during ice particles collisions. In particular, when high values of ambient temperature and liquid water content are reached, the ice particle surface temperature is expected to reach 0 °C, which is known as wet growth regime. Previous experimental results have shown that wet growth is not uniformly reached on the ice particle surface and a surface temperature distribution is developed. In order to know the surface temperature distribution of a fixed ice particle growing by accretion of supercooled water, numerical calculations were carried out. It was found that the surface temperature distribution has a strong dependence on liquid water content, ambient temperature, airflow velocity and water droplet size. The stagnation point always reaches higher temperatures and, in many cases, its temperature is near 0 °C. For some values of the liquid water content, ambient temperature and airflow velocity, it was possible to determine temperature differences up to 7 °C between the stagnation point and the equator. This variation in the surface temperature implies that the region near the stagnation point would experience wet growth, while the rest of the ice particle surface would remain in dry growth regime, supporting the partial wet growth hypothesis. This could also explain the charge transfer reported during ice particles collisions under wet growth conditions.Fil: Luque, Melina Yasmín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Burgesser, Rodrigo Exequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Castellano, Nesvit Edit. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentin

Quarterly Journal of the Royal Meteorological Society, 2016

Quarterly Journal of the Royal Meteorological Society, 2019

The sedimentation process of columnar ice crystals was evaluated using data obtained by the latti... more The sedimentation process of columnar ice crystals was evaluated using data obtained by the lattice Boltzmann method. The data used correspond to columnar ice crystals with maximum dimension less than 100 μm and aspect ratios between 1 and 3. The terminal velocity was computed for different ice-crystal bulk densities and for three falling orientations. The analysis corresponds to ice crystals falling in viscous flow regimes, where theoretical formulations overestimate the terminal velocity. Different characteristic lengths of columnar ice crystals and different theoretical proposals for the sedimentation process were tested in order to find the best representation of the data. Characteristic lengths reported in the literature do not represent the sedimentation process for all the falling orientations used in this study. Thus, it was not possible to obtain a unique relation between the Best and Reynolds numbers. In particular, columnar ice crystals falling with their longer dimension parallel to the vertical direction show a large dispersion that it does not seem possible to reduce. The theoretical and semi-empirical formulations of the terminal velocity evaluated show large deviations in the computed velocity, with a strong dependence on ice-crystal aspect ratio. The dispersion observed seems to be intrinsically related to the dimensionless variables used to parametrize the terminal velocity. To derive a unique scale law that could represent the sedimentation process of ice crystals, geometric, kinematic, and dynamic similarities are required. However, these conditions are not fulfilled in the sedimentation process. K E Y W O R D S characteristic length, columnar ice crystals, lattice Boltzmann method, sedimentation process, viscous flow regimes 1 INTRODUCTION Given the relevant role of the sedimentation process of solid precipitation particles in the temporal evolution of Equally contributing authors. clouds and in different microphysical processes occurring within them (Pruppacher and Klett, 1997), numerous investigations of this process have been reported in the literature (Böhm, 1989, 1992; Mitchell, 1996; Heymsfield and Westbrook, 2010; among others). These different studies concluded that the mass and cross-sectional area that the

En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incl... more En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incluye los procesos de condensación de vapor y que tiene en cuenta la variación de los parámetros físicos con la altura. Las ecuaciones pronósticas que se presentan son resueltas por diferencias finitas en una grilla tipo "stagger" con Ax = 500m y At = 10s. Se utilizan condiciones periódicas de contorno, en los contornos laterales, lo cual. hace posible la resolución, por un método pseudo-espectral, de la ecuación diagnóstica resultante para la perturbación de presión . Un análisis cualitativo de los resultados de obtenidos hasta el momento, indican que la performance del modelo, en la descripción de la etapa de formación de una nube convectiva, es satisfactorio. Se presenta además una discusión de las ecuaciones a utilizar para simular trayectorias y crecimiento de un granizo en los campos obtenidos con el modelo.

Atmospheric Research, 2014

This paper describes new laboratory observations about the size evolution of ice crystals and clo... more This paper describes new laboratory observations about the size evolution of ice crystals and cloud droplets immersed in a mixed-phase cloud. The experiments were performed by using a cloud chamber facility for three temperatures −6°C, − 10°C and −20°C, in order to explore the basic crystal growth habits (columns and hexagonal plates). The sizes of the cloud droplets, ice-columns and hexagonal ice-plates were examined for growth times between 50 and 300 s. The results show evidence that after ice crystal nucleation, the cloud droplets reduce gradually their sizes by the evaporation process; while the ice crystals grow as a consequence of the water vapor diffusion process. The ice crystal growths at different temperatures were compared with the results reported by other authors. The experimental data were also compared with a theoretical model of the growth rate of ice crystals. It was observed that the numerical model provides a description of the ice columns' growth in fairly good agreement with the laboratory observations, while it predicts that the hexagonal plates evolve with maximum sizes larger than those observed in the experiments. In general, it has been noted that the results obtained from the model are very sensitive to the parameter that denotes the ratio between the condensation coefficient for the basal face and prism face. It is a critical coefficient that needs to be carefully addressed in cloud modeling.

Journal of Geophysical Research: Atmospheres, 2015

Satellite observations of lightning flash distribution data are used to examine the diurnal cycle... more Satellite observations of lightning flash distribution data are used to examine the diurnal cycle of lightning activity over the tropical and subtropical regions of South America. A harmonic analysis is used to study the spatial variations in the peak and strength of diurnal lightning activity across this area. Results show that in the northern and central regions of South America, the times of maxima in lightning activity was concentrated from late afternoon to evening hours (between 14:00 and 18:00 local time), which may be associated with the peaking of the local convective activity connected with heating of the surface caused by daytime insolation. In subtropical South America, particularly in the area limited by 25°S, 35°S of latitude and 70°W, 50°W of longitude, the time of maximum lightning activity was shifted to nocturnal hours, extending from close to midnight to early morning hours. This behavior can be associated to the peak in mesoscale convective systems in the region which occurs in the morning hours. The annual flash densities in the tropical and subtropical parts of the continent were found to have comparable magnitudes. However, the contribution of the continental tropics to the global electric circuit dominates over the continental subtropics contribution throughout all seasons, since the surface covered by the tropical region is more than twice the area covered by the subtropical region.

Journal of Geophysical Research: Atmospheres, 2013

1] A laboratory investigation of the electric charge separated in collisions between vapor-grown ... more 1] A laboratory investigation of the electric charge separated in collisions between vapor-grown ice crystals and a target growing by riming is presented in this work, with the goal of studying the performance of the noninductive mechanism under microphysical conditions similar to some of those which occur in the stratiform regions of the mesoscale convective systems. A series of experiments were conducted by using a target of 2 mm in diameter, for ambient temperatures between À7°C and À13°C, effective liquid water content between 0.05 and 0.5 g m À3 , and air speeds between 1 and 3 m s À1 . Charge diagrams of the sign of the electric charge transfer on the rimer as a function of the ambient temperature and the effective liquid water content for each velocity are presented. The results show that the riming target charges positive for temperatures above À10°C. For temperatures below À10°C, the charging is positive for high liquid water content and negative for low liquid water content. The magnitude of the charge transfer per collision under the studied conditions ranges from 0.01 to 0.2 fC. The implications of these results to the electrification processes are discussed.

Quarterly Journal of the Royal Meteorological Society, 2003

The convection heat-transfer coef cient for circular cylinders, exposed to an air stream perpendi... more The convection heat-transfer coef cient for circular cylinders, exposed to an air stream perpendicular to their axis, is dependent on the Reynolds number, Re. There is experimental evidence showing that the heattransfer coef cient for hail or graupel growing by accretion of supercooled water droplets is greater than that corresponding to an equivalent smooth surface. Work carried out so far indicates that surface roughness could in uence this coef cient. This work presents an experimental study of the in uence of lobes on the heat-transfer process to the surroundings from metallic cylinder surfaces. The dependence of the heat-transfer coef cient on Re was studied for 2000 Re 20 000.

En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incl... more En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incluye los procesos de condensación de vapor y que tiene en cuenta la variación de los parámetros físicos con la altura. Las ecuaciones pronósticas que se presentan son resueltas por diferencias finitas en una grilla tipo "stagger" con Ax = 500m y At = 10s. Se utilizan condiciones periódicas de contorno, en los contornos laterales, lo cual. hace posible la resolución, por un método pseudo-espectral, de la ecuación diagnóstica resultante para la perturbación de presión . Un análisis cualitativo de los resultados de obtenidos hasta el momento, indican que la performance del modelo, en la descripción de la etapa de formación de una nube convectiva, es satisfactorio. Se presenta además una discusión de las ecuaciones a utilizar para simular trayectorias y crecimiento de un granizo en los campos obtenidos con el modelo.

Quarterly Journal of the Royal Meteorological Society, 1999

An experimental study of the heat balance of a stationary cylindrical collector accreting superco... more An experimental study of the heat balance of a stationary cylindrical collector accreting supercooled water droplets has shown a new dependence on the sizes of the water droplets in the laboratory cloud. In a study in 1967, MacMin and Payne related the steady-state heat-release during accretion to the heat loss by convection and conduction; their equation involved a numerical factor x for which they assumed a value of 0.28. The present study has shown that x is a function of the droplet spectrum with values around 0.5 at a velocity of 4 m s-' for a mean volume-weighted droplet-diameter of 18 pm, and that x approaches 0.3 for droplets greater than 30 p m diameter.

Quarterly Journal of the Royal Meteorological Society, 2005

The heat transfer coefficient of ice accretions was reported by Avila and co-workers who found th... more The heat transfer coefficient of ice accretions was reported by Avila and co-workers who found that it depends on the cloud droplet spectrum used for the accretion. In the present work, these data are re-examined and a new parametrization of the heat transfer coefficient is proposed as a function of accreted-ice density.

Quarterly Journal of the Royal Meteorological Society, 2006

Collisions between vapour-grown ice crystals and a riming target, representing a graupel pellet f... more 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.

Quarterly Journal of the Royal Meteorological Society, 2004

Laboratory studies of a thunderstorm charging mechanism involving rebounding collisions between i... more Laboratory 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 nonequilibrium 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.

Quarterly Journal of the Royal Meteorological Society, 2002

One aspect of hailstone growth that has received attention over the last twenty years is the impo... more One aspect of hailstone growth that has received attention over the last twenty years is the importance of low-density riming growth in hail. Density is an important parameter in riming growth because it regulates hail size and is also involved in other atmospheric processes, like crystal multiplication, charge transfer, etc. New numerical dynamic and cinematic models take into account the variability of density with growth parameters (such as size of cloud droplets, impact velocity and surface temperature) in order to attain more realistic results. These kinds of models need to have a function that describes the real low-density riming process.

Journal of Geophysical Research, 2011

1] New laboratory measurements of graupel pellet charging due to collision with ice particles at ... more 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 mm. 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.

Journal of Geophysical Research, 2000

Measurements have been made of charge transfer when vapor grown ice crystals rebound from a rimin... more 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 C6rdoba 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. 20,803 20,804 PEREYRA ET AL.' A STUDY OF GRAUPEL CHARGING 0.1

Journal of Geophysical Research, 1998

Further laboratory measurements of charge transfer between ice crystals and riming graupel pellet... more Further laboratory measurements of charge transfer between ice crystals and riming graupel pellets, which are thought to be associated with the electrification processes within thunderstorms, have been carried out in the University of Manchester Institute of Science and Technology cloud chamber. In experiments with clouds in the temperature range -6 øC to -26 øC, the supercooled droplet spectrum has been extended to larger droplet sizes, above 60/xm maximum diameter, representative of the broadest spectrum observed in some thunderstorm cloud charging regions. The results indicate that at temperatures from -6 øC to -18øC, broadening the droplet spectrum leads to negative graupel charging at higher values of cloud effective liquid water content than has been reported in previous laboratory studies. The significance of the result is that in order to ensure that laboratory experiments simulate as closely as possible the thunderstorm cloud microphysical environment, attention must be paid to the spectrum of droplets used. Two mechanisms of charge transfer that may account for this behavior are discussed, the relative growth rate theory and the surface splinter theory, and both are found to be compatible with the results on the assumption that the larger droplets lead to a reduction in the rate of vapor deposition to the timing surface. Analysis of the implications of these results to thunderstorm electrification requires more details of the evolution of droplet spectra in thunderclouds, their spatial and temporal development and location relative to observed regions of electrification.

Universidad Nacional de Córdoba, Mar 31, 2017

Quarterly Journal of the Royal Meteorological Society, 2019

Experimental evidence has demonstrated that temperature on the surface of an ice particle growing... more Experimental evidence has demonstrated that temperature on the surface of an ice particle growing by accretion is not uniform. This non-uniformity is relevant because the ice particle surface temperature is an important microphysical parameter that influences the sign and magnitude of the charge transfer during ice particles collisions. In particular, when high values of ambient temperature and liquid water content are reached, the ice particle surface temperature is expected to reach 0 °C, which is known as wet growth regime. Previous experimental results have shown that wet growth is not uniformly reached on the ice particle surface and a surface temperature distribution is developed. In order to know the surface temperature distribution of a fixed ice particle growing by accretion of supercooled water, numerical calculations were carried out. It was found that the surface temperature distribution has a strong dependence on liquid water content, ambient temperature, airflow velocity and water droplet size. The stagnation point always reaches higher temperatures and, in many cases, its temperature is near 0 °C. For some values of the liquid water content, ambient temperature and airflow velocity, it was possible to determine temperature differences up to 7 °C between the stagnation point and the equator. This variation in the surface temperature implies that the region near the stagnation point would experience wet growth, while the rest of the ice particle surface would remain in dry growth regime, supporting the partial wet growth hypothesis. This could also explain the charge transfer reported during ice particles collisions under wet growth conditions.Fil: Luque, Melina Yasmín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Burgesser, Rodrigo Exequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Castellano, Nesvit Edit. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentin

Quarterly Journal of the Royal Meteorological Society, 2016

Quarterly Journal of the Royal Meteorological Society, 2019

The sedimentation process of columnar ice crystals was evaluated using data obtained by the latti... more The sedimentation process of columnar ice crystals was evaluated using data obtained by the lattice Boltzmann method. The data used correspond to columnar ice crystals with maximum dimension less than 100 μm and aspect ratios between 1 and 3. The terminal velocity was computed for different ice-crystal bulk densities and for three falling orientations. The analysis corresponds to ice crystals falling in viscous flow regimes, where theoretical formulations overestimate the terminal velocity. Different characteristic lengths of columnar ice crystals and different theoretical proposals for the sedimentation process were tested in order to find the best representation of the data. Characteristic lengths reported in the literature do not represent the sedimentation process for all the falling orientations used in this study. Thus, it was not possible to obtain a unique relation between the Best and Reynolds numbers. In particular, columnar ice crystals falling with their longer dimension parallel to the vertical direction show a large dispersion that it does not seem possible to reduce. The theoretical and semi-empirical formulations of the terminal velocity evaluated show large deviations in the computed velocity, with a strong dependence on ice-crystal aspect ratio. The dispersion observed seems to be intrinsically related to the dimensionless variables used to parametrize the terminal velocity. To derive a unique scale law that could represent the sedimentation process of ice crystals, geometric, kinematic, and dynamic similarities are required. However, these conditions are not fulfilled in the sedimentation process. K E Y W O R D S characteristic length, columnar ice crystals, lattice Boltzmann method, sedimentation process, viscous flow regimes 1 INTRODUCTION Given the relevant role of the sedimentation process of solid precipitation particles in the temporal evolution of Equally contributing authors. clouds and in different microphysical processes occurring within them (Pruppacher and Klett, 1997), numerous investigations of this process have been reported in the literature (Böhm, 1989, 1992; Mitchell, 1996; Heymsfield and Westbrook, 2010; among others). These different studies concluded that the mass and cross-sectional area that the

En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incl... more En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incluye los procesos de condensación de vapor y que tiene en cuenta la variación de los parámetros físicos con la altura. Las ecuaciones pronósticas que se presentan son resueltas por diferencias finitas en una grilla tipo "stagger" con Ax = 500m y At = 10s. Se utilizan condiciones periódicas de contorno, en los contornos laterales, lo cual. hace posible la resolución, por un método pseudo-espectral, de la ecuación diagnóstica resultante para la perturbación de presión . Un análisis cualitativo de los resultados de obtenidos hasta el momento, indican que la performance del modelo, en la descripción de la etapa de formación de una nube convectiva, es satisfactorio. Se presenta además una discusión de las ecuaciones a utilizar para simular trayectorias y crecimiento de un granizo en los campos obtenidos con el modelo.

Atmospheric Research, 2014

This paper describes new laboratory observations about the size evolution of ice crystals and clo... more This paper describes new laboratory observations about the size evolution of ice crystals and cloud droplets immersed in a mixed-phase cloud. The experiments were performed by using a cloud chamber facility for three temperatures −6°C, − 10°C and −20°C, in order to explore the basic crystal growth habits (columns and hexagonal plates). The sizes of the cloud droplets, ice-columns and hexagonal ice-plates were examined for growth times between 50 and 300 s. The results show evidence that after ice crystal nucleation, the cloud droplets reduce gradually their sizes by the evaporation process; while the ice crystals grow as a consequence of the water vapor diffusion process. The ice crystal growths at different temperatures were compared with the results reported by other authors. The experimental data were also compared with a theoretical model of the growth rate of ice crystals. It was observed that the numerical model provides a description of the ice columns' growth in fairly good agreement with the laboratory observations, while it predicts that the hexagonal plates evolve with maximum sizes larger than those observed in the experiments. In general, it has been noted that the results obtained from the model are very sensitive to the parameter that denotes the ratio between the condensation coefficient for the basal face and prism face. It is a critical coefficient that needs to be carefully addressed in cloud modeling.

Journal of Geophysical Research: Atmospheres, 2015

Satellite observations of lightning flash distribution data are used to examine the diurnal cycle... more Satellite observations of lightning flash distribution data are used to examine the diurnal cycle of lightning activity over the tropical and subtropical regions of South America. A harmonic analysis is used to study the spatial variations in the peak and strength of diurnal lightning activity across this area. Results show that in the northern and central regions of South America, the times of maxima in lightning activity was concentrated from late afternoon to evening hours (between 14:00 and 18:00 local time), which may be associated with the peaking of the local convective activity connected with heating of the surface caused by daytime insolation. In subtropical South America, particularly in the area limited by 25°S, 35°S of latitude and 70°W, 50°W of longitude, the time of maximum lightning activity was shifted to nocturnal hours, extending from close to midnight to early morning hours. This behavior can be associated to the peak in mesoscale convective systems in the region which occurs in the morning hours. The annual flash densities in the tropical and subtropical parts of the continent were found to have comparable magnitudes. However, the contribution of the continental tropics to the global electric circuit dominates over the continental subtropics contribution throughout all seasons, since the surface covered by the tropical region is more than twice the area covered by the subtropical region.

Journal of Geophysical Research: Atmospheres, 2013

1] A laboratory investigation of the electric charge separated in collisions between vapor-grown ... more 1] A laboratory investigation of the electric charge separated in collisions between vapor-grown ice crystals and a target growing by riming is presented in this work, with the goal of studying the performance of the noninductive mechanism under microphysical conditions similar to some of those which occur in the stratiform regions of the mesoscale convective systems. A series of experiments were conducted by using a target of 2 mm in diameter, for ambient temperatures between À7°C and À13°C, effective liquid water content between 0.05 and 0.5 g m À3 , and air speeds between 1 and 3 m s À1 . Charge diagrams of the sign of the electric charge transfer on the rimer as a function of the ambient temperature and the effective liquid water content for each velocity are presented. The results show that the riming target charges positive for temperatures above À10°C. For temperatures below À10°C, the charging is positive for high liquid water content and negative for low liquid water content. The magnitude of the charge transfer per collision under the studied conditions ranges from 0.01 to 0.2 fC. The implications of these results to the electrification processes are discussed.

Quarterly Journal of the Royal Meteorological Society, 2003

The convection heat-transfer coef cient for circular cylinders, exposed to an air stream perpendi... more The convection heat-transfer coef cient for circular cylinders, exposed to an air stream perpendicular to their axis, is dependent on the Reynolds number, Re. There is experimental evidence showing that the heattransfer coef cient for hail or graupel growing by accretion of supercooled water droplets is greater than that corresponding to an equivalent smooth surface. Work carried out so far indicates that surface roughness could in uence this coef cient. This work presents an experimental study of the in uence of lobes on the heat-transfer process to the surroundings from metallic cylinder surfaces. The dependence of the heat-transfer coef cient on Re was studied for 2000 Re 20 000.

En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incl... more En este trabajo se presenta un modelo numérico de convección atmosférica tridimensional, que incluye los procesos de condensación de vapor y que tiene en cuenta la variación de los parámetros físicos con la altura. Las ecuaciones pronósticas que se presentan son resueltas por diferencias finitas en una grilla tipo "stagger" con Ax = 500m y At = 10s. Se utilizan condiciones periódicas de contorno, en los contornos laterales, lo cual. hace posible la resolución, por un método pseudo-espectral, de la ecuación diagnóstica resultante para la perturbación de presión . Un análisis cualitativo de los resultados de obtenidos hasta el momento, indican que la performance del modelo, en la descripción de la etapa de formación de una nube convectiva, es satisfactorio. Se presenta además una discusión de las ecuaciones a utilizar para simular trayectorias y crecimiento de un granizo en los campos obtenidos con el modelo.

Quarterly Journal of the Royal Meteorological Society, 1999

An experimental study of the heat balance of a stationary cylindrical collector accreting superco... more An experimental study of the heat balance of a stationary cylindrical collector accreting supercooled water droplets has shown a new dependence on the sizes of the water droplets in the laboratory cloud. In a study in 1967, MacMin and Payne related the steady-state heat-release during accretion to the heat loss by convection and conduction; their equation involved a numerical factor x for which they assumed a value of 0.28. The present study has shown that x is a function of the droplet spectrum with values around 0.5 at a velocity of 4 m s-' for a mean volume-weighted droplet-diameter of 18 pm, and that x approaches 0.3 for droplets greater than 30 p m diameter.

Quarterly Journal of the Royal Meteorological Society, 2005

The heat transfer coefficient of ice accretions was reported by Avila and co-workers who found th... more The heat transfer coefficient of ice accretions was reported by Avila and co-workers who found that it depends on the cloud droplet spectrum used for the accretion. In the present work, these data are re-examined and a new parametrization of the heat transfer coefficient is proposed as a function of accreted-ice density.

Quarterly Journal of the Royal Meteorological Society, 2006

Collisions between vapour-grown ice crystals and a riming target, representing a graupel pellet f... more 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.

Quarterly Journal of the Royal Meteorological Society, 2004

Laboratory studies of a thunderstorm charging mechanism involving rebounding collisions between i... more Laboratory 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 nonequilibrium 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.

Quarterly Journal of the Royal Meteorological Society, 2002

One aspect of hailstone growth that has received attention over the last twenty years is the impo... more One aspect of hailstone growth that has received attention over the last twenty years is the importance of low-density riming growth in hail. Density is an important parameter in riming growth because it regulates hail size and is also involved in other atmospheric processes, like crystal multiplication, charge transfer, etc. New numerical dynamic and cinematic models take into account the variability of density with growth parameters (such as size of cloud droplets, impact velocity and surface temperature) in order to attain more realistic results. These kinds of models need to have a function that describes the real low-density riming process.

Journal of Geophysical Research, 2011

1] New laboratory measurements of graupel pellet charging due to collision with ice particles at ... more 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 mm. 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.

Journal of Geophysical Research, 2000

Measurements have been made of charge transfer when vapor grown ice crystals rebound from a rimin... more 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 C6rdoba 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. 20,803 20,804 PEREYRA ET AL.' A STUDY OF GRAUPEL CHARGING 0.1

Journal of Geophysical Research, 1998

Further laboratory measurements of charge transfer between ice crystals and riming graupel pellet... more Further laboratory measurements of charge transfer between ice crystals and riming graupel pellets, which are thought to be associated with the electrification processes within thunderstorms, have been carried out in the University of Manchester Institute of Science and Technology cloud chamber. In experiments with clouds in the temperature range -6 øC to -26 øC, the supercooled droplet spectrum has been extended to larger droplet sizes, above 60/xm maximum diameter, representative of the broadest spectrum observed in some thunderstorm cloud charging regions. The results indicate that at temperatures from -6 øC to -18øC, broadening the droplet spectrum leads to negative graupel charging at higher values of cloud effective liquid water content than has been reported in previous laboratory studies. The significance of the result is that in order to ensure that laboratory experiments simulate as closely as possible the thunderstorm cloud microphysical environment, attention must be paid to the spectrum of droplets used. Two mechanisms of charge transfer that may account for this behavior are discussed, the relative growth rate theory and the surface splinter theory, and both are found to be compatible with the results on the assumption that the larger droplets lead to a reduction in the rate of vapor deposition to the timing surface. Analysis of the implications of these results to thunderstorm electrification requires more details of the evolution of droplet spectra in thunderclouds, their spatial and temporal development and location relative to observed regions of electrification.