Eldo E. Ávila

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 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.

A laboratory study of static charging by fracture in ice growing by riming

Charge transfer by fracture between colliding ice particles is measured using a wind tunnel inside a cold room. The charges were recorded one by one and the mechanics of each collision was studied in detail. The sign of the temperature gradient at a given point of impact on the surface influences the sign of the charge transfer. In this work we extend previous measurements to particles growing by riming. A cylinder growing by riming in a wind tunnel was used as target for collisions at 5 m s−1 with ice spheres of 100 μm in diameter in a wide range of temperatures. Charge transfers of either sign were also observed here and according to the ambient conditions, one of sign prevailed on average. The transferences ranged from ±50 fC at higher temperatures increasing in magnitude to −80 fC at lower temperatures. There was ample evidence of fragmentation following most of the collisions. The influence of the growth regime on the charging and the production of fragments were also studied. A physical model of the microscopical mechanism leading to the charging of the fragments is presented. The laboratory evidence, when applied to a thundercloud, suggests that the proposed mechanism could operate in a wide range of conditions and even explain some observations in situ related to the abundance of ice crystals. Comparisons are made with other experiments and models on static charging of ice.

Laboratory studies of the influence of cloud droplet size on charge transfer during crystal‐graupel collisions

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°C to −26°C, the supercooled droplet spectrum has been extended to larger droplet sizes, above 60 μm maximum diameter, representative of the broadest spectrum observed in some thunderstorm cloud charging regions. The results indicate that at temperatures from −6°C to −18°C, 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 riming 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.

A laboratory study of the influence of water vapour and mixing on the charge transfer process during collisions between ice crystals and graupel

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.

Temperature Dependence of Static Charging in Ice Growing by Riming

Abstract Charge transfer between colliding ice particles is measured using a wind tunnel inside a cold room. A cylinder growing by riming in a wind tunnel was used as a target for collisions between 5 and 6 m s−1 with ice spheres of 100-µm diameter. The target temperature was adjusted to simulate different liquid water concentrations. As the target temperature increased, for air temperatures below −18°C, initial positive target charging reversed sign to negative; with a further temperature increase the charging reversed sign again. These measurements, which are relevant to thunderstorm electrification, were carried out with and without riming and results are compared with other works. A novel approach is presented here suggesting a new pair of variables describing the charging.

Charging in ice-ice collisions as a function of the ambient temperature and the larger particle average temperature

Measurements of charge transfer between ice particles are reported. The ambient temperature and the average graupel temperature are presented as an alternative pair of variables describing the charge transferred between ice particles during a collision. These variables are alternative to the generally used air temperature and liquid water content. A new charging diagram is also presented here. The results suggest that other variables would be necessary to better describe the phenomena. Some of the current hypotheses on charge transfer are also discussed. The data gathered in the present experiment correspond to collisions made in a wind tunnel between a cylinder growing by riming and ice spheres of 100 μm in diameter; the velocity was between 5 and 6 m s−1. The ambient temperature was between −8° and −24°C. The target temperature was adjusted with a Peltier element keeping the target always equal or above than the ambient temperature. The implications of these results to the thunderstorm electrification are discussed.

Lightning in Western Patagonia

On the basis of 8 years (2005–2012) of stroke data from the World Wide Lightning Location Network we describe the spatial distribution and temporal variability of lightning activity over Western Patagonia. This region extends from ~40°S to 55°S along the west coast of South America, is limited to the east by the austral Andes, and features a hyper-humid, maritime climate. Stroke density exhibits a sharp maximum along the coast of southern Chile. Although precipitation there is largely produced by cold nimbostratus, days with more than one stroke occur up to a third of the time somewhere along the coastal strip. Disperse strokes are also observed off southern Chile. In contrast, strokes are virtually nonexistent over the austral Andes—where precipitation is maximum—and farther east over the dry lowlands of Argentina. Atmospheric reanalysis and satellite imagery are used to characterize the synoptic environment of lightning-producing storms, exemplified by a case study and generalized by a compositing analysis. Lightning activity tends to occur when Western Patagonia is immersed in a pool of cold air behind a front that has reached the coast at ~40°S. Under these circumstances, midlevel cooling occurs before and is more prominent than near-surface cooling, leading to a weakly unstable postfrontal condition. Forced uplift of the strong westerlies impinging on the coastal mountains can trigger convection and produces significant lightning activity in this zone. Farther offshore, large-scale ascent near the cyclones center may lift near-surface air parcels, fostering shallow convection and dispersing lightning activity.

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

Abstract 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.

Charge separation in updraft of convective regions of thunderstorm

analyzed for three different velocities: 6, 8 and 11 m s 1 . The ambient temperature was varied in the range 5t o 30� C and the effective water content between 0 to 2gm 3 . Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature and the effective liquid water content for each velocity are presented. The results indicate that increasing the velocity leads to negative particle charging during riming at higher velocity and the implications of these findings to non-severe

Ventilation coefficients for cylindrical collectors growing by riming as a function of the cloud droplet spectra

Abstract Laboratory measurements of the ventilation coefficient of ice particles growing by riming are presented in this work. The effect of the cloud droplet size spectrum after accretion on the ventilation coefficient was analyzed with droplets of mean volume diameter between 15 and 33 μm. The study was performed with cylindrical collectors of 2.8 and 4 mm diameter, the air temperature was varied from −5°C to −27°C, and three different velocities were used: 4.0, 7.0 and 8.5 m s −1 . The results show a significant dependence of the ventilation coefficient on the droplet sizes; in particular it was found that for small droplets the coefficient is increased and it can be twice its predicted theoretical value. It is suggested that this effect is produced by the different surface structure formed on the collector as a consequence of the different sizes of water droplets. The influences and effects of the cloud droplet size spectrum on the surface temperature and ventilation coefficient are discussed as a function of the Stokes number, which could be a more appropriate parameter to describe or simulate heat and mass transfer processes to accreting surfaces.