Nesvit E. Castellano

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.

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.

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.

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.

A comparative study on hailstone trajectories using different motion equations, drag coefficients and wind fields

Abstract The growth of hydrometeors to hailstone sizes is simulated by computer. The trajectories are calculated using the results of a three-dimensional cloud model as input. The model simulates the time evolution of the wind, the liquid water content and the air temperature fields in a cubic region of 16 km per side. These fields are recorded and later used in another program that calculates the hail growth and the aerodynamical forces involved. Embryos of 0.25, 0.5 and 1.00 mm of initial radii are released at 676 locations uniformly distributed in a region of 6×6×4 km3 around the updraft center. The trajectories of all the particles are followed all the way to the 0°C altitude. The final radii and residence times in the cloud of all particles are also stored. A systematic study on the changes brought about by the use of different expressions for the drag coefficient and for the equation of motion on both the computed trajectories and on the final size of hailstones is performed. The study includes the location of the most favored regions for the initiation of hail in each case. The effect of using stationary and time dependent fields is also considered in detail. A comparison of the results obtained using a realistic equation of motion and those employing an approximation frequently used by others show the respective final radii differing by up to 30%. It is observed that expressions predicting values for the drag coefficient that are not nearly as scattered as those found in nature can lead to final radii differing by as much as 50%. The use of stationary wind fields can produce an important overestimation of the maximum sizes achieved by hail inside the cloud.

Diurnal patterns in lightning activity over South America

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.

Surface temperature distribution for ice accreted on a cylindrical collector

Atmospheric icing has been studied by several authors due to its importance in many phenomena where a solid body grows by accretion of supercooled cloud or fog droplets. An important parameter, as for the deposit evolution is the surface temperature, Ts, which determines the deposit density and morphology in dry regime and the transition for dry to wet regime. In the present work an algebraic solution is given for the differential equation representing the equilibrium surface temperature of an ice accretion growing on a fixed cylinder. This solution differs from previous ones because it takes into account the heat flux occurring in a cylinder, due to the temperature gradient created at the surface. It is shown that, for an ice cylinder growing in normal atmospheric conditions, this flux is of the same order of magnitude of that representing the heat exchange with the environment. The proposed mathematical treatment requires the representation of the different heat fluxes in the form of Fourier series, the terms of which are functions of cos (nθ), where θ is the angular distance from the radius through the stagnation point. Special attention is given to the Nusselt number Nu(θ), which affects all heat fluxes related with ventilation. An expression for Nu(θ), valid for an heat conducting cylinder, in the range 4 x 103<Re<2x104, is proposed. Several Ts(θ) curves are given for different environmental conditions and the effects of varying the Stokes and the Reynolds numbers are considered. The results obtained for an isolating, an ice and a Cu collector are compared.

Requirements for low density riming and two stage growth on atmospheric particles

Abstract A theoretical study is carried out of the conditions that can be expected to determine low density riming on atmospheric ice particles. Using a growth simulation model, critical liquid water contents L wc and air temperatures T a are calculated, which correspond to a density ρ =0.5 g/cm 3 for rime deposit on ice particles with radii varying from 1 to 10 mm. Their dependence on the used laws for the ice density as a function of Macklins parameter and for the drag coefficient as a function of Reynolds number, is discussed. The evolution of the density and related parameters for free falling particles growing by accretion from initial values of the radius R and density ρ is studied in different environmental conditions. It is shown that the temperature of the deposit T s increases with R , up to the transition to wet growth, represented by T s =0°C. Only for L wc ≥2 g/m 3 the transition from low density ice to wet growth is found to occur rapidly, at a distance from the center R ≤1 cm. This distance is considered to represent the maximum radius of regions where two-stage growth, due to water penetration and freezing into pores of low-density layers, can be responsible for rapid variations of the particle density and consequently of its free-fall speed, which would characterize the effect of hail growth via microphysical recycling.

On the search for a representative drag coefficient law to be used in hail trajectory simulations

A new method to calculate the ice particle drag coefficient used in hailstone trajectory calculations is presented. The method takes into account the high variability of this parameter and also allows for the investigation of the different trajectories that equally initialized embryos may follow within a cloud due to non-deterministic changes in their surface texture. The method is applied in a hail trajectory simulation model. The results are compared with those obtained by means of curves fitted to the drag coefficient going through mean, low and high values. The aim of this work is to investigate the representativity of hail trajectories simulated using these curves for the assessment of the drag coefficient value.