Theoretical model of the Bergeron–Findeisen mechanism of ice crystal growth in clouds (original) (raw)
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Quarterly Journal of the Royal Meteorological Society, 2004
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, 1999
Experiments were conducted with a wind tunnel in a cold room, in order to investigate the influence of the cloud-droplet spectrum on the charges transferred when individual ice spheres collided with a fixed artificial graupel pellet growing by riming. The experiments were carried out with ice spheres of about 100 pm in diameter, impact velocities around 4 m s-', temperatures between -10 "C and -30 "C and effective water contents representative of real clouds. Two different cloud-droplet spectra were used. One had more than 30% of the droplets with sizes greater than 13 pm, and the other had more than 50% of the droplets greater than that. The new results show that the size distribution of the droplets is very important to the sign of electric charge transferred. The target graupel charged positively over all the temperature range covered when the smaller-droplet spectrum was used, but negatively at temperatures below -18 "C for the larger-droplet spectrum. These results show the importance of droplet sizes to thunderstorm charging.
On the relationship of thunderstorm ice hydrometeor characteristics and total lightning measurements
Atmospheric Research, 2005
Satellite-borne and ground-based devices for the detection of lightning offer the opportunity to explore relationships-on all significant scales up to global-between lightning frequency, f, and other thundercloud parameters. Calculations predict that f is proportional to the product of the downflux p of solid precipitation and the upward mass flux, I, of ice crystals. This prediction has received support from limited computational studies. The physical reasons for such a relationship are explained in terms of the paramount role of ice in the electrification of thunderstorms. Herein, this prediction is subjected to further, preliminary examination through analysis of lightning and dualpolarimetric radar data collected during the STERAO experiment conducted in Northern Colorado during the summer of 1996. The analysis has yielded some highly provisional support for this flux hypothesis. Computed trends of radar derived hydrometeor fractions of solid precipitation and small 0169-8095/$ -see front matter D ice show correlation to the total lightning frequency and raise the possibility of determining values of p and/or I from lightning measurements.
Small ice crystals and the climatology of lightning
Geophysical Research Letters, 2006
Vigorous debate still surrounds the cloud electrification process and unexplained regional variations in lightning activity. Here, we show that climatological maxima in lightning activity are associated with small effective diameter D e of ice crystals near cumulonimbus cloud tops. This relationship, unlike lightning's more well-known relationship with cloud top height, is consistent over land and ocean. Since multiple studies indicate that D e is reduced by atmospheric aerosol, this relationship strengthens previous suggestions of a role for aerosols as well as dynamics in electrification. Moreover, the angular distribution of backscattered radiance shows that modest ($10%) D e decreases reflect large ($2Â) increases in the number of small (<$30 mm) particles N, a finding supported by cloud model simulations. Both relationships provide an important new test of cloud microphysics and/ or electrification models.
Atmospheric Research, 2001
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.
Aircraft observations of the influence of electric fields on the aggregation of ice crystals
Quarterly Journal of the Royal Meteorological Society, 2005
Aircraft observations of ice-crystal size and habit distributions in the cirrus outflow from deep convection at several geographic locations are reported. In situ measurements were made in the outflow from maritime thunderstorms near Kwajalein, part of the Marshall Islands and of thunderstorms with more continental aerosol concentrations both in the United States and near Darwin, Australia over the Tiwi Islands. Images of chain-like aggregates of small ice crystals, some with plate-like shapes were observed with a state-of-the-art microphysics probe in the outflow regions of continental storms that were typically highly electrified, displaying lightning. The 'chains' were not found in the outflow regions of maritime storms that are typically less electrically active. The striking similarity between these images and previous laboratory measurements of ice aggregation in electric fields are remarked upon. This evidence is used to support the theory that chain aggregates of ice crystals may be common in fully glaciated regions of continental thunderstorms, where ice-particle number densities are high, and their presence is due to the electric field alignment of ice crystals with subsequent enhancement of the aggregation process by dipole induction resulting in short-range attractive inter-particle forces. It is not confirmed where in the storm the aggregates were typically formed; however, in the Darwin thunderstorms they were noted to occur with the highest frequency towards the cirrus outflow base when the cirrus base altitude was high, and generally decreased in frequency with increasing distance from the storm. The potential consequences of electrically enhanced aggregation in continental storms and related electric field mechanisms along with the role of homogeneous freezing in intense thunderclouds are discussed.
Vapour density field of mixed-phase clouds
Atmospheric Research, 2008
This work presents a theoretical model based on the electrostatic image charges method to calculate the steady-state water vapour density field for a population of supercooled cloud droplets and ice crystals. The model allows a determination of the vapour density among the cloud droplets and ice particles and obtains a representative vapour density of the system which is called the ambient vapour density of the mixed-phase cloud. The results show that the ambient vapour density is close to the saturated value over water in clouds where the cloud droplet concentration is much higher than the ice crystal concentration and close to the ice saturation value in glaciated clouds. A parameterization of the ambient vapour density is given as a function of the sizes and concentrations of the cloud droplets and ice crystals.
Quarterly Journal of the Royal Meteorological Society, 1999
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, 2006
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.
A numerical study of thunderstorm electrification: Model development and case study
Journal of Geophysical Research, 1991
We have developed a numerical model for examining the thunderstorm electrification process in which we assume the electrification is entirely due to noninductive charge transfer between colliding ice crystals and hail. Since this ice-hail charge mechanism is very dependent on particle sizes and distributions, we use an explicit microphysical framework. To maintain simplicity, the electrification model is kinemati• thus the temperature and velocity fields are input into the electrification model. These fields can be either calculated by a background model or retrieved from observations. For this study, we have used the cloud model of Taylor (1989) to generate the temperature and velocity fields to examine the July 19, 1981, CCOPE thundercloud. Using these fields, the electrification model produced time-dependent ice particle concentrations, radar refiectivities, charge and vertical electric field distributions in good general agreement with those observed. The model produced a maximum electric field strength of 1.27 kV cm -•, which is on the order of that needed for lightning initiation, and this maximum occurred very close to the time of the observed discharge (as inferred by the sailplane measuremenu). Thus the ice-hail charge mechanism a•rs to have played an important role in the electrical development of the July 19 cloud. The details of the electrification depended on the liquid water content and the glaciation proceases, and particularly on the ice crystal characteristics. Rapid growth of the crystals to riming sizes (> 400 g) yielded the most efficient charging. The electrification was also sensitive to the ice-ice sticking efficiency but not to the characteristics of the large riming ice. , 1974], and cloud top entrainment of ice nuclei. The relative importance of these contributing glaciation processes depends on environmental conditions, and they produce very different vertical distributions of small ice particles. While hail and supercooled drop concentrations can be significant at temperatures not too far below 0øC, the observed concentrations of small ice particles are often much higher than the ice nucleus concentration at that temperawe. Therefore, ice particles are probably formed at colder temperatures higher in the cloud and transported to warmer temperatures by downdrafts. Finally, the concentrations of ice particles within clouds depend directly and indirectly upon entrainment, which brings dry air and nuclei into the cloud and may modify the development of ice particles Hobbs and Rangno, 1985]. Because these processes are linked,