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Laboratory studies of the effect of cloud conditions on charge transfer in thunderstorm electrification |
| C. P. R Saunders and H
Norman E. E. Avila |
| The pioneering thunderstorm charging studies of Marx Brook and his colleagues in 1957 have been continued in experiments in several laboratories across the world. The 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 over the years have found the sign of the charge transferred to be dependent on the cloud liquid water content and on cloud temperature, Takahashi 1978 and Jayaratne et al 1983. Subsequently the dependence of charge transfer on velocity and ice crystal size was identified and incorporated in a parameterisation of charge transfer sign and magnitude in a form suitable for use in numerical models of thunderstorm electrification by Saunders et al 1991. There are marked differences between the results of Takahashi and Jayaratne in the dependence of the sign of graupel charging as a function of cloud water and temperature. More recently Pereyra et al 2000 have shown that results somewhat similar in form to those of Takahashi may be 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 obtained in experiments, work has continued in UMIST in Manchester with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently and monitored by means of a SPEC particle probe. Results so far 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 two clouds, graupel may be charged negatively by rebounding ice crystal collisions at cloud water contents high enough to be approaching graupel wet growth - a result not encountered in any previous studies. A range of cloud conditions is being used in order to help determine the reasons for the spread of charge transfer results reported previously. References: Jayaratne, Saunders and Hallett, QJRMS 109, 609-630, 1983
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Lightning and In-Cloud Ice Phases in the East Asian Monsoon |
| Tsutomu Takahashi |
| Data from the TRMM-LIS satellite shows a considerable difference between lightning activity over a maritime continent and that observed over the open ocean of the western Pacific in eastern Asia (Christian et al., 2000). Results from data acquired from videosondes indicate that lightning flash activity is closely related to the cloud's ice particle number concentration. Data from videosondes launched from the shore of the Japan Sea during the winter monsoon and from southern Kyushu during the Baiu season revealed several important findings. 1) Intense space charges appear in a cloud as pairs consisting of graupel in the lower levels and ice crystals in the upper ones. When graupel are below -6$B!n(J they carry predominantly positively charged and above -9$B!n(J the predominant charge is negative. 2) Space charge increases as the number concentrations of ice crystals and graupel increase. For accumulated space charges greater than 5 pC/$B&I(J, the graupel concentrations exceeded 1/$B&I(Jand ice crystals more than about 20/$B&I(J. These results support the importance of riming electrification mechanisms as primary charging processes in thunderstorms. The numbers of graupel and ice crystals are of crucial importance in charge accumulation. During the past fifteen years, more than 200 videosondes have been launched into monsoon rains from 13 different locations in East Asia. The numbers of graupel and of ice crystals in rainwater contents of 1 g m-3 were compared with lightning activity in the storms. There were more graupel and crystals in areas of increased lightning activity. Lower graupel and ice crystal number densities extended from the equatorial western Pacific to Shanghai. Much higher crystal and graupel number densities existed within narrow bands along the maritime continent. Ice particle concentrations differed by two orders of magnitude between the maritime continent and the open ocean as lightning activity also varied by the same magnitude.
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Electrification and Lightning in Simulated Storms |
| Edward R. Mansell (CIMMS/Univ. of Oklahoma) Edward Mansell |
| Results will be presented from the recently upgraded OU/NSSL thunderstorm electrification and lightning model. The model now includes an explicit treatment of small ion processes, allowing for a more strict accounting of net charge in the model domain. Sensitivity experiments find a wide range of electrical and lightning characteristics. The same microphysical/dynamical evolution can, depending on the choice of charge separation parameterizations (both noninductive and inductive), produce noticeably different lightning behavior. For example, a storm might produce CG lightning of either or both polarities or none at all (intracloud lightning only). One particularly interesting result is a multicell simulation that produced -CG lightning from the main convective region and a few +CG flashes from the forward anvil, as is often observed in nature. This outcome supports the hypothesis that +CG flashes from the forward anvil originate from descending layers of charge (positive above negative) and opposes the idea that the positive charge must somehow be 'exposed to ground.' The included figure shows two minutes of lightning activity in a multicell storm that produced both +CG (yellow) and -CG (white) lightning. Red diamonds indicate initiation locations. Simulated base-scan radar reflectivity is overlaid with near-surface electric field contours (intervals of 5kV/m, negative values are dashed). |
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Fine Structure
of Thunderstorm Electrified Field: |
| E. A. Mareev, D.I.
Iudin, A.E. Sorokin, V.Yu. Trakhtengerts T. C. Marshall, M. Stolzenburg |
| It is well known that turbulence in the atmosphere leads to Kolmogorov-type spectra of wind and temperature fluctuations. More recently, we have investigated spectra of fluctuations in electric field. Several electric field soundings through stratiform clouds and convective regions of mesoscale convective systems, made with balloon-borne electric field meters and radiosondes, have been examined. All these soundings demonstrate the presence of fine structures in the electric field distribution, with characteristic spatial scales of irregularities ranging from hundreds to tens of meters. Fourier analyses of the measured in-cloud electric fields give power-law spectra that depend on the wave number, with the spectral index close to -2 and with the transition to white noise occurring for spatial scales shorter than 30-35 m. We have also analyzed the mechanisms of spectra formation. Specific electrodynamical properties of thunderstorm clouds, arising due to intensive multi-flow motion of charged particles (heavy charged droplets, light ice crystals, etc.) have been investigated. Our studies to date have shown that a thundercloud has the ability to self-organize. This self-organization is manifested as small-scale electrical stratification, where intense electrical cells with the scales of order of 10 - 100 m are generated [1,2]. The cells are of particular interest in understanding cloud electricity because the electric field within the cells may exceedthe average field value substantially. In addition, we have developed a model that deals with the analysis of a nonlinear diffusion equation for electric field strength evolution [3]. Using this approach, different parameterizations of inductive and non-inductive mechanisms lead to different expressions for the charge separation current. Point discharge dissipation current and turbulent diffusion current are also taken into account. Stationary states and their stability have been investigated with the model.We have found that a nontrivial stationary state is eventually stable with respect to large-scale perturbations, but this large scale state has superimposed on it smaller scale perturbations having characteristic scales that are dependent on the charging intensity parameters. The latter phenomenon has been considered numerically under a more complicated four-component (ions-hydrometeors) model and applied to common thundercloud conditions. Characteristic scales of order of hundred meters have been found for this nonequilibrium system. Taking into account the random sources of external currents that describe the charge exchange due to particle collisions, we get a Langevine-type equation for electric field. In the framework of this equation and with reasonable assumptions concerning the correlation properties of the source, we are able to get electric field variation spectra close to the experimental ones we have found from the balloon soundings. References: V.Yu. Trakhtengerts, Electrical cells in thunderstorm clouds, Doklady
Academii Nauk, 308, 584, 1989 (in Russian). |
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Simulation of the Development and Structure of the Electric Field in a 3-Dimensional Electrically Active Cloud Field using the RAMS Model |
| Orit Altaratz and Zev
Levin Tamir Reisin Yoav Yair |
| This work describes the implementation of numerical schemes that formulate the electrification processes occurring in thunderstorms into the RAMS mesoscale forecast model using real topography of the part of the Israeli coast on the eastern shores of the Mediterranean Sea. We simulated the charge separation and the development of thunderstorms on a regional scale, and tracked the evolution of the electric field until the conditions for lightning were met. The development of the model involved the parameterization of the non-inductive graupel-ice charge separation mechanism that operates inside the clouds. The charge is transferred between graupel - pristine ice, graupel - snow and graupel - aggregates particles during collisions and rebound. The parameterization of this process required a determination of the charge transferred per collision and the calculation of the number of collisions of these particular hydrometeor categories at each grid point during the time step. In the model, charge is being transferred (on the micro scale) and then separated by convection, advection and sedimentation (on the macro scale). The time evolution of the total space charge distributions is computed. The resulting electric field is calculated separately by solving the 3d Poisson's equation at each time step and at each point in the domain. Assuming a threshold value for the electric breakdown field we simulated the initiation of lightning flashes by artificially reducing the space charge in the clouds. We will present the model and its initial results for the conditions prevailing during the occurrence of winter thunderstorms in our region. |
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Development and first results of an explicit electrical scheme in the 3D French mesoscale model "MésoNH" |
| Christèle Barthe Jean-Pierre Pinty Gilles Molinié Frank Roux |
| The electrification of convective clouds is simulated by an explicit scheme based on the dynamical and microphysical core of a 3D cloud resolving model. The electric charges are carried individually by cloud droplets, rain drops, cloud ice, snow/aggregates and graupel/hail. Only non-inductive charge separation mechanisms (ice-ice rebounds) are considered. Charge transfers are consistent with microphysical transfers of mass and include a variable power law for the charge-size relationship. The electric field E is solved from an elliptical equation for the electric potential with appropriate boundary conditions. The lightning parameterization is based on the representation of a bi-leader channel followed by a stochastic branching algorithm to simulate streamers leading to an enhanced horizontal propagation of the flashes as generally observed. The model is run for a 3D case of deep convective storm on a large grid with high horizontal and vertical resolution. Numerical results will illustrate both the microphysical and electrical aspects of the simulated storm in the extended abstract. |
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| Last Update : June 3,
2003 |
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