LABORATORY STUDIES OF THE EFFECT OF CLOUD CONDITIONS ON CHARGE TRANSFER IN THUNDERSTORM ELECTRIFICATION

C. P. R. SAUNDERS; HOLLY BAX-NORMAN; E. E. AVILA

Versalles - Francia

Conferencia; XII International Conference of Atmospheric Electricity; 2003

International Comission of Atmospheric Electricity

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 [1957] 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 on 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 various 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 higher cloud water contents than have been noted previously.  A range of cloud conditions is being used in order to help determine the reasons for the spread of charge transfer results reported previously.