CHARGE-TRANSFER BY PRECIPITATION BETWEEN THUNDERCLOUD AND GROUND

Simultaneous measurements of precipitation current, electric field at ground level and 44 m above, and rain rate have been performed during summer experiments in southwestern France. The total precipitation cur rent density was measured with a new instrument consisting of an elect rometer connected to a plane conductor surface shielded by a grounded grid. Thus several characteristics of the precipitation current below thunderstorms have been pointed out: (1) The precipitation current can have both polarities, and the polarity can change during the same eve nt. However, it is more often positive, that is, negative raindrops ar e dominant. (2) The current density reaches 5 nA m(-2) in absolute val ue. (3) The beginning of the detection of a precipitation current gene rally corresponds to a reverse of the surface electric field or a decr ease of the field 44 m above ground. (4) During most of the charged ra in periods we observe the mirror image relation between surface field and precipitation; that is, their polarities are opposite. (5) The pre cipitation seems to carry the charge of the cloud that nias dominant i n the creation of the field close to the ground before the rain. When this charge reaches the ground, the field decreases at each level and it reverses at the ground. (6) The precipitation current detected at t he ground is low compared with that usually measured at cloud base. Th e Wilson capture across the charge layer above ground seems to be effi cient by limiting the precipitation current density. So the total prec ipitation current at the ground evaluated from our data and for an ave rage thunderstorm is weak; however, because of the Wilson capture, it is not possible to conclude that the charge transfer by precipitation between thundercloud and ground is not important.