LIGHTNING FLASHES FROM A PRESCRIBED FIRE-INDUCED CLOUD

A prescribed fire was ignited near Chapleau, western Ontario, Canada, on the afternoon of August 10, 1989. The fire, covering approximately 400 ha, burned vigorously over a period of 3 hours, from 1400 to 1700 EDT, generating a plume cloud structure including a portion resembling an anvil. During the period 1500-1630, 21 ground flashes lowering positive charge from the plume cloud were observed simultaneously by an electric field mill near the fire and a lightning location system. Four negative flashes that occurred at the beginning of the time period were not recognized as ground flashes by the location system. These four flashes out of a total of 37 flashes exceeding 90 V/m at the electric field meter were the only flashes perceived as lowering negative charge; all the others lowered positive charge, whether recognized by the locating system as ground flashes or not. The median current of the positive flashes to ground as calculated from the location system output was 36 kA. Application of simple models of charge distribution to the plume structure indicate that the plume and associated cloud had a monopolar charge distribution, with positive charge at altitude and that the fire was most probably the source of this charge. A charge generation rate of 2 nC/m2/s by the fire could generate enough charge to satisfy the flash discharge rate and amount using the simplest model. The locations for the ground termini of the flashes are downwind of the fire location, some by as much as 30 km. They seem to be associated with the anvillike structure of the plume, although it is known that positive ground flashes commonly occur at considerable horizontal distances from their origin. There is no time organization to the distances of the termini from the fire.