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the negative strike can be considered substantiated. The calculated positive CG
strikes were observed only in the parts of the thundercloud, where two charged
layers located near the Earth had an approximately similar
“
normal dipole
”
struc-
ture (i.e., positive charges were above negative ones).
Brown et al. (2002) studied the relationships between CG lightning strikes and
the stages of the vertical motions development in the region of thunder centre,
taking as an example the data of observations of a multi-cell thundercloud centre
formed on 11 July 1989 in the region of the city of Elgin (North Dakota). Radar
observations of re
ectance and vertical velocity enabled one to identify some cells
within the thunder centre and trace the evolution of each cell. The evolution process
took place in accordance with the model of thunder centre developed in the late
1940s and in the following succession (Br
fl
é
on 2004):
maturing of upward motions and increase of the cells vertical extent at the stage
of cumulus clouds development;
maximum development of vertical motions in the upper part of a cell and
downward
fl
fluxes with rains from the middle part of the cell; and
light rains at the stage of the cell
'
s dissipation.
On 11 July there was observed a trend in the formation of the cell clusters, each
cluster consisting of both growing (at the stage of maturing) and disintegrating
cells. There were no CG strikes when the zone of storm
s convection contained
only one cluster of cells. The strikes took place only in the presence of two or more
clusters. Except from two cases, lightning strikes were observed in the zone of the
storm
'
s convection, as a rule, closer to the growing (mature) than to dissipating
cells. The discussed observational data favor the hypothesis according to which
rains falling at the sites of downward
'
fluxes determine the conditions favoring the
formation of lightning strikes that reach the Earth. However,
fl
it follows from
observations that the complicated structure of the electrical
field caused by super-
imposed
fields of several cells especially favors an appearance of CG strikes. This
situation can promote an earlier formation of CG strikes in the process of new storm
cells formation.
Although studies of lightning were started in the late 1700s, only in the early
20th century observations of the electric
field were carried out to retrieve the spatial
distribution of charges in thunderclouds responsible for the formation of lightning.
Then intensive studies began of the distribution of low-frequency electromagnetic
waves generated by lightning strikes (atmospherics). Random natural
elds
received on the Earth
s surface in the frequency range from several Hz to MHz are
created mainly by thunderstorm sources. Their study for a formalized description in
the interests of radar observations has been carried out by Remizov (1985). Radio-
noise caused by thunderstorms is disastrous for communication systems just as
thunderstorms themselves are disastrous for population as a factor that can lead to
'
fires and destructions.
Chronis and Anagnostou (2003) discussed the preliminary results of the func-
tioning of the experimental network recording the lightning strikes (ZEUS), which
consists of six receivers located in Western Europe and functions from July 2001.
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