The Velocities of Charged Hydrometeors and the Production of High Fields in Thunderstorms
Calculations of the terminal velocities of charged hydrometeors in the presence of electric fields have been confirmed experimentally. They have formed the basis of computations of the charging current J flowing through a thundercloud as a result of the operation of a precipitative mechanism of cloud electrification. Values of J were calculated for a range of values of field strength E, precipitation rate p 0, precipitation content L, cloud water content C, charge distribution, total separated charge, and the fraction f of the small particles that have undergone a charging event.
As E increases, J increases initially, reaches a maximum value J max and then decreases, becoming zero when the field reaches a value E max. At this point the dissipative effect of the field in increasing the downwards velocity of the positively charged small particles and slowing down, levitating or even driving upwards negatively charged precipitation exactly balances the effect of gravitational separation of oppositely charged hydrometeors.
The values of E max and J max are very sensitive to variations of these listed parameters, and it is found that the estimated field required for the initiation of a lightning stroke (~350 kV/m) can be achieved only over a narrow range of conditions. The ease with which precipitative mechanisms can produce breakdown fields is considerably increased, however, if account is taken of spatial inhomogeneities in the field.
KeywordsPrecipitation Rate Terminal Velocity Cloud Droplet Precipitative Mechanism Downward Velocity
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