Maxwell current density characteristics below isolated thunderstorms in tropics

Abstract

Surface electric field and the Maxwell current density (J_M ) have been measured beneath isolated thunderstorms at a tropical station, Pune, (18°32'N, 73°51'E, 559 m above mean sea level). The data collected over a period of 5-6 years have been broadly divided under three categories. In Category I, J_M remains positive for most of the duration of the storm, most of lightning-induced field changes are negative, and the flash rate quickly increases in the initial stage, remains high during the mature stage, and slowly decreases to low values in the dissipating stage of the storm. In contrast, in Category III, J_M remains negative for most of the duration of the storm, most of the lightning-induced field changes are positive, and both J_M and the average flash rate quickly decrease in the dissipation stage of the storm. In Category II, variations in J_M and flash rate are not parallel and J_M peaks in the dissipation stage of the storm when the flash rate is low. When the contributions due to lightning, precipitation, and coronae can be neglected, J_M mainly consists of displacement current during the active stage and of conduction current during the dissipation stage of the storm. In the dissipation stage of the storm, J_M nearly follows the electric field with a time lag of ~4 min, and the space charge released into the atmosphere by coronae currents from the ground can increase the conductivity of the subcloud layer by an order of magnitude. The presence of the extensive lower positive charge center or of inverted dipole structure in the bases of the Category II and III storms has been associated with the lower relative humidity and dew point depression and higher cloud base height computed for these storms as compared to that of a Category I storm.