Channel Base Current Model for Negative Multiple Cloud-to-ground Lightning

Approximately 50% of all negative cloud-to-ground lightning flashes have one single ground connection. For this type of flash, the current intensity at the channel base is modeled here as an electronic circuit with especial focus on the current cut-off process. The fundamental concept used is the current instability criteria proposed by Stan Heckman (1992). Considering the total channel resistance (R) and capacitance (C) and the thermodynamic (cooling) channel constant (tau), he hypothesizes that when RC > tau, the channel is unstable and the current cut-off (discrete stroke); and when RC < tau, the channel is stable and there is continuing current as long as there is charge available in the channel. Laboratory arc discharge data based on current measurements are used for R and tau as a function of current. In order to simulate this circuit, three processes were taken into account: (1) power source (lightning upper channel development inside the cloud), (2) electrical breakdown at the channel top (return-stroke initiation) and (3) channel base decay and cut-off (comparing tau, R and C). All processes have a correspondent circuit element/subsystem. The fundamental electronic elements are respectively: (1) a controlled current source as a function of charge density and cloud leader length and cross area; (2) a breakdown device (surge arrester) combined with a cut-off elapsed time-dependent resistance performing the resistive interrupted channel; and (3) a resistance in parallel with a capacitor in series with a switch controlled by a comparator RC versus tau. The primary objective of this modeling is to simulate the current waveform in time, representing the cut-off and transition to another subsequent return-stroke with realistic values for interstroke interval, continuing current and M-components amplitude and duration. Typical values for these parameters will be represented as well as distribution extreme values. These current waveforms will be compared with cloud-to-ground lightning current measurements in towers (e.g. San Salvatore, Gaisberg, Cachimbo).