On the Intensity of First Return Strokes in Positive Cloud-To-Ground Lightning in Winter

Intensities of about 700 first return strokes (RSs) in positive cloud-to-ground lightning flashes observed by a 14-site Fast Antenna Lightning Mapping Array in one winter season are analyzed. Peak currents estimated from range-normalized peak amplitudes of electric field change (E-change) waveforms of RSs are used to represent their intensities. It is found that peak currents of positive RSs are positively correlated with their waveform parameters including pulse width, rise time, fall time and half-peak width. Time differences between lightning initiation and first RSs are closely related with peak currents of positive RSs, with strong positive RSs usually associated with small time differences. Peak currents of positive RSs are also related with amplitudes of preceding leader pulses. Out of 674 positive RSs, 232 are preceded by positive leader pulses, and amplitudes of these pulses are positively correlated with peak currents of RSs. Strong positive RSs with peak currents larger than 150 kA are analyzed and compared with strong negative RSs. While strong negative RSs usually produce abnormal E-change waveforms that are generally different from typical RS waveforms, strong positive RSs do not have such special features. Strongest positive RSs usually occur within a very short time (usually smaller than 10 ms) after lightning initiation and are more likely to be associated with positive preliminary breakdown pulses. It is inferred that these strongest RSs are usually preceded by a fast downward positive leader with a speed on the order of 10(6) m/s. Charge structures responsible for strong positive RSs are discussed.