[1] Electrical characteristics of thunderstorms on the central Tibetan Plateau at an altitude of 4508 m have been studied. The evolution of surface electric (E) field and the E field changes produced by lightning flashes under a representative thunderstorm revealed a tripole charge structure with a larger-than-usual lower positive charge center (LPCC). The storms appear to begin with the lower dipole of a normal tripole structure, rather than with the upper dipole followed by the development of a weaker lower positive charge. The flash rate is quite low and the average value is usually 1 fl/min. The IC flashes were usually polarity-inverted and occurred in the lower dipole. The large LPCC did not cause positive CG flashes to occur during the whole storm lifetime, and only negative CG flashes were observed in the late stage of the storm.
Diurnal and seasonal variation, intensity, and structure of deep convective systems (DCSs; with 20-dBZ echo tops exceeding 14 km) over the Tibetan Plateau–South Asian monsoon region from the Tibetan Plateau (TP) to the ocean are investigated using 14 yr of Tropical Rainfall Measuring Mission (TRMM) data. Four unique regions characterized by different orography are selected for comparison, including the TP, the southern Himalayan front (SHF), the South Asian subcontinent (SAS), and the ocean. DCSs and intense DCSs (IDCSs; with 40-dBZ echo tops exceeding 10 km) occur more frequently over the continent than over the ocean. About 23% of total DCSs develop into IDCSs in the SHF, followed by the TP (21%) and the SAS (15%), with the least over the ocean (2%). The average 20-dBZ echo-top height of IDCSs exceeds 16 km and 9% of them even exceed 18 km. DCSs and IDCSs are the most frequent over the SHF, especially in the westernmost SHF, where the intensity—in terms of strong radar echo-top (viz., 40 dBZ) height, ice-particle content, and lightning flash rate—is the strongest. DCSs over the TP are relatively weak in convective intensity and small in size but occur frequently. Oceanic DCSs possess the tallest cloud top (which mainly reflects small ice particles) and the largest size, but their convective intensity is markedly weaker. DCSs and IDCSs show a similar diurnal variation, mainly occurring in the afternoon with a peak at 1600 local time over land. Although most of both DCSs and IDCSs occur between April and October, DCSs have a peak in August, whereas IDCSs have a peak in May.
Leader process in a positive single‐stroke cloud‐to‐ground lightning flash was captured by a high‐speed video camera with a time resolution of 1000 frames/s. The positive leader process and the intracloud discharge process before the return stroke, which lasted about 600 ms, are studied in detail by combining the data from fast and slow antenna. The channel branches seemed to develop horizontally with a speed of the order of 104 m/s during the initial stage just outside the thundercloud. The luminous duration of the leader was about 12 ms, and the luminous intensity at the tip was much stronger than the channel behind it, presented obviously a stepped‐like characteristic. The 2‐D propagation speed of the stepped‐like leader ranged from 0.1 × 105 m/s to 3.8 × 105 m/s. The time interval between 26 leader pulses during the last 0.5 ms just before the stroke was about 17 μs according to E‐field changes.
[1] Current, luminosity, and electric field pulses in a rocket-triggered negative lightning flash have been analyzed based on the channel base current, high-speed video images, and electric field changes at 30 m from the channel. Among the 31 distinct current pulses, there are 4 return strokes, 18 typical M components, 5 large M components with unusual large peak current in a range of kiloamperes, 3 initial continuing current (ICC) pulses, and 1 stroke-M component (RM) event which exhibits both return stroke and M component features. The geometric mean of peak current is 13.5 kA, half peak width is 28.4 ms, and risetime from 10% to 90% peak is 1.1 ms for the 4 return strokes, while the corresponding values are 243 A, 400 ms, and 319 ms, respectively, for the 18 typical M components and 5.1 kA, 76.3 ms, and 34.6 ms, respectively, for the 5 large M components. The electric field and current waveforms of ICC pulses exhibit features similar to those of the M components, indicating the similarity of their mechanisms. Detectable optical luminosity is found just prior to all the pulse events, even return strokes. The M components are superimposed on a slowly varying continuing current, while the directly measured current prior to the return stroke is not significant. The simultaneous electric field and current waveform of RM implies a superposition of dart leader and M incident wave in the channel, and the possible reason is that two branches with common lower portions coexist simultaneously in the upper part of the discharge channel.
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