John C. Willett

Rocket and balloon observations of electric field in two thunderstorms

Instruments that measure the intense electric field strengths in thunderclouds (∼100 kV m−1) are designed to minimize the production of ions by small electrical discharges (coronas) emanating from the instruments themselves. The nearby charge of these ions would unpredictably disturb the natural field of the cloud. In an attempt to assess this disturbance, two different instruments (one carried by a rocket and one carried by a balloon) were launched on two occasions into thunderstorms. In spite of differing trajectories, the soundings were similar, which gives us some confidence in both instruments. In addition, the measurements revealed some interesting features of the two storms. Each storm appeared to have six significant and distinct regions of charge. The balloon soundings also revealed that lightning flashes temporarily increased the electric field strength above the thunderclouds (at altitudes from 9.7 to 14.3 km) by amounts up to 10 kV m−1, after which the fields decayed away in 50 to 125 s. One pair of ascent and descent rocket soundings, separated in time by a maximum of 60 s and horizontally by 1 to 3 km, showed little change in the thunderstorm electric field between ground and 7.5 km altitude.

Lightning‐channel morphology revealed by return‐stroke radiation field waveforms

Simultaneous video and wideband electric field recordings of 32 cloud-to-ground lightning flashes in Florida were analyzed to show that the formation of new channels to ground can be detected by examination of the return-stroke radiation fields alone. The return-stroke E and dE/dt waveforms were subjectively classified according to their fine structure. Then the video images were examined field by field to identify each waveform with a visible channel to ground. Fifty-five correlated waveforms and channel images were obtained. Of these, all 34 first-stroke waveforms (multiple jagged E peaks, noisy dE/dt), 8 of which were not radiated by the chronologically first stroke in the flash, came from new channels to ground (not previously seen on video). All 18 subsequent-stroke waveforms (smoothly rounded E and quiet dE/dt after the initial peak) were radiated by old channels (illuminated by a previous stroke). Two double-ground waveforms (two distinct first-return-stroke pulses separated by tens of microseconds or less) coincided with video fields showing two new channels. One “anomalous-stroke” waveform (beginning like a first stroke and ending like a subsequent) was produced by a new channel segment to ground branching off an old channel. This waveform classification depends on the presence or absence of high-frequency fine structure. Fourier analysis shows that first-stroke waveforms contain about 18 dB more spectral power in the frequency interval from 500 kHz to at least 7 MHz than subsequent-stroke waveforms for at least 13 μs after the main peak.

Observed Enhancement of Reflectivity and Electric Field in Long-Lived Florida Anvils

Abstract A study of two long-lived Florida anvils showed that reflectivity >20 dBZ increased in area, thickness, and sometimes magnitude at the midlevel well downstream of the convective cores. In these same regions electric fields maintained strengths >10 kV m−1 for many tens of minutes and became quite uniform over tens of kilometers. Millimetric aggregates persisted at 9–10 km for extended times and distances. Aggregation of ice particles enhanced by the strong electric fields might have contributed to reflectivity growth in the early anvil, but is unlikely to explain observations farther out in the anvil. The enhanced reflectivity and existence of small, medium, and large ice particles far out into the anvil suggest that an updraft was acting, perhaps in weak convective cells formed by instability generated from the evaporation and melting of falling ice particles. It is concluded that charge separation must have occurred in these anvils, perhaps at the melting level but also at higher altitudes, in o...

The influence of channel geometry on the fine scale structure of radiation from lightning return strokes

Evidence is presented indicating that channel geometry is a factor in determining the shape of the electric field waveforms radiated by subsequent return strokes in cloud-to-ground lightning. The data consist of 61 subsequent return strokes in both triggered flashes and natural lightning recorded at the NASA Kennedy Space Center during the summer of 1987. In the case of the triggered flashes, the data include the electric field change, the current at the channel base, and video images of the channel. Video images were obtained for about half of the natural flashes. The fine scale structure of the electric field changes radiated by these strokes has been compared during the first 10 μs. In cases where unambiguous identification of the channel could be made from the video recordings, this fine scale structure is similar for return strokes in the same channel and differs significantly for return strokes in different channels. This is true for both natural and triggered flashes. On the other hand, the current waveforms measured at the channel base (available for the triggered flashes) are similar in shape for all strokes, regardless of the channel. Similar results were also obtained for the natural flashes without video images, in which case strokes from the same flash exhibit a greater similarity than do strokes from different flashes (and therefore different channels).