Abstract. Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short-duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 gs. The HF emissions associated with NPBPs were broadband noise-like radiation bursts with a mean duration of 2.8 gs and amplitudes 10 times larger than emissions from typical intracloud and cloud-to-ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes.

Sources of charge for the individual strokes of four multiple‐stroke flashes to ground have been determined, using measurements of the electrostatic field change obtained at eight locations on the ground beneath the storm. The resulting charge locations have been compared to 3‐cm radar measurements of precipitation structure in the storm. The field changes of individual strokes were found to be reasonably consistent with the lowering to ground of a localized or spherically symmetric charge in the cloud. The centers of charge for successive strokes of each flash developed over large horizontal distances within the cloud, up to 8 km, at more or less constant elevation between the −9° and −17°C environmental (clear air) temperature levels. Comparison with the radar measurements has shown that the discharges developed through the full horizontal extent of the precipitating region of the storm and appeared to be bounded within this extent. In one instance where cellular structure of the storm was apparent, the strokes selectively discharged regions where the precipitation echo was the strongest. Vertical extent of the stroke charge locations was small in comparison with the vertical extent of the storm. The field changes in the intervals between strokes have been found to exhibit many of the features which Malan and Schonland used to infer that ground flashes discharge a nearly vertical column of charge in the cloud. This and other evidence is used to show that their observations, which were made at a single station, could instead have been of horizontally developing discharges. The interstroke field changes have been analyzed using a point dipole model and found to correspond to predominantly horizontal charge motion that was closely associated with the ground stroke sources for the flashes. The interstroke activity served effectively to transport negative charge in the direction of earlier stroke volumes and often persisted in the vicinity of an earlier stroke volume, while subsequent strokes discharged more distant regions of the cloud. Long‐duration field changes that sometimes preceded the first stroke of a flash have been analyzed and found to correspond to a series of vertical and horizontal breakdown events within the cloud, prior to development of a leader to ground. These events were associated in part with the negative charge region that became the source of the first stroke and effectively transported negative charge away from the first stroke charge volume and from the charge volumes of subsequent strokes. Several continuing current discharges were found also to progress horizontally within the cloud and sustained currents in the range of 580 A to less than 50 A. The continuing current field changes were consistently better fitted by the monopole charge model than the field changes of discrete strokes within the same flash.

No abstract

Lightning discharges were investigated with high time‐resolution equipment on both electric‐field and electric‐field‐change meters. The analysis of the electrical records reveals that the late stages of intracloud discharges are very similar to those of cloud‐to‐ground discharges during the periods between successive return strokes (junction process) and during the period after the last return stroke (final process). In contrast, the initial portion of the field change of an intracloud discharge bears little or no resemblance to the initial portion of the leader field change of a discharge to ground. It is suggested that the difference in the initial breakdown characteristics results from variations in the relative populations of water drops and ice particles as they affect the internal impedence of the region of cloud where breakdown occurs. The difference in the initial field‐change characteristics of intracloud and cloud‐to‐ground discharges is so distinct that from the first 10 msec of the electric‐field‐change record one can predict with over 95 per cent certainty whether a discharge will reach ground or remain within the cloud.

Lightning charges, locations, and currents have been determined for 12 flashes from four winter storms observed on the Hokuriku coast of Japan during December 1977 through January 1978. Additional data is available from a total of eight winter storms. The heights and magnitudes of the charges in strokes‐to‐ground were calculated from simultaneous measurements of electric field changes made at seven stations covering an area of about 150 km2. Discharges lowering positive charge to earth often exhibit large continuing currents (>104 A) for periods up to 10 ms. One positive discharge involved a peak current value of ≃105 A and a charge that exceeded 300°C after 4 ms. Negative continuing current strokes are generally an order of magnitude smaller, ≃4×103 A, involving charges of 100°C or less. The positive lightning charges are located higher than the negative charges of the same storm, constituting a ‘normal’ bipolar system similar to the charge configuration found in summer thunderstorms. For the eight storms observed during this study, 26 out of a total of 63 strokes‐to‐ground were positive. A strong correlation exists between the fraction of positive ground strokes and the vertical wind shear in the cloud layer. On the basis of this study we suggest that the occurrence of positive strokes‐to‐ground is a consequence of the wind shear. The shear provides a horizontal displacement between the charges that helps to ensure that an initiating positive streamer will continue down to ground rather than into the negatively charged region that would normally be located directly below it. The data suggest that positive strokes‐to‐ground should appear at a threshold shear value in the cloud layer of about 1.5 m/s/km.