W. Rison

A GPS‐based three‐dimensional lightning mapping system: Initial observations in central New Mexico

A GPS-based system has been developed that accurately locates the sources of VHF radiation from lightning discharges in three spatial dimensions and time. The observations are found to reflect the basic charge structure of electrified storms. Observations have also been obtained of a distinct type of energetic discharge referred to as positive bipolar breakdown, recently identified as the source of trans-ionospheric pulse pairs (TIPPs) observed by satellites from space. The bipolar breakdown has been confirmed to occur between the main negative and upper positive charge regions of a storm and found to be the initial event of otherwise normal intracloud discharges. The latter is contrary to previous findings that the breakdown appeared to be temporally isolated from other lightning in a storm. Peak VHF radiation from the energetic discharges is observed to be typically 30 dB stronger than that from other lightning processes and to correspond to source power in excess of 100 kW over a 6 MHz bandwidth centered at 63 MHz.

Energetic radiation associated with lightning stepped‐leaders

With the use of a NaI scintillation detector, bursts of radiation with energies in excess of 1 MeV were recorded at a mountain-top observatory immediately before three, nearby cloud-to-ground, negative lightning strikes. Coincident recordings of the electric field changes due to the discharges showed that, in each case, the bursts began between 1 and 2 milliseconds before and continued until the onset of the first return stroke. This radiation was associated with approaching stepped-leaders and may have influenced their development.

A distinct class of isolated intracloud lightning discharges and their associated radio emissions

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 μs. The HF emissions associated with NPBPs were broadband noise-like radiation bursts with a mean duration of 2.8 μs 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.

Radio interferometric observations of cloud-to-ground lightning phenomena in Florida

Radio frequency observations of cloud-to-ground lightning in Florida have been analyzed to document a number of features of the lightning. The observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges. Observations are presented for about 50 radiating events during five multiple-stroke, normal-polarity flashes to ground. The results confirm and extend results of a similar study of New Mexico lightning by Rhodes et al. (1994). Dart leaders, in-cloud K events, and attempted leaders are found to be the same phenomena, namely, a negative-polarity streamer that propagates horizontally and/or vertically to ground at estimated speeds of about 10 7 ms −1 down to 10 6 ms −1 . The step or impulsive fast electric field change produced by some K events is found to be caused by positive breakdown back at the starting point of the negative streamer, after the streamer has progressed over some or all of its distance. This breakdown appears to initiate a forward stroke along the streamer channel which can renew the breakdown at the front end of the channel. K breakdown which happens to occur during a continuing current discharge to ground produces an M event (channel brightening) when it connects with the conducting channel to ground. The step fast field change of the M events is found to be produced at the time of connection inside the cloud ; no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke. M events can also be initiated by fast (10 7 ms −1 ) positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region. These streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground, as the M event. In all cases the current increase or channel brightening produced by K and M events is predicted to occur in a forward direction, that is, toward or down the channel to ground, in the same direction as the initiating negative streamer. Positive breakdown is detected only immediately following return strokes or during some K streamers. In both cases the electric stresses are expected to be very large, suggesting that positive breakdown which produces a conducting channel is difficult to initiate. Results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground ; this forced the subsequent leaders to develop a new path to ground, first as series of attempted leaders and then as a dart-stepped leader.

GPS‐based mapping system reveals lightning inside storms

Great Plains storms are known for their ability to produce severe weather. They are also prodigious producers of lightning; just how prodigious has been vividly illustrated by observations in central Oklahoma with a new Global Positioning System (GPS)-based lightning mapping system.The observations are useful not only for studying storm electrification but also provide a valuable indicator of storm structure and intensity. The system maps lightning in three spatial dimensions by measuring the times at which impulsive VHF radiation events arrive at a network of ground-based measurement stations. Low-cost GPS receivers provide sufficient timing accuracy to produce high-quality pictures of the total lightning activity over a large area.

High speed video of initial sprite development

High speed video of sprites show that they are typically initiated at an altitude of about 75 km and usually develop simultaneously upwards and downwards from the point of origin with an initial columniform shape. The initial development of sprites appears to be dominated by corona streamers with velocities in excess of 107 m/s. Many of the observed characteristics are consistent with a conventional breakdown mechanism for both sprite initiation and initial sprite development.

Observations of VHF source powers radiated by lightning

Three-dimensional lightning mapping observations have been used to estimate the peak source powers radiated by individual VHF events of lightning discharges. The peak powers vary from minimum locatable values of about 1 W typically up to 10–30 kW or more in the 60–66 MHz passband of the receivers. An energetic positive bipolar event radiated in excess of 300 k W peak power. The strongest radiation sources tended to be observed in the upper part of storms, corresponding to the upper positive charge region, where the breakdown is of negative polarity. The results illustrate the bidirectional nature of intracloud discharges, with the largest source powers being along the negative portion of the discharge and an order of magnitude greater than the source powers along the positive portion. Overall, the source powers follow an approximate P−1 distribution for powers above about 100 W. The radiation sources indicate the location of the main charge regions in a storm; sample comparisons with radar data show that the main negative charge coincided with the precipitation core

THE SEVERE THUNDERSTORM ELECTRIFICATION AND PRECIPITATION STUDY

Abstract During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

TELEX The Thunderstorm Electrification and Lightning Experiment

Measurements during TELEX by a lightning mapping array, polarimetric and mobile Doppler radars, and balloon-borne electric-field meters and radiosondes show how lightning and other electrical properties depend on storm structure, updrafts, and precipitation formation.

The Electrical Structure of Two Supercell Storms during STEPS

Abstract Balloon soundings were made through two supercell storms during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in summer 2000. Instruments measured the vector electric field, temperature, pressure, relative humidity, and balloon location. For the first time, soundings penetrated both the strong updraft and the rainy downdraft region of the same supercell storm. In both storms, the strong updraft had fewer vertically separated charge regions than found near the rainy downdraft, and the updraft’s lowest charge was elevated higher, its bottom being near the 40-dBZ boundary of the weak-echo vault. The simpler, elevated charge structure is consistent with the noninductive graupel–ice mechanism dominating charge generation in updrafts. In the weak-echo vault, the amount of frozen precipitation and the time for particle interactions are too small for significant charging. Inductive charging mechanisms and lightning may contribute to the additional charge regions found at lower a...