Michael Stock

Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms

A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.

Preliminary breakdown of intracloud lightning: Initiation altitude, propagation speed, pulse train characteristics, and step length estimation

Using a low-frequency lightning location system comprising 11 sites, we located preliminary breakdown (PB) processes in 662 intracloud (IC) lightning flashes during the summer of 2013 in Osaka area of Japan. On the basis of three-dimensional location results, we studied initiation altitude and upward propagation speed of PB processes. PB in most IC flashes has an initiation altitude that ranges from 5 to 10 km with an average of 7.8 km. Vertical speed ranges from 0.5 to 17.8 × 105 m/s with an average of 4.0 × 105 m/s. Vertical speed is closely related with initiation altitude, with IC flashes initiated at higher altitude having lower vertical speed during PB stage. Characteristics of PB pulse trains including pulse rate, pulse amplitude, and pulse width are also analyzed. The relationship between pulse rate and vertical speed has the strongest correlation, suggesting that each PB pulse corresponds to one step of the initial leader during the PB stage. Pulse rate, pulse amplitude, and pulse width all show decreasing trends with increasing initiation altitude and increasing trends with increasing vertical speed. Using a simple model, the step length of the initial leader during the PB stage is estimated. Most of initial leaders have step lengths that range from 40 to 140 m with an average of 113 m. Estimated step length has a strong correlation with initiation altitude, indicating that leaders initiated at higher altitude have longer steps. Based on the results of this study, we speculate that above certain altitude (~12 km), initial leaders in PB stages of IC flashes may only have horizontal propagations. PB processes at very high altitude may also have very weak radiation, so detecting and locating them would be relatively difficult.

Near-field, low frequency interferometric imaging of lightning

In the past, lightning interferometric mapping systems assumed that a source is very far from the measurement location. The assumption greatly simplifies the mathematics needed to locate the source, but the resulting source positions are limited to two spatial dimensions (azimuth and elevation). For short baseline systems, this assumption is very good because the source is almost always much farther away than the diameter of the array, making three-dimensional location all but impossible. Location in 3 spatial dimensions was primarily achieved by solving the time-difference of arrival equations [e.g. 1]. Even [2] used interferometry techniques only to determine the time-difference of arrival, and then solved the standard set of equations. However, using it is possible to adapt the interferometry imaging techniques described in [3] into the near-field case, allowing interferometry techniques to be used to locate sources in 3-D.

Estimating detection efficiency in the absence of satellites: ENTLN detection efficiency in 2015 and 2016

To estimate the detection efficiency of a ground based lightning location system, typically the ground based locations are compared to those seen by satellite. This works well because the detection efficiency of the satellite is relatively uniform with space, and reasonably good. However, this technique does not work for the 2015 or 2016 calendar years because the LIS satellite used is no longer in orbit, and has only recently been replaced by ISS-LIS. In this study, we will investigate applying a machine learning regression algorithm Lasso to lightning parameters determined by ENTLN to see if it is possible to estimate the detection efficiency of the network without satellite data.

Lightning interferometer via VHF Emission (LIVE)

The Lightning Interferometer via VHF Emission (LIVE) is the next iteration of broadband, interferometric lightning mapping systems. LIVE is a real-time capable instrument, producing images of lightning with a latency of about 5 seconds. Even under real-time operation conditions, LIVE will locate between 5000-50,000 sources in a single lightning flash, mapping out both the positive and negative breakdown portions of the flash.

Improvements to the BOLT lightning location system

The Broadband Observational network for Lightning and Thunderstorms (BOLT) is a network of low frequency electric field antennas which is capable of mapping lightning flashes in 3D. We are currently investigating improvements to both the hardware design of the BOLT antennas, and to the algorithm used to determine each lightning source location. The antenna revision increases the sensitivity of the antenna to frequencies most beneficial to lightning location. The processing algorithm improvements have increased the number of lightning sources located, and produce 3D maps of sufficient quality for charge analysis.