Helin Zhou

Characteristics of upward positive lightning flashes initiated from the Gaisberg Tower

We report the measured current characteristics of positive lightning discharges to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. On the basis of the recorded current waveforms, a total of ...

Characteristics of upward bipolar lightning flashes observed at the Gaisberg Tower

We analyze current records for 21 natural upward-initiated bipolar lightning flashes observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. Thirteen (62%) of 21 bipolar flashes occurred in non-convective season (September- March) and twelve (57%) of them occurred in seasonal transmission periods of March, August, and November in Austria. Thirteen (62%) of them belong to Type 1 associated with a polarity reversal during the initial stage current, based on the classification as suggested by Rakov and Uman [1]. We also find that the initial polarity reversal from negative to positive occurs more often (16 of 21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean total absolute charge transfer is 99.5 C with a relatively short total duration of 320 ms.

The influence of meteorological conditions on upward lightning initiation at the Gaisberg Tower

Meteorological parameters associated with the initiation of upward lightning discharges from a tall tower were investigated. The parameters, including temperature, wind speed, air pressure, and relative humidity, were measured at the Gaisberg Tower top and at a distance of 170 m from the tower. A comparison of parameters associated with self-initiated upward flashes (initiated without any nearby preceding lightning activity) and nearby-lightning-triggered upward flashes (triggered by nearby preceding lightning activity) was made. It shows that at the Gaisberg Tower (GBT), relatively low ambient temperature facilitates to initiate upward flashes without any nearby preceding lightning activity. Relatively high ambient temperature (8.5 °C to 15.5 °C) facilitates to initiate upward positive stepped leaders, rather than the common continuous leaders, in the self-initiated upward negative flashes. No reliable correlation between wind speed and upward initiation of lightning was found.

Distribution of charge along the tower and lightning channel when lightning strikes a tall tower

We derive expressions to calculate the charge density along the tower and lightning channel based on the assumptions of the transmission line model of the lightning strikes to a tall tower and a series point current source placed at the tower top. These expressions are used to calculate the very close-range electric fields in the monopole (continuity equation) technique in terms of the retarded current and charge density along the tower and lightning channel and their results are compared with those calculated from the traditional dipole (Lorentz condition) technique in terms of the retarded current along the tower and lightning channel. Alternative explanations are provided to the inversion of polarity of the vertical electric field at very close range based on distribution of charge density along the tower and lightning channel.

Simultaneous observations of electric field changes, wideband magnetic field pulses, and VHF emissions associated with K processes in lightning discharges

We studied simultaneous electric field changes, microsecond-scale (VLF/LF) magnetic field pulses, and VHF emissions associated with K processes in 37 cloud and 54 cloud-to-ground lightning flashes ...

Mixed mode of charge transfer to ground for initial continuous current pulses in upward lightning

In this paper, we discuss properties of initial continuous current (ICC) pulses superimposed on the slow varying initial stage current in upward flashes initiated from the Gaisberg Tower (GBT) based on simultaneous current, near/far electric field measurements. ICC pulses are usually associated with M-component mode of charge transfer to ground, if only one branch of the lightning channel is active. However, there are several cases in which ICC pulses involve a predominantly leader/return-stroke mode of charge transfer to ground in a new channel that connects to the old channel carrying continuous current. With the help of high-speed video camera records, we find that this mixed mode of charge transfer for ICC pulses, leader-return stroke in one channel and continuous current in another channel, occurring simultaneously during the initial stage current in upward lightning from tall towers is very common. This is in contrast to triggered-lightning where usually only one channel is dominant and ICC pulses are associated predominantly with M-component mode of charge transfer. Also, ICC pulses with mixed-mode of charge transfer have shorter risetimes, larger peaks, and shorter half-peak widths.

Review of upward positive and bipolar lightning flashes at the Gaisberg Tower

First instrumentation for lightning measurements at the Gaisberg Tower (GBT) has been installed in 1998. During the 10-years period from 2000–2009 a total of 652 lightning events have been recorded. Vast majority of the lightning to the GBT was upward initiated and only one record is a candidate for a downward flash when the recorded current waveform is used for discrimination between upward and downward lightning. 3% (21/652) of the flashes were bipolar and about the same number of 4% (26/652) were positive. 73% of positive and 63% of bipolar lightning was triggered by the GBT during cold season in Austria.

Simultaneous current and electric field observations of upward negative leaders initiated from the Gaisberg Tower

In this paper, we present simultaneous current and electric field records of two upward negative leaders initiated from the Gaisberg Tower (GBT). It is shown that bipolar current pulses occur at the beginning of the upward negative leaders followed by regular unipolar leader pulse trains, and the corresponding electric field exhibits asymmetrical V-shaped pulses with a hump superimposed at the end of the pulse. The stepped leader characteristics in electric field waveforms at close range are similar to that of downward stepped leader pulses in altitude-triggered lightning flashes. Distinct electric field changes prior to the upward negative leader inception are indicative of nearby lightning discharges.

Close electric field changes associated with upward-initiated lightning at the Gaisberg Tower

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lighting location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, we find that most of upward flashes (179/205 or 87%) are initiated at the tower top without any nearby preceding discharge activity, 26 flashes (13%) are initiated by nearby triggering lightning discharges, including 10 positive cloud-to-ground lightning, 1 negative cloud-to-ground lightning, and 15 cloud discharges. The possible reasons for self-triggered upward flashes dominating at the GBT would be the field enhancement due to the Gaisberg Mountain above 800 m of the surrounding terrain of the city of Salzburg and low cloud base during cold season.

Calculation of electromagnetic fields in free space when lightning strikes a tall object

We calculate vertical electric field and azimuthal magnetic field at different elevation angles and distances associated with lightning strikes a tall object. Simple and exact expressions for electromagnetic fields are derived when the current reflection coefficient at tall object top is zero and return stroke propagation speed in the lightning channel is equal to the speed of light. Further, we investigate the effects of current reflection coefficient at tall object top being not zero and the propagation speed is less than the speed of light (e.g., one half) on electromagnetic fields. Interestingly, we find that the vertical electric field has its largest peak value either at the smallest elevation angle or at the largest elevation angle. While for the azimuthal magnetic field, we note that its largest peak value is always at the smallest elevation angle or relatively small elevation angles.