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Experimental Observations and Theoretical Modeling of Lightning Interaction with Tall Objects
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates various discharge processes associated with upward lightning initiated from tall objects, both experimentally and theoretically, trying to bridge the knowledge-gap in understanding the mechanisms of the phenomena.

Upward lightning initiation has attracted a great deal of interest during recent years due to the proliferation of tall communication towers and wind turbines.  Simultaneous lightning current waveforms, close electric field changes, and lightning location system data associated with upward lightning measured at the instrumented Gaisberg Tower (GBT) near Salzburg in Austria from 2005 to 2009 is studied. It was observed  that a majority of upward flashes (87%) were self-initiated, namely they were initiated at the tower top without any nearby preceding discharge activity, whereas only  26 (13%) upward flashes were nearby-lightning-triggered, namely they were triggered by nearby preceding lightning discharges occurring within 100 milliseconds or so. This observation is different from the study conducted in Rapid City in South Dakota of USA where they observed the majority of upward flashes (80/81) initiated from ten tall towers were nearby-lightning-triggered. The possible reasons for self-initiated upward flashes dominating at the GBT could be (1) the field enhancement due to the shape of Gaisberg Mountain which is more than 800 m above the surrounding terrain of the city of Salzburg and (2) low altitude of charge region in the cloud during non-convective season (September to March) in Austria.

Three modes of charge transfer had previously been identified in cloud-to-ground lightning, namely that happens during leader-return stroke sequence, during continuing current and during M-components. On the basis of the analysis of the simultaneous current, electric field changes, and high-speed video images measurements, a mixed mode of charge transfer to ground for ICC pulses and M-components in tower-initiated (upward) lightning is proposed. Compared to classical rocket-triggered lightning, occurrence of multiple branches of upward lightning from tall towers is very common. A newly illuminated or re-illuminated branch connecting to the already luminous channel attached to the tower during initial continuous current and continuing current following return strokes is frequently observed. A mixed mode of charge transfer to ground is composed of a new or decayed branch involving the leader/return-stroke mode of charge transfer to ground, and superimposing on the old channel with continuous (continuing) current mode of charge transfer to ground. This mode can explain ICC pulses exhibiting shorter risetimes, larger peaks, and shorter half-peak widths than “classical” M-components as reported in previous studies.

Charge density expressions for the tall object and lightning channel when lightning strikes a tall object are derived based on continuity equation and current expressions given by the series point current source model representation of the tall object and lightning channel. Validation of charge density expressions was performed by comparing the total vertical electric field at ground level calculated by the traditional dipole technique with that calculated by the monopole technique using charge density expressions derived above. Distribution of charge density versus height along the tall object and along the lightning channel by considering the different values of current reflection coefficient at strike object base and the return stroke front speed in the lightning channel has provided an alternative way to explain the polarity inversion of vertical electrical field at very close range from strike object base.

Characteristics of upward positive and bipolar lightning flashes observed from the GBT during 2000-2009 are studied in detail. A total of 26 upward positive flashes and 21 upward bipolar flashes were identified, which only accounted for 4% and 3%, respectively, of the total 652 flashes measured at the GBT during the 10-year observation period. There are only very few previous studies of upward positive and bipolar flashes. Bipolar flashes lower both negative and positive charges during different phases with the same flash. In the positive flashes, median values for flash peak current, flash duration, flash charge transfer, and flash action integral were determined as 5.2 kA, 82 ms, 58 C and 0.16×103 A2s, respectively. From simultaneous current and high-speed video measurements of one bipolar flash it is inferred that sub-branches are connected to one branch transferring both positive and negative charges successively, running into charge source regions of opposite polarity in the thundercloud. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xiv, 102 p.
Series
Trita-EE, ISSN 1653-5146 ; 2013:002
Keyword [en]
Tall objects, Upward lightning initiation, Charge density distribution, Upward positive lightning, Upward bipolar lightning
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-109770ISBN: 978-91-7501-595-8 (print)OAI: oai:DiVA.org:kth-109770DiVA: diva2:584130
Public defence
2013-01-21, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130109

Available from: 2013-01-09 Created: 2013-01-08 Last updated: 2013-01-15Bibliographically approved
List of papers
1. Measured current and close electric field changes associated with the initiation of upward lightning from a tall tower
Open this publication in new window or tab >>Measured current and close electric field changes associated with the initiation of upward lightning from a tall tower
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2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, D08102- p.Article in journal (Refereed) Published
Abstract [en]

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lightning location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, most of them (87% or 179/205) were initiated from the tower top without any nearby preceding lightning activity (called "self-initiated"), whereas 26 upward flashes (13%) were initiated from the tower top with immediately preceding nearby lightning activity (called "nearby-lightning-triggered"), including 15 positive ground flashes, one negative ground flashes, and 10 cloud discharges. The possible reasons for self-initiated upward flashes dominating at the GBT could be the field enhancement due to the Gaisberg Mountain above the surrounding terrain and low altitude of charge region during non-convective season (September to March), since we note that self-initiated lightning at the GBT occurred predominantly (79% or 142/179) during non-convective season. On the other hand the majority (85% or 22/26) of nearby-lightning-triggered upward flashes at the GBT occurring during convective season (April to August) and 80 nearby-lightning-triggered upward flashes out of 81 upward flashes observed at the ten tall towers in Rapid City in South Dakota of USA occurring during summer seasons, could be due to the result of high altitude of charge region. The triggering flashes were detected to be within 1 and 18 km distance and the time intervals between them and upward lightning initiation are in the range of 0.3 to 90.7 ms.

Keyword
Charge regions, Cloud discharge, Electric field change, Field enhancement, Ground flash, High altitude, Lightning activity, Lightning currents, Lightning location systems, Low altitudes, Measured currents, South dakotas, Summer season, Time interval, Upward lightning
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:kth:diva-59213 (URN)10.1029/2011JD017269 (DOI)000303122600005 ()2-s2.0-84859909925 (Scopus ID)
Funder
StandUp
Note

QC 20120522. Updated from submitted to published.

Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08Bibliographically approved
2. Mixed mode of charge transfer to ground for initial continuous current pulses in upward lightning
Open this publication in new window or tab >>Mixed mode of charge transfer to ground for initial continuous current pulses in upward lightning
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2011 (English)In: 2011 7th Asia-Pacific International Conference on Lightning (APL), 2011, 677-681 p.Conference paper, Published paper (Refereed)
Abstract [en]

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.

Keyword
ICC pulse;M-component mode;charge transfer mixed mode;high-speed video camera record;initial continuous current pulse;leader-return-stroke mode;lightning channel;near-far electric field measurement;shorter half-peak width;simultaneous current based Gaisberg tower;slow varying initial stage current;upward lightning;charge exchange;earthing;electric field measurement;lightning protection;video cameras;
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-82367 (URN)10.1109/APL.2011.6110212 (DOI)2-s2.0-84862967946 (Scopus ID)
Conference
2011 7th Asia-Pacific International Conference on Lightning, November 1-4, 2011, Chengdu, China
Note
QC 20120214Available from: 2012-02-11 Created: 2012-02-11 Last updated: 2013-01-09Bibliographically approved
3. Distribution of charge along the tower and lightning channel when lightning strikes a tall tower
Open this publication in new window or tab >>Distribution of charge along the tower and lightning channel when lightning strikes a tall tower
2012 (English)In: 2012 31st International Conference on Lightning Protection, ICLP 2012, IEEE , 2012, 6344342- p.Conference paper, Published paper (Refereed)
Abstract [en]

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.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
charge density distribution, electric field, lightning, tall tower
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:kth:diva-109802 (URN)10.1109/ICLP.2012.6344342 (DOI)2-s2.0-84870666483 (Scopus ID)978-146731897-6 (ISBN)
Conference
31st International Conference on Lightning Protection, ICLP 2012; Vienna;2 September 2012 through 7 September 2012
Funder
StandUp
Note

QC 20130109

Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2013-09-05Bibliographically approved
4. Characteristics of upward positive lightning flashes initiated from the Gaisberg Tower
Open this publication in new window or tab >>Characteristics of upward positive lightning flashes initiated from the Gaisberg Tower
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2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, no 6, D06110- p.Article in journal (Refereed) Published
Abstract [en]

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 26 flashes consisting of initial stage only were identified as upward positive discharges initiated by an upward negative leader from GBT. They accounted for 4% (26/652) of the total flashes recorded at the GBT. Nineteen (73%) out of the 26 positive flashes occurred during nonconvective season (September-March). Median values of flash peak current, flash duration, charge transfer, and action integral were determined as 5.2 kA, 82 ms, 58 C, and 0.16 x 10(3) A(2) s, respectively. Current pulses of high repetition rate superimposed on the initial portion of initial continuous current are inferred to be associated with the upward negative stepped leader process. The weighted arithmetic means of leader pulse peak current, leader pulse duration, leader interpulse interval, and leader pulse charge are 3 kA, 31 mu s, 32 mu s, and 42 mC, respectively. On the basis of an assumed stepped leader speed in the range of 8 x 10(4) to 4.5 x 10(5) m/s an upward negative stepped leader channel charge density of 15-87 mC/m, a leader length of 168-945 m, and an average leader step length of 2.4-13.3 m were estimated. The upward negative stepped leader channel charge density and length are significantly larger and smaller than their counterparts in downward negative stepped leaders, respectively, while the upward leader step length is consistent with previous studies. Possible reasons for this are discussed.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:kth:diva-59211 (URN)10.1029/2011JD016903 (DOI)000302236900004 ()2-s2.0-84859451112 (Scopus ID)
Funder
StandUp
Note

QC 20120507. Updated from submitted to published.

Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08Bibliographically approved
5. Characteristics of upward bipolar lightning flashes observed at the Gaisberg Tower
Open this publication in new window or tab >>Characteristics of upward bipolar lightning flashes observed at the Gaisberg Tower
Show others...
2011 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, D13106- p.Article in journal (Refereed) Published
Abstract [en]

We analyze current records for 21 upward initiated bipolar lightning flashes observed at the Gaisberg Tower (GBT) in Austria from 2000 to 2009. A bipolar lightning flash occurrence of 3% (21/652) is found during the 10-year observation period. Thirteen (62% or 13/21) of them occurred in nonconvective season (September-March). On the basis of the classification suggested by Rakov and Uman (2003), 13 (62%) of the 21 bipolar flashes belong to Type 1 associated with a polarity reversal during the initial stage (IS) current, five belong to Type 2 associated with different polarities of the IS current and the following return strokes, one belongs to Type 3 associated with return strokes of opposite polarity following the IS, and two of them are not assigned. We also find that the initial polarity reversal from negative to positive occurs more often (76% or 16/21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean (GM) and arithmetic mean (AM) of the total absolute charge transfer are 99.5 C and 125 C, with the GM and AM total flash duration of 320 ms and 396 ms, respectively. From simultaneous current and high-speed video measurements of one bipolar flash, within the field of view, the positive charge was transferred along one branch initially, followed by the negative charge transfer after cessation of the luminosity for 142 ms, while the other two branches connected to the main channel always contributed to the negative charge transfer during the whole process.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-37158 (URN)10.1029/2011JD015634 (DOI)000292609200002 ()2-s2.0-79960249760 (Scopus ID)
Available from: 2011-08-03 Created: 2011-08-02 Last updated: 2017-12-08Bibliographically approved

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