Lightning Parameters Evaluation in the Colombian ...

Lightning Parameters Evaluation in the Colombian Highest Atmospheric Activity Zone C. Younes, H. Torres, E. Pérez, L. Gallego, G. Cajamarca, A. Pavas Research Program on Acquisition and Analysis of Signáis - PAAS-UN National University of Colombia Abstract: This paper presents an evaluation o f the lightning parameters characteristic o f Nechí which is the place with the highest atmospheric activity in the Colombian territory. Such place presents a very special characteristic of its lightning parameters (GFD, polarity, multiplicity, etc). Atmospheric activity of this place agrees with the N A L S A ' S Lightning Imaging Sensor ( L I S ) which data evaluation shows the same place in Colombia with the highest activity, Lightning parameters have been studied for a long time especially in European countries and the United States o f America, tropical countries like Colombia start to study •this phenomena just two or three decades ago. The research group P A A S - U N has been studying lightning phenomena for almost 20 years; during last 8 years PAAS-UN has analyzed the Colombian L L S data and with its lightnmg direct measurement experimental station called Ilyapa a characterization of the lightnmg activity has been done successfully showing places like Neehi with a very particular atmospheric activity characteristics that deserves an especial analysis.

was installed in 1997 and has been working properly smce that time, manipulated by I S A . Several studies have been made by PAAS-UN smce that time, and using the LLS-RECMA data it was identified the Colombian place with the highest lightning activity called Nechí and an evaluation o f its lightning parameters was done finding a very interesting results about its GFD, Polarity and Multiplicity specially. Additionally, using the Lighting Imaging Sensor LIS" data it was confirmed that Nechí is the place with the highest activity i n Colombian territory. 2

2. Study Methodology 2.1 Data Manipulation

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Keywords: Lightnmg parameters, L L S , Lightning activity.

Tropical zone,

1. Introduction Lightning activity varíes a lot around the world bemg more prevalent m the continents than i n the oceans; additionally such activity is not uniformly distributed, there are some places where the activity is higher. Colombia, which is located between 4°S and 12°N of latitude and between 68°W and 79°W o f longitude in the top comer o f South America in the middle of the tropical zone, has a lightning activity very particular. There are some places with one of the highest Ground Flash Density (GFD) of the world [1], but at the same time, i t has some places with GFD valúes very low. Lightning parameters in Colombia are evaluated using the Colombian lightning location system ( L L S - R E C M A ) that

Research Program on Acquisition and Analysis of Signal of the National University o f Colombia

There are two kind of data used i n this study, L L S R E C M A data and LIS data. 2.1.1 LLS-RECMA data Colombian L L S data is collected form a network composed by 6 LPATS series I V sensors which detects cloud to ground and cloud to cloud lightning strokes so a data manipulation had to be done i n order to grouping such strokes into flashes and to genérate some data bases with the data information. Figure 1 shows the spatial distribution of the L L S RECMA. 2.1.2 LIS data LIS has efficiency higher than LLS-RECMA, nevertheless due to it is located into a low orbit satellite ít can just observes áreas o f 580km x 580km along a band that cross Colombian territory i n different positions one or twicc a day, LIS observes just a few o f the total lightning that

Interconexión Eléctrica S.A. E S P . is the largest Colombian Power Utility. LIS is a space based instrument used to detect the distribution and variability o f total lightning (cloud-tocloud, intracloud, and cloud-to-ground lightning) that occurs in the tropical regions o f the globe aboard of the Tropical Rain Measurement Mission Observatory.

strikes the earth, however w i t h data of 4 years o f mission (December 1997 to December 2001) it is possible to calcúlate a Flash Density with LIS data.

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* Figure 1: Colombian L L S sensors distribution. Highest efficiency zone LIS detects events that créate optical emissions on the top of the cloud and by means o f grouping those, using an special algorithm. a flash datábase is created. Despite a flash detected by LIS is not exactly the same o f one flash detected by a L L S , it is possible to affirm that they are almost the same phenomena because when a cloud to ground flash is generated it begins as an intracloud discharge [2].

2.2 Parameters selection The parameters under study are Polarity, GFD and Multiplicity, such parameters where chosen because o f they can describe properly the lightning activity characteristics especially for engineering purposes.

Figure 3. Nechí polarity variation year to year Figure 2 shows the polarity variation in Nechí month to month using the 1997-2001 mean valué for each month. Figure 3 shows the relation between positivo and negative lightnings year to year in the área under study. It is very interesting that this región presents a valué of positive lightning very different than the one usually accepted by intemational researching about lightnmg, such valué use to be 5% - 10%. 3.2 Ground Flash Density 3.2.1 Spatial Variation Analysis The Figure 4 shows a GFD map o f the área under study made with the data of GFD for áreas of 3kmx3km; despite there is a zone that contains a very high densitv valué, it is also density valúes between 0 and 5 flashes/km year at just 30 km from the máximum valué. 2

2.3. Lightning parameters analysis Once the lightning parameters are chosen, then an analysis in time and space was carried out. Time variation analysis was done in some parameters for variations month to month for polarity and year to year for polarity and GFD.

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3. Lightning Parameters Analysis

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3.1 Polarity

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Figure 4. 1997 G F D map for Nechí in áreas of 3kmx3km. [SPOSITIVES HNEGATIVÉS]

Figure 2. Nechí polarity variation month to month

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Figure 5. Highest Ground Stroke Density valúes in Nechí's área from 1997 to 2001 Figure 5 shows the displacement o f the highest Ground Stroke Density (GSD) valúes since 1997 to 2001. Such valúes are the highest ones o f the Colombian territory which are very concentrated around N e c h í ' s área.. Figure 6 shows two Colombian lightning activity maps. The left map was made using L L S - R E C M A data and the other one was made using LIS data. The information that was used to créate such maps was the one of the whole period (1997-2001). The maps do not have scale because it was done with the aim o f comparing observationally the lightning activity.

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Figure 7. Left. G F D map using L L S - R E C M A data. Right. G F D map using L I S data.

3.2.2 Temporal Variation Analysis In the next figure, i t can be observed the GFD temporal variation for Nechí between 1997 and 2001. As it is shown, despite the variations year to year, the valúes are very high related to other regions o f the world with a great lightning activity. The GFD valúes were calculated using áreas o f 3km x 3km.

Figure 7 shows two maps of GFD o f the highest efficiency zone. On the left is a map created using L L S - R E C M A information and calculated i n áreas o f 30km x 30km and the other was created using L I S information. It was done with the aim of make a more detailed map of lightning activity with valúes using both systems. In the middle o f this zone is N e c h í ' s área which has the highest lightning activity, additionally it can be observed that both maps agrees each other despite it was used information from systems that detect different kind of phenomena (see [3]). -78

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1997

1998

1999

2000

2001

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Figure 8. G F D variation between 1997 and 2001

3.3 Multiplicity In the figures 9 - 1 1 i t can be observed llie multiplicity for each year of analysis, the multiplicity for both positive and negative lightning and the total multiplicity for all positive and negative lightning in the área under study.

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Figure 6. Left. Lightning activity map using L L S R E C M A data. Right. Lightning activity map using L I S

There are two main aspects to be considered, first that multiplicity does not change practically anythmg year to year and i t is very l o w compared with studies carried out in other countries_[4]

4. Results Analysis 4.1. Polarity Polarity is the parameter under study with has the most important results; i t is very different compared with studies carried out i n other countries even i n different places of Colombia [4].

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STROKE NUMBER 1997 B 1 9 9 8 H1999 Q 2000 H 2 0 0 l |

Figure 9. Total multiplicity variation between 1997 and 2001

Positive flashes percentages have a valué between 57% and 8 1 % which differ a lot with the percentage accepted by the international community that is about 5% to 10%, it demands an especial analysis due to the effects that it can cause i n engineering applications like lightning protection of structures specially to transmission lines that only considers negative flashes. The temporal variation during the year (figure 2) shows that all the months have more positive lightnmg percentage than the negative ones except for January, nevertheless this month is the month with the lowest lightning activity, in fact there were very few flashes i n such month compared with the other months and the few ones liad negative polarity.

4.2 Ground Flash Density As it can be observed i n figure 4, in the área of Nechí exists a place with a very high GFD (49 flashes/km year) in 1997, nevertheless there are some places quite cióse (30 km) with GFD valúes around 5 flashes/km year, so it is a place with a very especial characteristic about GFD valúes. 2

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4 5 STROKE NUMBER

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Figure 10. Negative and positive lightning multiplicity

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It is important to clarify that this place is located in the área which has the highest efficiency o f the LLS-RECMA, very cióse to Cerromatoso (see figure 1), so a possible problem with the LLS performance is excluded. In figure 5, it can be observed the displacement of the point with the highest GSD valúes in the área under study and in the whole Colombian territory, all this points are in the vicinity o f Nechí and its displacement is about 40 km around a middle point.

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Figure 11. Total lightning multiplicity

In order to be sure o f the lightning activity ni this área, a correlation between LIS data and LLS-RECMA data was done, figures 6 and 7 show two maps each one made with the referred systems data, and i t can be concluded that the área under study effectively has the major lightning activity i n Colombia( see [3]). Figure 8 shows the variation o f GFD valúes year despite it varíes between 30 flashes/km year flashes/km year, the valúes are very high and temporal variation is observed which demands an of more years o f study to identify possible óyeles. 2

The next table shows the multiplicity valúes for negative and positive lightning for the period 1997 - 2001

Table 1. Lightning multiplicity in Nechí 1997

1998

1999 2000 2001

POSITIVES

1.20

1,19

1,17

1,17

1,10

NEGATIVES

1,76

1,92

1,75

1,63

1,81

TOTAL

1,32

1,34

1,32

1,23

1,32

2

to year, and 50 again a analysis

4.3 Multiplicity Multiplicity does not have a significantly change year to year around 1.3 which is very low compared with different studies that declare this parameter with a valué between 3 and 5.

The highest valué is cióse to 2 strokes/flash in 1998 (see table I i for negative strokes. nevertheless due to negativo strokes are lower than positive ones, the total multiplicity is cióse to 1. and due to the distance that can oceur helueen subsequent strokes. it is possible to study strokes instead flashes m this /one. 5. Conclusiuns Lightning parameters such as polarity, GFD multiplicity are ver\t in Nechí than in other studies around the world,

and

1 ''or enginecring purposes, it demands an especial analysis specially for design of lightning protcction systems of structures, Lightnmg parameters characteri/ation in tropical /one like Colombia musí be done and keep in continuously study due lo i K particular behaviour. Lightning parameters in tropical zone present a variation in time and space that must be study constantly in order to get more information and improve the knowledge o f this phenomenon. The lightning activity characterization of this área was correlated with LIS data i n order to be sure o f the particular lightnmg activity distribution, the correlations has a reasonably agreement. 6. Acknowledgment The Colombian lightning data were provided by Interconexión Eléctrica S.A. E.S.P. ISA (Power Utility) and LIS data were provided by the N A S A Lightning Team. This work has been supported by the Colombian Science Council -COLCIENCIAS, Research Program PAAS-UN and National University of Colombia. 7. References [1]

H. Torres, et al "Experiences and first results of Colombian lightning location network" Proceedings 23rd International Conference on Lightning Protection, pp. 186-190, Firenze, 1996.

[2]

H. J. t hastian. R.J. Blakeslee, S.J. Goodman, D.M. Douglas, "Algorithm Theoretical Basis Document (ATBD) for the Lightning Imaging Sensor (LIS)". Technical Report, NASA, George C. Marshall Space Flight Center, Science Directorate, Earth Science Department. Revised February 1, 2000..

[3]

C. Younes, H. Torres, et. al. "a comparison between satellital and terrestrial lightning detection systems" Proceedings of ISH 2003. Delf. The Netherlands.

[4]

C. Younes, H. Torres, "Lightning Parameters Evaluation in Colombia by means of Terrestrial and Satellital Measurements", Master's Thesis. Engineering Faculty. National University of Colombia Bogotá. 2002.