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DOCUMENTATION

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Dataset Software

Vaisala US NLDN Lightning Stroke Data

These data are restricted to collaborators that have a working relationship with the NASA Marshall Space Flight Center (MSFC) Lightning Group.

Introduction
About Vaisala
Data Collection
Spatial Coverage
Temporal Coverage		 Stroke Data File Format
File Naming Convention
Citing NLDN Lightning Flash Data
References
Contact Information

Introduction

Distinguishing Flashes and Strokes:

NLDN lightning flash data provides time, location, polarity, first-stroke amplitude, and multiplicity of each cloud-toground lightning flash. However, research has shown that there can be up to 20 return strokes that make up a flash and these strokes often strike the earth in different locations up to several kilometers apart. NLDN stroke data provides time, location, polarity, and amplitude of these strokes, providing the very detailed data needed for in-depth lightning analysis and lightning incident investigations. Flash data is most often used for general trending of lightning events; stroke data is critical for understanding specific incidents.

A lightning flash is the result of a transfer of significant charge between two charged objects. Lightning discharges can occur inter-cloud, cloud-to-cloud, cloud-to-air and cloud-to-ground. Generally, cloud-to-ground (CG) lightning has the greatest immediate impact on our lives. A CG stroke can kill, destroy equipment, start fires and disturb power delivery systems. Lightning detection in real-time is used to track, record and anticipate the occurrence of lightning. With adequate leadtime, preventative measures can be taken which can help to minimize its destructive potential.

A CG flash is typically composed of a sequence of individual cloud-to-ground return strokes which transfer significant charge from the cloud to ground, each stroke exhibiting peak currents in the range of 5 kA to 300 kA. These strokes have a nominal duration of 20-50 microseconds, and are typically separated in time by 20 to 100 msec. A flash will typically be comprised of 2-3 strokes, but may contain as few as one and as many as twenty strokes. The number of strokes in a flash is frequently referred to as multiplicity. For most flashes, the subsequent strokes (strokes which occur after the first stroke of a flash) will contact the Earth at the same strike point as the first stroke because they travel through the ionized channel of air established by the first stroke. However, approximately one third of all flashes can contain strokes with different ground strike points, separated by a few hundred meters to several kilometers. For practical purposes, researchers have typically defined a flash as consisting of all CG discharges which occur within 10 km of each other within a one second interval.

IMPACT sensors that determine the ground point of the CG lightning flash make use of both direction finding (DF) and time-of-arrival (TOA) methods of location. Direction finding provides azimuth information to the ground point, and any two df antennas can be used to locate that point. The more antennas, the better the location. TOA antennas record the precise (based upon GPS timing) time that they 'hear" the characteristic CG electromagnetic signal. Using the signal from two TOA antennas produces an infinite number of possible ground point solutions. These solutions produce a locus of points on the ground in the shape of an hyperbola. If another antenna is employed with either of the original two, a second hyperbola is generated, with the ground point at the intersection of the two curves. Occasionally, a third hyperbola is required to create a unique ground point solution.

Since the IMPACT sensors make use of both types of locating methods, their accuracy has been shown to out perform either method by itself. This is true for both CG location accuracy and probability of detection because each method provides for an independent solution.  The only constraint for a location solution is that there are more independent observations of angle and/or time than there are variables to estimate (latitude, longitude, and stroke time). The NLDN is designed to have an average of 6-8 sensors contributing to the detection and location of a stroke.

When only two stations detect a flash, there is redundant information for an optimized estimate of location. In that case there are four measured parameters (two azimuths and two arrival times), while only three parameters are calculated (latitude, longitude, and time). Location and time are determined by iteratively adjusting initial estimates of the parameters so that differences between observed and calculated azimuths and propagation times are minimized. Although IMPACT systems detect and analyze individual return strokes from each flash, they group all strokes that belong to the same flash and provide only one data record per flash. This record contains time, location, and peak signal amplitude of only the first return stroke, but provides multiplicity or number of strokes that made up the flash.

About Vaisala

Vaisala Thunderstorm is the lightning-specialty business unit within the Vaisala Group. Vaisala's US NLDN® is the most reliable lightning information system monitoring cloud-to-ground lightning activity across the continental United States, 24 hours a day, 365 days a year. The NLDN consists of more than 100 remote, ground-based Vaisala IMPACT ESP Lightning Sensors. These sensors instantly detect the electromagnetic signals given off when lightning strikes the Earth's surface. Within seconds of each strike the NCC's central analyzers process information on location, time, polarity, and amplitude of each stroke, and this information is sent to customers across the country. NLDN sensors use both magnetic direction finding and time-of-arrival recording methods to provide multiple identifiers for each lightning event. Highly refined algorithms, the result of over twenty years of lightning research, are used to process sensor information and calculate accurate lightning solutions. Since 1989, the NLDN has reported more than 20 million cloud-to-ground lightning flashes that occur every year. The development and maturity of the data product can be followed via the following timeline:

NLDN Timeline

1984-1989: Three separate regional lightning networks develop and operate at various locations. These networks used early direction finding methods for lightning detection
1989: Regional networks share data to establish a national network, the NLDN. This cooperative project is funded by the Electric Power Research Institute (EPRI) and operated by the State University of New York (SUNY) at Albany. For the first time, real-time data is available to users across the country.
1991: Real-time and historic lightning data becomes commercially available.
1993: NLDN Network Control Center is moved to its current location in Tucson, Arizona.
1994: Comprehensive customer research leads to the development of new, more powerful lightning display and lightning analysis software.
1995: First major system-wide upgrade completed with project partner EPRI. This upgrade added new lightning sensors that combined magnetic direction finding and time-of-arrival detection methods in a single sensor, the original IMPACT Lightning Sensor. NLDN began reporting flashes and individual return strokes within a flash. Flash detection efficiency increased to 80-90% with median stroke location accuracy of 500 meters.
1996-1999: Commercial applications of historic lightning data proliferate in electric power, insurance, and other industries as a result of improved location accuracy and application-specific software developments.
1998: The Canadian Lightning Detection Network, owned by Environment Canada, is completed. Operations for the CLDN are combined with NLDN operations in Tucson. The lightning data from the NLDN and CLDN sensors – close to 200 sensors – are processed on a single processing platform.
2000: NLDN real-time and historic lightning data is available on the Internet in several application-specific formats.
2003: Second major system-wide upgrade completed with replacement of aging sensors and earlier sensing technology with new, more advanced, third generation Vaisala IMPACT ESP Lightning Sensors throughout the network. Preliminary evaluations indicate overall minimum 90% flash detection efficiency and 60-80% stroke detection efficiency.

Data Collection

Vaisala's US NLDN Lightning Stroke Data consists of more than 100 remote, ground-based lightning sensors. Sensors send raw data via satellite to the Network Control Center (NCC) in Tucson, Arizona. Sensors instantly detect the electromagnetic signals created when lightning strikes the Earth's surface. Within seconds, the NCC's central analyzers process information on location, time, polarity, and amplitude of each stroke. Quality checks are performed for each location, and the lightning solutions are then archived and sent to the real time subscribers. Monthly quality controlled data are subsequently sent to subscribers on CD. These QC'ed data files received monthly via CD-ROM are then archived at the GHRC.

Spatial Coverage

The spatial coverage of the Vaisala US NLDN Lightning Stroke Data includes continental United States and extends 200-300 km off the coastline. The detection accuracy decreases with distance from the antenna networks.

Temporal Coverage

The temporal coverage of the Vaisala US NLDN Lightning Stroke Data is from January 1, 2007 to the present.

Stroke Data File Format

All GHRC "v0" Stroke data will only have the first 12 fields found in the following format table.. The "v1" Stroke data includes the 13th field in the table. The multiplicity field will always be "0" for stroke data.

Sample data line:

06/27/07 16:18:21.898 18.739 -88.184 0.0 kA 0 1.0 0.4 2.5 8 1.2 13 G

Column Number
Field name
Example
1
Date of strike (mm/dd/yy GMT)

06/27/07

2

Time of strike (to milliseconds) in GMT of flash event

16:18:21.898

3

Latitude (Deg)

18.739

4

Longitude (Deg)

-88.184

5

Polarity and strength of strike (kA)

0.0 kA

6

Multiplicity of flash

0

7

Semimajor Axis in Kilometers, of 50% probability ellipse for each flash

1.0

8

Semiminor Axis in Kilometers,of 50% probability ellipse for each flash

0.4

9

Ratio, Semimajor to Semiminor

2.5

10

Angle of 50% probability ellipse from North (Angle of Orientation )

8

11

Chi-squared value of statistical calculation

1.2

12

Number of sensors reporting the flash

13

13

Cloud-to-Ground / In-Cloud Discriminator (G, C)

G

File Naming Convention

A raw data file is produced for each day. The naming convention for the Vaisala US NLDN Lightning Stroke raw data file is:

Nstrokeyyyymmdd_daily_vx_lit.raw

where,

yyyy = Yea r
mm=Month
dd = Day
vx =  version number, where:
v0= Long milisecond format (12 fields)
v1=Added Cloud-to-Ground / In-Cloud Discriminator (G, C) as 13th field

As an example, the raw data file containing the cloud-to-ground lightning strokes detected on 22 July 2007 from 00:00:000 to 23:59:599 UTC would be contained in the file named "Nstroke20070722_daily_v2_lit.raw".

Citing NLDN Lightning Stroke Data

Our data sets are provided through the NASA Earth Science Data and Information System (ESDIS) Project and the Global Hydrology Resource Center (GHRC) Distributed Active Archive Center (DAAC). GHRC DAAC is one of NASA's Earth Observing System Data and Information System (EOSDIS) data centers that are part of the ESDIS project. ESDIS data are not copyrighted; however, in the event that you publish our data or results derived by using our data, we request that you include an acknowledgment within the text of the article and a citation on your reference list. Examples for general acknowledgments, data set citation in a reference listing, and crediting online web images and information can be found at: http://ghrc.nsstc.nasa.gov/uso/citation.html

References

Cummins, K.L., R.O. Burnett, W.L. Hiscox, and A.E. Pifer, 1993: Line reliability and fault analysis using the National lightning detection network. Preprints, Precise Measurements in Power Conference, Arlington, VA, Oct. 27-29, 1993.

Cummins, K.L., W.L. Hiscox, A.E. Pifer, and M.W. Maier, 1992: Performance Analysis of the U.S. National Lightning Detection Network. Proceedings, 9th International Conference on Atmospheric Electricity. St. Petersburg, Russia. A.I. Voeilkov Main Geophysical Observatory, Karbysheva 7, 194018, St. Petersburg, Russia.

Maier, M. W., et al., 1983: Locating Cloud-To-Ground Lightning With Wideband Magnetic Direction Finders. Presented at 5th Symposium on Meteorological Observations and Instrumentation. Toronto, Ontario, Canada, April 11-15, 1983.

Orville, R.E., 1994: Cloud-to-ground Lightning Flash Characteristics in the Contiguous United States: 1989-1991. Journal of Geophys. Res., Vol. 5., 10833-10841.

Reap, R.M., D.R. MacGorman, 1989: Cloud-to-Ground Lightning: Climatological Characteristics and Relationships to Model Fields, Radar Observations, and Severe Local Storms. Monthly Wea. Review, 117, 518-535.

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To order this data or for further information, please contact:

Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/

 

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