An Introduction to the National Lightning Detection Network

Summary

The U.S. National Lightning Detection Network is a commercial lightning detection network operated by the Vaisala Group. (LightningStorm.com). A network of more than 150 antenna stations are connected to a central processor that records the time, polarity, signal strength, and number of strokes of each cloud-to-ground lightning flash detected. The Improved Performance Combined Technology (IMPACT) Antenna system uses a combination of time-of-arrival of radio frequencies and direction finding technology to geo-locate the flash. Depending on the location within the network, Vaisala claims a location average accuracy of 500 meters, with a detection probability between 80-90 percent, varying slightly by region.

The NLDN data set is a continuous record from 3 Jan 1988 through the present.

Table of Contents

1. Discussion
2. Data Collection and Dissemination
3. Spatial and Temporal Coverage
4. Temporal Coverage
5. Flash Format Information for Standard Format
6. File Naming Convention
7. Citing NLDN Data
8. References
9. Contact Information

1. Discussion

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.

Note: There has been an improvement in the method of calculation for Peak Current Amplitude, which takes effect on july 1, 2004. Peak Current Amplitude Improvements: When estimating peak-current amplitudes, the NLDN uses a simple power-law model to compensate for signal attenuation due to propagation over finite-conductivity soil. This is sufficient for lightning events within 400 km of a sensor but underestimates propagation losses for more distant events. By modifying the parameters used in the algorithm, we can significantly reduce the random error for individual sensor measurements. At the same time, we will re-calibrate the NLDN peak-current estimate using rocket-triggered lightning data obtained in Florida in 2002-2003. The mean value for return strokes will increase by approximately 15% as a result of these changes, once implemented. With this re-calibration and propagation correction, we will reduce the overall expected error in peak current down to 15-20early a factor-of-two improvement from earlier years.

2. Data Collection and Dissemination

The Network Control Center is the primary hub for all NLDN communications, computation and archiving activity. The data to be archived are sent to dual redundant magneto optical recording media. Following archival, the raw data are sent to the lightning location calculation processes. The data are sorted by sensor report times, then a location is calculated for time coincident sensors. Quality checks are performed for the location. The lightning solutions are then archived and sent to the real time subscribers. Monthly data is subsequently sent to subscribers on CD. These files are then processed at the GHRC. The processing algorithm steps through the millisecond data provided by Vaisala and bundles the data into one second data blocks which are then archived.

3. Spatial Coverage

The spatial coverage of the NLDN includes continental United States and extends 200-300 km off the coastline. The spatial coverage of the Long Range Detection Network covers the western half of the northern hemisphere with the conterminous United States masked out. The detection accuracy decreases with distance from the antenna networks.

4. Temporal Coverage

The temporal coverage of the NLDN begins on January 3, 1988 to the present. Data for dates prior to 1 Jan 1994 were collected with either a direction finding system (Lightning Location and Protection, Inc. sensors) or time of arrival system (Lightning Position and Tracking System, LPATS sensors) or a combination of both.

The temporal coverage of the Long Range Detection Network is July 12, 1996 to the present.

5. Flash Format Information for Standard Format

The fields that comprise the parameters given in standard format are listed with their associated lengths.  A sample line from a data file in standard format follows.

Sample data line:

01/30/95 00:02:33 24.347  -82.469   -80.0  1
 

Parameter	Field Width	From Sample
Date	9	01/30/95
Time (msec.)	7	00:02:33
Latitude (degrees)	7	24.347
Longitude (degrees) W=negative	9	-82.469
Signal Strength (kA)	8	-80.0
Multiplicity (#strokes/flash)	3	1

6. File Naming Convention

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

Nflashyy.ddd_daily.lit.raw

where

yy     =  Year
ddd    =  Day Of Year

As an example, the raw data file containing the cloud-to-ground lightning flashes detected on 22 July 1994 ( day of year, 94203) from 00:00:00 to 23:59:59 UTC would be contained in the file named Nflash94.203_daily.lit.raw.

Similarly, the naming convention for the Long Range Detection Network raw data file is:

Lrflashyyyy.ddd_daily.lit.raw

where

yyyy    =  Year
ddd    =  Day Of Year

As an example the raw file consisting of Long Range Detection network data
for 31 January 1997 is Lrflash1997.031_daily.lit.raw.

7. Citing NLDN data

When NLDN data obtained from the GHRC is used in publication, the following acknowledgment shall be used:

NLDN data provided by the NASA Lightning Imaging Sensor (LIS) instrument team and the LIS data center via the Global Hydrology Resource Center (GHRC) located at the Global Hydrology and Climate Center (GHCC), Huntsville, Alabama through a license agreement with Vaisala, Inc. The data available from the GHRC are restricted to LIS science team collaborators and to NASA EOS and TRMM investigators.

8. 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.

9. Contact Information:

Data can be ordered and questions addressed at http://ghrc.nsstc.nasa.gov/.

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 FAX: 256-961-5859 E-mail: ghrc@eos.nasa.gov

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