1. Introduction
2. Citing NLDN Data
3. U.S. Composite Lightning Product
4. HDF File Structure
    4.1 Fifteen minute Lightning Product HDF file structure
    4.2 Daily Lightning File Structure
    4.3 Latitude/Longitude Values
5. File Naming Convention
    5.1 Fifteen minute lightning product file naming convention
    5.2 Daily lightning product file naming convention
6. How to Get the HDF Library and Tools
7. References
8. Contact Informatio

1. Introduction

This README file contains information on cloud-to-ground lightning data produced by the U.S. National Lightning Detection Network (NLDN). The cloud-to-ground lightning product, the file format (Hierarchical Data Format or HDF), instructions for accessing the HDF library, and pertinent scientific references are provided here. The U.S. National Lightning Detection Network is a commercial lightning detection network operated by the Vaisala Group whose webpage is at (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. These data are ingested in real-time and stored in a raw data file.

NOTE: DISTRIBUTION OF THESE DATA ARE RESTRICTED TO EARTH OBSERVING SYSTEM (EOS) AND TRMM AFFILIATED INVESTIGATORS. WHEN CONTACTING THE GHRC FOR THE FIRST TIME WITH REQUEST FOR NLDN DATA, PLEASE PROVIDE YOUR NASA CONTRACT NUMBER.

2. Citing NLDN Data

When NLDN  data obtained from the GHRC is used in publication, the following acknowledgement 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 Global Atmospherics, Inc (GAI). The data available from the GHRC are restricted to LIS science team collaborators and to NASA EOS and TRMM investigators.

3. U.S. Composite Lightning Product

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.

 

The Global Hydrology Resource Center (GHRC) generates a gridded cloud-to-ground lightning product from the data collected from the U.S. National Lightning Detection Network. This lightning product is created by binning the number of flashes that occur over a 15 minute period to pixels. Each pixel is 0.0718954 degrees latitude by 0.0765027 degrees longitude (approximately 8 km x 8km). The data spatial range is from 20N to 53N (459 pixels) and from 60W to 130W (915 pixels). Therefore, the northwest corner is located at 53N latitude, 130 W longitude.

Pixel values can range from 0-254 which corresponds to the number of flashes detected in that pixel in a 15 minute period. A value of 255 denotes that 255 or more flashes occurred within that pixel during the 15 minute interval. Although the pixel values can range from 0-254 flashes, generally the maximum pixel value observed in the products is about 100 flashes. This product is produced four times each hour for the following periods past the hour:

(1) 00:00:00 - 00:14:59.99 UTC,
(2) 00:15:00 - 00:29:59.99 UTC,
(3) 00:30:00 - 00:44:59.99 UTC,
(4) 00:45:00 - 00:59:59.99 UTC.

The 8 km x 8 km gridded product is produced in near real time and is available approximately five minutes after the end time for each 15 minute period. For example, for the 15 minute time period ending at 01:14:59.99 UTC, the data would be available at 01:20 UTC. These real time products have not been quality assured as discussed below.

A daily composite is also produced from the 15 minute data over the continental United States. For the daily product, the value in each pixel is the number of flashes occurring in that pixel area during a 24 hour period (from 00:00:00 UTC to 23:59:59 UTC). The binned values are scaled by 3 such that a value of 1 corresponds to 1-3 flashes, 2 from 4-6, etc. A value of 255 indicates 765 or more flashes occurred in the pixel over the 24 hour period.

Both the 15 minute and daily products are generated in real time and are available on our FTP site. The annotation (in the HDF file) identifies files run in real time, or from quality assured data. These products (daily and 15 minute) are re-processed and archived the following month, from Quality checked data received from Vaisala.

4. HDF File Structure

The sole purpose of the HDF raster image format is for a space-saving technique. Although an image may be created with default parameters, this HDF file contains only pixel data and summary information. If an image is desired, the corresponding product in the GIF (Graphical Interchange Format) should be ordered.

4.1 Fifteen minute Lightning Product HDF file structure

Each 15 minute lightning product file contains:

The summary file indicates whether the image was generated in real time or was reprocessed using quality assured data.

The RIS8 image contains the following levels:

4.2 Daily Lightning File Structure

Each daily lightning product file contains:

The summary file contains the indication whether the image was generated in real time or was reprocessed using quality assured data.

The RIS8 image contains the following levels:

4.3 Latitude/Longitude Values

The navigation for the RIS8 images (15 minute and daily) is as follows:

The lightning array has a horizontal (East-West) dimension of 915 and a vertical (North-South) dimension of 459. The latitude bounds are from 20N to 53N in the North-South direction and from 130W to 60W in the East-West direction. The latitude and longitude can be computed for the center of each pixel in the following manner:

N latitude = 53. - ((row# - 1/2) x (0.07189542484))
W longitude = 130. - ((col# - 1/2) x (0.07650273224))

For example:

The (row#,col#) of the upper left hand (northwestern corner) pixel is (1,1). Therefore, the latitude and longitude of the center of pixel (1,1) is:

N latitude = 53. - ((1 - 0.5) x (0.07189542484)) = 52.96405228759
W longitude = 130. - ((1 - 0.5) x (0.07650273224)) = 129.9617486339

The (row#,col#) of the lower right hand (southeastern corner) pixel is (459,915). Therefore, the latitude and longitude of the center of pixel (459,915) is:

N latitude = 53. - ((459 - 0.5) x (0.07189542484)) = 20.03594771242
W longitude = 130. - ((915 - 0.5) x (0.07650273224)) = 60.03825136612

5. File Naming Convention

5.1 Fifteen minute lightning product file naming convention

The 15 minute products have the following naming convention:

NhhmmZyy.ddd_cglitn.hdf

where

hh = hour (UTC)
mm = minutes
yy = year
ddd = day of year

For example, N1600Z94.132_cglitn.hdf. Using the example file name, the file contains the lightning product for the 15 minute period ending 1600 UTC on day 94132 (May 12, 1994 from 15:45:00 to 15:59:59).

5.2 Daily lightning product file naming convention

The daily products have the following naming convention:

Ndailyyy.ddd_cum_litn.hdf

where

yy = year
ddd = day of year

For example, Ndaily94.182_cum_litn.hdf.

In the daily case, the day corresponds to the 24 hour period beginning at 00:00:00 UTC on the day of year indicated. In the example given then the daily file contains the binned 24 hour cloud-to-ground lightning for the period beginning 00:00:00 UTC on 94182 (July 1, 1994) and ending 23:59:59.

6. How to Get the HDF Library and Tools

See http://hdf.ncsa.uiuc.edu.

7. References

Cummins, K. L., M. J. Murphy, E. A. Bardo, W. L. Hiscox, R. B. Pyle, and A. E. Pifer, 1998. A Combined TOA/MDF Technology Upgrade of the U. S. National Lightning Detection Network, J. Geophys. Res., 103, 9035-9044. Idone, V. P.,

D. A. Davis, P. K. Moore, Y. Wang, R. W. Henderson, M. Ries, and P. F. Jamason, 1998. Performance evaluation of the U. S. National Lightning Detection Network in eastern New York, 1, Detection Efficiency, J. Geophys. Res., 103, 9045-9056.

8. 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 E-mail: ghrc@eos.nasa.gov

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NASA Information Contact: Michael Goodman, Global Hydrology and Climate Center GHRC Web Curator: GHRC Web Team Last update: Tuesday, 29-Mar-2005 13:08:04 CST If you have trouble viewing or navigating this page, please contact GHRC User Services. U.S. Government Compliance report.