NAMMA ATD Lightning Data

Table of Contents

Introduction

The Arrival Time Difference (ATD) Thunderstorm detection system is a low-cost innovation that has grown out of a requirement placed on the Met Office to locate thunderstorms for general weather prediction [public safety], the national Electricity supply Grid and Defense operations.

The NAMMA ATD Lightning data was provided by the UK Meteorological Office. These data are derived from multiple outstations, and contain lightning stroke data, latitude and longitude information,  as well as accuracy and weighting codes. These data provided lightning coverage for the African Coast during the NAMMA experiment. Time and location was determined by the Arrival Time Difference (ATD) of the reporting stations.

These data files were generated during support of the NASA African Monsoon Multidisciplinary Analyses (NAMMA) campaign, a field research investigation sponsored by the Science Mission Directorate of the National Aeronautics and Space Administration (NASA). This mission was based in the Cape Verde Islands, 350 miles off the coast of Senegal in west Africa. Commencing in August 2006, NASA scientists employed surface observation networks and aircraft to characterize the evolution and structure of African Easterly Waves (AEWs) and Mesoscale Convective Systems over continental western Africa, and their associated impacts on regional water and energy budgets. For more information about the NAMMA Campaign, go to the NAMMA web site: http://namma.nsstc.nasa.gov/

This UK ATD Lightning Detection Network was developed, based on VLF Arrival Time Difference (ATD) principles. At very low frequencies the electromagnetic pulse from the return stroke of a lightning strike is able to propagate over extremely large distances by using the earth-ionosphere waveguide. Thus a network can be designed to cover large areas of the Earth.

The ATD system at the Met Office works by accurately measuring the arrival time of the lightning pulse at several widespread locations. The system consists of 7 remote stations and one central control station (at Bracknell, UK). At each remote station there is a whip antenna, which is sensitive to radiation from 10-14KHz. The ATD system works by detecting the vertical component of the electromagnetic field generated by a lightning discharge. Strong electromagnetic emissions at these frequencies are caused by rapid neutralization of charge in the lowest few hundred meters of cloud to ground (C-G) strokes. Atmospheric attenuation at these frequencies is very low and the electromagnetic discharge (SFERIC) can propagate over thousands of kilometers along the earth-atmosphere wave guide. After filtering, the waveform is Fourier analyzed, time-stamped by a rubidium oscillator and then sent to the control station. If the lightning strike is strong enough to be detected by several remote stations, the control station then receives waveforms from all of these stations. At the control station, the waveforms are correlated and the arrival time differences between each remote station and a reference station are calculated. Each time difference defines a locus of constant ATD between two stations. Using the loci from four or more outstations, it is possible to fix the location of a lightning strike on the surface of the Earth. The accuracy of lightning fixing is dependent mostly on the accuracy of time keeping at the remote stations. To ensure the best possible time accuracy, there is a back-up rubidium oscillator at each remote station, and they are both checked regularly by GPS. The final location error is also dependent on the number of remote stations contributing waveforms and the geometry of the fix in relation to the remote stations. If the fix is inside the quasi-triangle formed by the UK stations, Gibraltar and Akrotiri, then the loci should be intersecting at right angles, and the fix is well determined. For example, over the UK, the error is typically about 2km. But if the fix is well outside the network, say in South America, then the loci all intersect at shallow angles, and the error is therefore greater, perhaps as much as 100km.

Each file represents a 24 hour timeframe of lightning data. These data cover the time period of August 14 through September 20, 2006. The files are named according to the example shown below:

namma_UK_ATD_yyyymmdd.dat

where, yyyy is the year, mm is the month, and dd is the day.

These data are arranged in ASCII data files, space delimited, and each file supplies data for a 24 hour period of time. Below is the first line of a data file shown as an example.

Example data:

2006-08-27 00:00:00 141 34.95 -56.15 1 11 39 0 11 754 0

Where each column of the ATD Lightning data file is defined below:

key to columns:

column 1 - Flash Number (FLASH NUM) of original position in the 5 min message
column 2 - Latitude (LAT)
column 3 - Longitude (LON)
column 4 - Rejection code 1 or -1 (RCODE)
column 5 - Accuracy (ACC) values from 0 to 15 typically 2-15
column 6 - Original Status (STAT)
column 7 - In (0) or Out (1) of area flag (OUT)
column 8 - weighting used for fading in flashes from out of area if requested (SCORE)
column 9 - Max. distance  "In" region (IMAXIN)  fade process not currently used
column 10 - Max. distance "Out" region  (IMAXOUT) fade process not currently used
column 11 - REJECTED or blank = default accept (REJECT) you do not see this
column 12 - (STATUS)  1 or blank = 0   you do not see this column

NOTE: accuracy code(ACC)  is 0-15,giving 1, 1.5, 2.25, 3.38, 5.07, 7.6, 11.4, 17.09, 25.63, 38.45, 57.67, 86.5, 129.75, 194.62, 291.93, 437.9 kilometer errors based on the maximum of the ellipse formed by the loci of the detecting stations.

J. Nash et. al, Progress in Introducing New Technology sites for the Met Office long range lightning detection system, Paper 2.9 WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2005), Instruments and Observing Methods Report No. 82, WMO/TD-No.1265

2. S. Keogh, E. Hibbett, J. Nash and J. Eyre, The Met Office Arrival Time Difference (ATD) system for thunderstorm detection and lightning location. Met Office, Numerical Weather Prediction : Forecasting Research Technical Report No. 488, e-mail:nwp_publications@metoffice.gov.uk

Questions related to these data should be directed to the data provider listed below:

Paul Taylor
paul.taylor@metoffice.gov.uk

To order these 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/