3. Verifying Spatial Coverage of GPATS Lightning, and Creating a Thunderstorm Activity Grid
- what it is and the network
- how gpats is detected
- strengths and limitations
- what it is and the network
- how lis is detected
- strengths and limitations
1. GPATS Verification with LIS
- Extract and process LIS dates and times, accounting for leap seconds
- 8 days (4 summer 4 winter), of LIS subjectively chosen for spatially large amounts of lightning activity
- gpats restricted to LIS viewbox space/time windows resulting in #X boxes total
A list of determined ES events is crucial to developing an ES climatology and synoptic classification. To discriminate U.S. ES events from SS, Colman (1990a), Moore et. al. 2003, Horgan et. al. 2007 and others used National Weather Service observer reports of thunderstorms to identify thunderstorm locations. Unfortunately, Australia does not have a dense spatial and temporal surface and upper air observing network. Using thunderstorm reports at upper air sounding locations and times may result in too few ES to create an Australian ES climatology and classify synoptic environments. In fact, a search of Melbourne Airport “Thunderstorm” Present Weather Observations at 00 and 12UTC between 2008-2015 yielded only 6 reports. An effective alternative to Bureau of Meteorology observer thunderstorms reports could be the location and timing of observed lightning strokes from the Global Position and Tracking System (GPATS), as a proxy for thunderstorm locations provided the network’s stroke location accuracy is within the typical thunderstorm spatial length scale of 4 to 40km (meso-beta scale, Fujita 1981). The European ADTNET sensor network with similar density and location calculation methods to GPATS, has published location accuracies of 5-6 km (Cummins and Murphy 2009). In addition, Kumar et. al. 2013 found almost all (94%) of GPATS strokes near Darwin over two wet seasons (October 2005–April 2006 and October 2006–April 2007), were within 10km of radar-derived convective cells. These results, although pertaining to the Australian tropical wet seasons, gives confidence to GPATS stroke location accuracies being within typical thunderstorm length scales.
A limitation of the GPATS ground-based network is its ability to detect all types of lightning strokes (intra-cloud strokes (IC) and cloud to ground (CG) or ground to cloud (GC) strokes constituting “total lightning”). In a comparison of GPATS stroke types to stroke types from a high-quality research lightning flash counter (CGR4) in Brisbane, Kuleshov (2012) showed GPATS grossly under reported IC strokes, with the majority of detected strokes being either CG or GC types. Given IC to CG (or IC to GC) ratios range from 2-10 in most thunderstorms (Cummins and Murphy 2009), GPATS may be under-detecting much of the total lightning spatial coverage.
A spatial comparison between GPATS strokes and stroke data from reliable total lightning sensors could quantify how much spatial coverage GPATS is missing. The Lightning Image Sensor (LIS) on the TRMM polar orbiting satellite is one such total lightning sensor, considered to be very accurate in detecting total lightning (Cummins and Murphy 2009). LIS stroke data, and spatio-temporal observation window data of the TRMM satellite swath (0.5 degree, 2 second each window respectively) is readily available. The spatial coverage of GPATS restricted to the same spatio-temporal observation windows of LIS, can be compared to the spatial coverage of LIS over Australia and , if comparable, GPATS can replace observed thunderstorm reports with superior spatial and temporal coverage. Given the vast amount of daily GPATS and LIS lightning stroke data (typically 100,000 to 300,000 strokes per day), only four summer and four winter days will be analysed.
Lastly a GPATS dataset covering Australian and ranging from March 2008 to December 2014 is readily available from the Bureau of Meteorology. Any ES and SS climatology using this GPATS dataset to identify thunderstorm locations will similarly range from March 2008 to December 2014.
The Global Position and Tracking System (GPATS) is a commercial ground-based lightning detection system, with 40 ground-based sensors throughout Australia. The sensors are separated by average distances of 600km (at most 1200km separation for a few sites in central Australia), and fairly even coverage except for a lack of sensors in eastern and southwestern WA, (www.gpats.com.au). The sensors accurately detect the time of a lightning stroke event (accurate to within 25 ns), in the very low frequency (VLF) radio band using Time Group of Arrival (TGOA) methods to geo-locate the lightning strike (Cummins and Murphy 2009). Strokes with the high currents (typically Type 1 and 2 CG stokes, rarely IC strokes), emit strongly in the VLF band and thus are better detected by TGOA sensors. Kuleshov (2012) compared GPATS stroke types to stroke types from a high-quality research lightning flash counter (CGR4) in Brisbane for years 2005, 2007, 2008. GPATS grossly under reported IC strokes observed by the CGR4 counter for those years, with GPATS IC stroke percentages from GPATS total strokes 0.4, 3.8 and 1.6% for 2005, 2007 and 2008 respectively, compared to CGR4 IC stroke percentages from CGR4 total strokes 57.4, 36.8 and 23% for those years respectively. Whilst GPATS location accuracies have not been published, the European ADTNET (with similar ground radio frequency sensor separation and the same location calculation methods as GPATS), has published location accuracies of 5-6km (Cummins and Murphy 2009).
Kumar et al (2013) identified radar-dervied convective cells near Darwin over two wet seasons (October 2005–April 2006 and October 2006–April 2007), for various monsoon wind flow regimes identified by Pope et al (2008). To assess lightning flash density per wind flow regime over the two wet seasons, Kumar et al used a subset of GPATS strokes which had one or more convective cells within a 10km radius and +-10 minute window of the lightning stroke. Of the 153125 GPATS strokes occurred during the 2 wet seasons, the convective cell criteria rejected only 6% of strokes. These results, although pertaining to the Australian tropical wet seasons, gives confidence to GPATS stroke location accuracies less than 10km.
GPATS CG, GC IC stroke data of date, time, latitude, longitude and peak current values in kilo-amps is available from 1998-2014, with positive peak current values denoting positive ground strokes (CG or GC strokes), and negative values denoting negative ground strokes (CG or GC strokes). Strokes with peak current values of 0.0 denote IC stokes. Kuleshov (2012) however identified GPATS negative stroke peak currents prior to 2007 erroneously reported as positive peak currents. GPATS negative and positive ground stroke numbers for 2007 and 2008 were comparable to CGR4 positive and negative strokes for those years, with the suggestion of processing changes in January 2007 corrected false recording of negative ground strokes as positive. A GPATS dataset from 2008-2014 was deemed sufficient to create a climatology of elevated thunderstorms.
Dowdy & Kuleshov (2014) developed an Australian ground stroke density map (number of strokes per kilometre per year), based on high-quality research lightning flash counter data (CGR4) and detected lightning from the TRMM polar orbiting satellite’s Lightning Image Sensor (LIS) over the period 1995-2002.
High values of GPATS ground stroke densities over 2008-2014 (Figure 1) compare well to Figure 1 (Dowdy & Kuleshov 2014), giving broad confidence in the spatial accuracy of GPATS. However a dearth of GPATS strokes in south-west Western Australia not seen in Figure 1 (Dowdy & Kuleshov 2014) is most likely due to a lack of GPATS ground sensors in that region. Also differing observation periods between the two plots could account for differing highest density values (GPATS: 17 strokes per km per yr, Figure 1: 12 strokes per km per yr).
para re GPATS density plot and CG/km/yr creation
Australian ground stroke density plots exist (Dowdy & Kuleshov 2014), derived from Lightning Flash Counter and TRMM LIS estimated ground stroke FLASH densities from total lightning FLASH densities (using Z = cloud/ground = 2 +-30% from Kuleshov (2006) emperical fraction of total to ground lightning flashes for Australia, Ng = Nt/(Z+1). Others have used this ratio to estimate number of ground strokes from TRMM (Given TRMM gives total lightning).
wanted to compare existing Australian ground stroke densities to GPATS ground stroke densities.
Over a 0.1 degree resolution grid covering Australia and the topics (98E, 180E, 65S, 17N), counted the number of GPATS lightning ground strokes within each 0.1 by 0.1 degree box. Then calculated number of strokes per km per year by the following: # strokes/((2014-2008)*10*10)
regions of highest density west of Darwin compare very well with D & K plot.
GPATS max density number is higher - suspect due to different period observation between to 2 plots (GPTAS 2008-2014 verses K & D 1995-2002), and different instruments used.
GPATS density shows derth of strokes in SW WA not seen in K & D plot - lack of sensors
large area off east Aust coast agrees well