The North American Lightning Detection Network (NALDN): Analysis of Flash Data – 2001-2009

Cloud-to-ground lightning data have been analyzed for the years 2001-2009 for North America, which includes Alaska, Canada, and the USA 48 states. We examine flashes recorded within the North American Lightning Detection Network (NALDN). We make no corrections for detection efficiency variability over the nine years or over the large geographical area comprising North America. There were network changes in the NALDN during the nine years, but we have not corrected or altered the recorded data in any way with the exception of deleting all positive lightning reports with peak currents less than 15 kA. Thus, the reader should be aware that secular changes are not just climatological in nature. All data were analyzed with a spatial resolution of 20-km. Our analyses provide a synoptic view of the inter-annual variability of lightning observations in North America, including the impact of physical changes in the network during the nine years of study. These data complement and extend previous analyses that evaluate the U.S. NLDN during periods of upgrade (e.g. Jerauld et al. 2005, Rudlosky and Fuelberg 2010). We analyze total (negative and positive) flashes for ground flash density, the percentage of positive lightning, and the positive flash density. Furthermore, we examine the negative and positive first stroke peak currents and the flash multiplicity. The highest flash densities in Canada are along the USA – Canadian border, 1-2 flashes km-2, and in the United States along the Gulf of Mexico coast from Texas through Florida, exceeding 14 flashes/km2 in Florida. The Gulf Stream is “outlined” by higher flash densities off the East Coast of the United States. Maximum annual positive flash densities in Canada range primarily from 0.01 to 0.3 flashes km2, and in the United States to over 0.5 flashes/km2 in the Midwest and in the states of Louisiana and Mississippi. The annual percentage of positive lightning to ground varies from less than 2% over Florida to values exceeding 25% off the West Coast, Alaska, and the Yukon. A localized maximum in the percentage of positive lightning in the NALDN occurs in Manitoba and western Ontario, just north of North Dakota and Minnesota. When averaged over North America, first stroke negative median peak currents range from 19.8 kA in 2001 to 16.0 kA in 2009 and for all years average 16.1 kA. First stroke positive median peak currents range from a high of 29.0 kA in 2008 and 2009 to a low of 23.3 kA in 2003 with a median of 25.7 kA for all years. There is a relatively sharp transition from low to high median negative peak currents along the Gulf and Atlantic coasts of the USA. No sharp transitions are observed for median positive peak currents. Relatively lower positive peak currents occur throughout the southeastern USA. The highest values of mean negative multiplicity exceed 3.0 strokes per flash in the NALDN with some variation over the 9 years. Lower values of mean negative multiplicity occur in the western USA. Positive flash mean multiplicity is slightly higher than 1.1 with the highest values of 1.7 observed in the southwestern states. We note that CG lightning has significant variations from storm to storm as well as between geographical regions and/or seasons and consequently, a single distribution for any lightning parameter, such as multiplicity or peak current may not be sufficient to represent or describe the parameter.