Negative first stroke leader characteristics in cloud‐to‐ground lightning over land and ocean

Abstract

We examine downward leader characteristics for negative first return strokes, along with estimated first stroke peak currents, for lightning occurring over land and ocean reported by the U.S. National Lightning Detection Network (NLDN). For the first time, to the best of our knowledge, we report independent evidence that supports the observations by lightning locating systems of higher first stroke peak currents for lightning occurring over ocean than land. We analyzed lightning occurring in five circular regions, each with 50 km diameter. In western Florida, the median stepped‐leader duration was 17% shorter over ocean than over land and in eastern Florida the median durations were 21% and 39% shorter over two oceanic regions than over land. We infer that the shorter durations over ocean simply reflect the higher (25% in western Florida and 11%–16% in eastern Florida) oceanic return stroke peak currents reported by the NLDN. These findings indicate that the cloud charge structure for (at least some) oceanic storms are different than those for storms over land. The percentage of flashes that had at least one NLDN‐reported negative cloud pulse prior to the first negative cloud‐to‐ground stroke was found to be about the same over land and ocean. Using regression analysis, we found no evidence that the relationship between leader duration and first return stroke peak current is different over land and ocean.

The Intracloud Lightning Fraction in the Contiguous United States

“…However, in our work, we employ the NLDN data because 1) its type classification accuracy is thought to be better (Nag et al 2015), 2) it does not suffer from the sampling limitations of the OTD/LIS datasets described later, and 3) the results can be directly compared to the climatology produced by B01.…”

“…The flash DE decreases in all directions outside the contiguous United States (CONUS) except Canada, where the Canadian Lightning Detection Network (CLDN) has been used in conjunction with the NLDN since 1998 (Orville et al 2011). As a result of a 2013 upgrade to the NLDN, the estimated CG flash DE improved from about 90% for the period of 2003-12 to over 95% throughout CONUS [see Nag et al (2014) for details]. Somewhat variable performance over earlier years is addressed in this work by employing corrections for detection efficiency, as described in the online supplemental material.…”

“…Additionally, the existence and nature of the lower positive charge center has a significant influence on the occurrence of CG flashes (Jacobson and Krider 1976). When there is a large positive charge center at low altitude above ground level, this can also inhibit the production of negative CG flashes (Nag and Rakov 2009;Mansell et al 2010). Storms that exhibit inverted and/or complex charge structures frequently have high IC flash rates, discourage the production of negative CG flashes, and encourage the production of positive CG flashes within the convective region of the storm Rutledge 1998, 2003;Wiens et al 2005;Carey and Buffalo 2007;Qie et al 2009;Bruning et al 2014;Fuchs et al 2015).…”

“…Advances in lightning detection and geolocation (summarized in Cummins and Murphy 2009;Nag et al 2015;Ushio et al 2015) have made essential contributions to our increasing knowledge in these areas. In addition, satellite observations of the diffuse cloud-top optical emissions produced from lightning since 1995 provide global climatologies of total lightning [(TL); all lightning flashes, including those that produce channels to ground] (Christian et al 2003;Mach et al 2007;Cecil et al 2014, hereafter C14;Cecil et al 2015).…”

“…First, type classification (IC vs CG) by the NLDN is a known and characterized problem, with regional variations. Overall, the classification accuracy is expected to be about 90% (Nag et al 2015;Zhu et al 2016), but with poorer accuracy for low-current negative first strokes (,10 kA) in anomalously electrified storms that produce large percentages of CG flashes with positive polarity (Fleenor et al 2009). We note that in our study the observation time period for most of the central Great Plains is limited to the OTD years (1995 to early 2000).…”