NCSU Partners with WFO Raleigh to Investigate Extreme Lightning Days in NC

Lightning is recognized as a significant weather threat to life and property, responsible for multiple deaths and injuries as well as billions of dollars in damage per year. Given these potential dangers, accurate predictions of lightning activity, particularly excessive cloud-to-ground (CG) lightning events, would be of great benefit to the general public and other affected groups such as the aviation, forestry, and recreation communities. Toward this goal of developing day-one outlooks for excessive lightning events, Dr. Gary Lackmann from North Carolina State University and his students Michael Graves and Lindsey Anderson have been working with the National Weather Service Forecast Office in Raleigh to study past significant lightning events in North Carolina.

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Composite 500 hPa map of “Eastern Trough” cases. Contours depict height lines; shading shows relative vorticity.

Using a thorough climatology of CG lightning spanning the years 2003 through 2012, a database of daily CG lightning strikes was produced and examined. Days with prolific lightning-producing convection (defined as more than 7000 strikes in 24 hours over central NC) were identified. For each prolific event, the researchers subjectively assessed the surface and upper-air synoptic patterns and categorized each event into four main synoptic patterns: Eastern Trough, Central Trough, Eastern Ridge, and Zonal. In addition, a set of non-prolific lightning events were identified, in order to compare and contrast “ordinary” lightning events with the prolific lightning events. Composites of upper air analyses and soundings were created for each category, as well as for the non-prolific cases.

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Composite dew point trace for 1500 UTC, at Raleigh-Durham (KRDU), from RUC gridded data, for prolific “Eastern Trough” cases. Note the positive moist anomaly in the mid levels, around 650 to 450 hPa, encompassing the “mixed-phase” region.

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Composite 1500 UTC RUC sounding at Raleigh-Durham (KRDU) for “Eastern trough” prolific lightning events. Shading represents estimated swath of “mixed phase” CAPE.

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Composite 1500 UTC RUC sounding at Raleigh-Durham (KRDU) for non-prolific “Eastern trough” lightning events. Shading represents estimated swath of “mixed phase” CAPE.

With the focus on the Eastern Trough category, which comprised 60 percent of the prolific lightning events, and comparing these cases to the composites of non-prolific events, researchers identified several characteristics that distinguished the prolific Eastern Trough events from those that produced much less lightning. In particular, a positive anomaly of mid-level humidity in the prolific lightning events was found to be statistically significant. This supports past research suggesting a high correlation between prolific lightning events and both high ice mass flux in the so-called mixed-phase region aloft (from -10°C to -30°C) and high values of precipitable water [see: McCaul et al. 2009 (WAF), Mazany et al. 2002 (WAF), Deierling and Petersen 2008 and Deierling et al. 2008 (J. Geophys. Res.), Kehrer et al. 2008 (WAF)]. A notably larger convective available potential energy (CAPE) in the -10°C to -30°C mixed-phase layer was also indicated with these excessive lightning cases, suggesting that strong instability yielding strong updrafts in the lightning-production region supports rapid charge separation and subsequent lighting production.

Future plans in the coming months include development of real-time diagnostics for the mid-level humidity anomaly and -10°C to -30°C CAPE anomaly, which will hopefully be folded into the experimental lightning activity forecasts that are completed daily each summer at WFO Raleigh. Expansion of the composite sample sizes with more cases is also planned.

We gratefully acknowledge the COMET program for funding that has made this important research possible.

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