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FORECASTERS' FORUM Introducing Lightning Threat Messaging Using OCTOBER 2019 F O R E C A S T E R S ’ F O R U M 1587 FORECASTERS’ FORUM Introducing Lightning Threat Messaging Using the GOES-16 Day Cloud Phase Distinction RGB Composite CYNTHIA B. ELSENHEIMER NOAA/NWS Southern Region Headquarters, Fort Worth, Texas Downloaded from http://journals.ametsoc.org/doi/pdf/10.1175/WAF-D-19-0049.1 by NOAA Central Library user on 11 August 2020 a CHAD M. GRAVELLE Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma (Manuscript received 9 March 2019, in final form 1 July 2019) ABSTRACT In 2001, the National Weather Service (NWS) began a Lightning Safety Awareness Campaign to reduce lightning-related fatalities in the United States. Although fatalities have decreased 41% since the campaign began, lightning still poses a significant threat to public safety as the majority of victims have little or no warning of cloud-to-ground lightning. This suggests it would be valuable to message the threat of lightning before it occurs, especially to NWS core partners that have the responsibility to protect large numbers of people. During the summer of 2018, a subset of forecasters from the Jacksonville, Florida, NWS Weather Forecast Office investigated if messaging the threat of cloud-to-ground (CG) lightning in developing convection was possible. Based on previous CG lightning forecasting research, forecasters incorporated new high-resolution Geostationary Operational Environmental Satellite (GOES)-16 Day Cloud Phase Distinction red–green–blue (RGB) composite imagery with Multi-Radar Multi-Sensor isothermal reflectivity and total lightning data to determine if there was enough confidence to message the threat of CG lightning before it occurred. This paper will introduce the Day Cloud Phase Distinction RGB composite, show how it can add value for short-term lightning forecasting, and provide an operational example illustrating how fusing these datasets together may be able to provide confi- dence and extend the lead time when messaging the threat of cloud-to-ground lightning before it occurs. 1. Introduction in the United States (e.g., Huffines and Orville 1999; Rudlosky and Fuelberg 2010; Holle et al. 2016). Be- The National Weather Service (NWS) has recorded tween 2008 and 2018, 54 lightning fatalities occurred in fatalities due to weather-related hazards in the United Florida—more than twice the number in Texas, which was States since 1940 (https://www.weather.gov/hazstat/)and, the second most during the same period. Although Florida based on total fatalities, lightning is the leading cause of is susceptible to lightning fatalities year-round, they occur deaths. Currently, the 10-yr average is 27 deaths annually, most often during the warm season (June–August) when which is ranked seventh and behind the two leading causes convection and lightning flash densities are greatest of weather-related fatalities: heat (10-yr average of 101 due to diabatic processes and the development of sea- annual fatalities) and flooding (10-yr average of 95 annual breeze convergence zones (Hodanish et al. 1997). fatalities). When lightning fatalities are ranked by state, Overall, lightning-related fatalities in the United they occur most frequently in Florida and coincide with States have decreased dramatically since the 1970s the highest cloud-to-ground (CG) lightning flash densities and dropped an additional 41% after 2001 when the NWS increased lightning public awareness through a Current affiliation: NOAA/NWS Southern Region Headquar- the NWS Lightning Safety Awareness Campaign ters, Fort Worth, Texas. (Jensenius 2016). This is likely due in part to the ‘‘When Thunder Roars, Go Indoors’’ slogan that was Corresponding author: Cynthia B. Elsenheimer, cindy.elsenheimer@ developed to mitigate the threat of lightning by en- noaa.gov couraging people to take shelter at the first sign a DOI: 10.1175/WAF-D-19-0049.1 For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/ PUBSReuseLicenses). 1588 WEATHER AND FORECASTING VOLUME 34 thunderstorm was approaching. However, Lengyel et al. 2013). Studies such as Goodman et al. (1988), Jameson (2005) state the majority (54%) of victims in lightning ca- et al. (1996), and Bringi et al. (1997) used DP differen- sualty cases they studied between 1995 and 2002 had little tial reflectivity ZDR ($0.5–1.0 dB) to identify columns or no warning the threat of CG lightning was approaching. of supercooled raindrops within the mixed phase of Therefore, it would be valuable to message the threat updrafts to anticipate initial lightning. In Woodard of lightning before it occurs and recent research has et al. (2012), ZDR columns were used with horizon- demonstrated short-term lightning forecasting skill. tal isothermal reflectivity to investigate the initia- To assist with short-term lightning forecasting, nu- tion of total lightning in 31 thunderstorms and 19 merous studies have shown the applicability of a radar- nonthunderstorms across northern Alabama. When based approach (e.g., Buechler and Goodman 1990). compared against the 40-dBZ threshold at the 2108C The foundation for these applied research studies is isotherm benchmark used in previous research, they the noninductive graupel–ice collision thunderstorm found ZDR (1.0 dB with 40 dBZ at 2108C) was effective Downloaded from http://journals.ametsoc.org/doi/pdf/10.1175/WAF-D-19-0049.1 by NOAA Central Library user on 11 August 2020 charging theory (e.g., Takahashi 1978; Saunders et al. at reducing false alarms but provided little advancement 2006). In the presence of supercooled liquid water in lead time and did not significantly improve current within a convective updraft, noninductive charging ex- operational reflectivity-based lightning prediction algo- ists when nonsticking collisions occur between graupel rithms. These results were supported by Scott and Carey and smaller, upward moving ice crystals. Baker and (2014) where only modest improvements when using Dash (1994) state when pieces of ice with different DP observations were found. Although the ZDR mass (i.e., ice crystal and graupel) are coated with a columns identified in Woodard et al. (2012) were negatively charged quasi-liquid layer of water (i.e., found using quasi-horizontal cross sections, the most supercooled liquid) and momentarily collide, the layer common approach is to use vertical cross sections of water is responsible for the exchange of mass. The through the storm’s core. In an operational environ- smaller ice crystal grows at a faster rate due to diffu- ment, using vertical cross sections are time consuming sion and charges positively. These collisions lead to an when interrogating multiple cells and integrating mul- exchange of charge and storm-scale charge separation tiple datasets. Furthermore, horizontal cross sections (e.g., Chylek et al. 2004; Petersen et al. 2005; Mosier can be challenging when using temperature thresholds et al. 2011). The result is the development of a strong because the height of radar bins increase away from electric field within the updraft and subsequent light- the radar on constant elevation scans. To eliminate ning (Zipser and Lutz 1994; Gremillion and Orville the difficulty with using horizontal cross sections, it is 1999). Therefore, the majority of short-term lightning common for to compute and use isothermal Z or ZDR forecasting research has been based on using radar (Schultz et al. 2013). However, isothermal ZDR is not data to identify the presence of graupel within the available to operational meteorologists at this time. mixed-phase region (i.e., between 2108 and 2208C) Intracloud (IC) lightning detections, from the of a developing updraft. Studies such as Vincentetal. National Lightning Detection Network (NLDN; (2003), Wolf (2007), Mosier et al. (2011),andSeroka Cummins and Murphy 2009) and Earth Networks et al. (2012) used radar reflectivity thresholds at iso- Total Lightning Network (ENTL; Liu and Heckman thermal levels to objectively and statistically determine the 2012), have also been used for short-term CG light- best CG lightning forecast criteria. While the results from ning forecasting. The strong electric field that de- those studies are slightly different, the majority of recent velops within a convective updraft often initiates IC radar-based research suggests using a 40-dBZ re- lightning before CG lightning. Krehbiel (1986),and flectivity threshold at the 2108C isotherm produces the others, state this occurs because 1) the electric field best lightning forecasts in terms of the critical success above the negative charge region in an updraft is index (Mosier et al. 2011). The Multi-Radar Multi- stronger than the electric field below, and 2) the Sensor (MRMS) system, developed at the National Se- critical electric field required for discharges is smaller vere Storms Laboratory and the University of Oklahoma, at lower atmospheric pressure higher in the updraft. provides operational isothermal radar reflectivity at the Furthermore, Bruning and MacGorman (2013) and 08, 2108, and 2208C levels using WSR-88D reflectivity Calhoun et al. (2013) formulated that high IC flash and the vertical profile of environmental temperature rates occur due to turbulent motions near the updraft from Rapid Refresh model analysis (Smith et al. 2016). core where noninductive charging from graupel–ice Dual-polarization (DP) radar observations provide crystal collisions is favored. These reasons are why the ability to identify hydrometers relevant to non- IC lightning tends to occur 5–10 times more often inductive charging, such as graupel, hail, and super- during the life cycle of a thunderstorm (Krehbiel cooled raindrops (Woodard et al. 2012; Kumjian 1986) and why IC lightning is directly correlated with OCTOBER 2019 F O R E C A S T E R S ’ F O R U M 1589 updraft intensity (e.g., Goodman et al. 2005; Tessendorf 2. Day Cloud Phase Distinction RGB composite et al. 2005; Deierling and Petersen 2008; Gatlin and The DCPD RGB composite is created by using Goodman 2010).
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