3.2 Total Lightning As an Indication of Convectively Induced Turbulence Potential in and Around Thunderstorms

3.2 TOTAL LIGHTNING AS AN INDICATION OF CONVECTIVELY INDUCED TURBULENCE POTENTIAL IN AND AROUND THUNDERSTORMS Ryan Rogers*1, Lawrence Carey1, Kris Bedka2, Wayne Feltz3, Monte Bateman4, Geoffrey Stano5 1 Atmospheric Science Department, University of Alabama-Huntsville 2 SSAI/NASA LaRC 3 CIMSS, University of Wisconsin-Madison 4 USRA, Huntsville, Alabama 5 NASA SPoRT, Huntsville, Alabama 1. INTRODUCTION statistics and found that thunderstorm related flight delays cost the commercial aviation industry Thunderstorms and their associated hazards approximately 2 billion dollars annually in direct pose a serious risk to general, commercial, and operating expenses. All threats to aviation military aviation interests. The threat to aircraft associated with thunderstorms are a result of the from thunderstorms primarily manifests itself in kinetic energy contained in the thunderstorm three areas: icing, lightning, and turbulence. updraft. Just the presence of lightning within a These hazards can be mitigated by avoiding convective cell gives some indication as to the thunderstorms by as wide a margin as is possible. strength of the updraft. Total Lightning (IC+CG) With aviation hazard reduction in mind, the information can reveal not only the location of the Federal Aviation Administration (FAA) provides convective updraft but can also give clues as to some guidelines for pilots to follow to maintain a the strength of the convection. Thunderstorms, by safe distance from thunderstorms in their 2012 definition, contain lightning and the ability to detect publication of the Aeronautical Information Manual and interpret lightning information is a valuable (AIM) in section 7-1-29 dealing with thunderstorm tool to determine which air space has increased flying. A few particularly relevant rules are: potential for Convectively Induced Turbulence 1. Do avoid by at least 20 nautical miles any (CIT), particularly in regions where radar coverage thunderstorm identified as severe or giving an is sparse or lacking altogether. It is hoped that the intense radar echo. This is especially true findings of this research will be applicable to data under the anvil of a large cumulonimbus. received from the Geostationary Lightning Mapper (GLM) on board the next generation GOES-R 2. Do clear the top of a known or suspected satellite scheduled for launch in early 2016. The severe thunderstorm by at least 1000 feet GLM will allow total lightning detection capability altitude for each 10 knots of wind speed at the across most of the western hemisphere at roughly cloud top. This should exceed the altitude 8 km2 resolution from geostationary orbit capability of most aircraft. 2. BACKGROUND 3. Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage. Convectively induced turbulence (CIT) is an aviation term encompassing all convectively Adherence to these guidelines can render induced turbulent motions that cause a turbulent large areas of airspace unusable in particularly response in aircraft (Lane et al. 2003). Cornman busy convective environments and in especially and Carmichael (1993) found that CIT may congested airspace. Therefore, it is not always account for over 60% of turbulence encounters practical or possible to explicitly follow the FAA over the United States. Additionally, CIT events guidelines but the fact remains that direct and tend to be highly transient in both space and time indirect effects from thunderstorms are (Hamilton and Proctor 2002; Lane et al. 2003) responsible for significant numbers of injuries and making them especially difficult to track and monetary losses to the aviation industry as a forecast. Kaplan et al. (2004) studied the whole. Weber et al. (2006) analyzed FAA meteorological conditions present in 44 severe, accident-producing turbulence encounters and *Corresponding author address: Ryan H. Rogers, Earth found that well over 80% of the cases they studied System Science Center, UAHuntsville, 320 Sparkman occurred within 100 km of moist convection. It Drive, Huntsville, AL 35805; e-mail: follows that decreases in the distance between [email protected] aircraft and convective updrafts should result in characteristics will manifest themselves in greater chances for CIT as has been found in changes in the total lightning flash rate and these numerous other studies (e.g. Bedka et al. 2010; changes can be used to assess the strength of the Lane et al. 2012). In fact, Lane et al. (2012) state updraft and, by extension, the potential for CIT. that, “the risk of moderate or greater (MOG) turbulence is almost twice the background value 3. DATA as far as 70 km from the storm”. A common framework is necessary to define The present study necessarily involves the turbulence magnitude with values ranging from exploration and manipulation of a wide range of light turbulence at the low end to extreme meteorological and aviation information including turbulence at the high end. Pilot reports (PIREPs) lightning, radar, and turbulence datasets. Radar have been the traditional means of quantifying the data for this study comes from the National turbulence found in and around thunderstorms. Weather Service’s Weather Surveillance Radar- Unfortunately, PIREPs are dependent on the 1988 Doppler (WSR-88D) network and includes pilot’s subjective assessment of turbulence radar reflectivity as well as Vertically Integrated magnitude as well as the specific response Liquid (VIL). characteristics of the particular aircraft. In Total lightning data were primarily observed addition, the pilot must radio his or her with the North Alabama Lightning Mapping Array observations to the ground, which can result in (NALMA) which has been operated by NASA significant temporal and spatial deviations from the continuously since 2001 (Goodman et al. 2005). precise location where the turbulence was The NALMA is a 3 dimensional Very High encountered. These factors render PIREP data Frequency (VHF) Time of Arrival (TOA) network less than ideal for research applications. consisting of 11 sensors dispersed across Cornman et al. (1995) outlined a way in which northern Alabama with a base station located at many of the drawbacks of the PIREP system could the National Space Science and Technology be eliminated and result in a research-quality Center (NSSTC) on the campus of the University metric of aviation turbulence that is suitable for of Alabama in Huntsville. Typical location errors verification of forecasts and algorithms. The within 150 km of the network center are on the Eddy Dissipation Rate (EDR) is calculated using order of 50 m in the horizontal and 100 m in the existing sensors, avionics, and communication vertical, with location errors increasing with the technology to produce and disseminate an square of the range from network center at objective, aircraft independent, state-of-the- distances greater than ~150 km (Koshak et al. atmosphere turbulence metric (Cornman et al. 2004). While overall in-network flash detection 1995). These objective measures of turbulence efficiency is excellent, the aforementioned location are now available on a significant number of errors, line of sight issues and general decrease in aircraft using the FAA’s automated turbulence sensitivity at large ranges can result in reporting system and the National Center for degradation of detection efficiency (Rison et al. Atmospheric Research (NCAR) in-situ algorithms. 1999). For these reasons the current study will Eddy dissipation rates are calculated and reported only examine lightning flashes that initiated within every minute regardless of meteorological 150 km of the NALMA network center. conditions or pilot/ground support staff workload Additionally, we utilized the flash clustering and thus EDR data offer a significant improvement algorithm detailed in McCaul et al. (2005; 2009) to over PIREP-derived turbulence data. group individual radiation sources into coherent Numerous studies have shown that there are flashes. For the purposes of this study we useful correlations between trends in lightning selected only those flashes with 8 or more sources characteristics and thunderstorm strength (e.g. per flash for inclusion in the analysis. Goodman 1988; Steiger et al. 2007; Schultz et al. In order to test the findings in this study at GLM 2011). Recent work has also shown that total resolution we have utilized data from the GLM lightning is a much more complete indicator of proxy (Bateman et al. 2008). This product utilizes thunderstorm strength than CG data alone, since a combination of LMA and Lightning Imaging increases in updraft strength and resultant Sensor (LIS) lightning data in order to make a microphysical modifications are first manifested statistical best guess as to what the GLM will electrically in the form of increases in the IC flash detect when launched. The GLM proxy algorithm rate, which is a component of total lightning (e.g. matches LMA source data with a database of prior Williams et al. 1999; Wiens et al. 2005). Findings LIS flashes to produce estimates of flash of this nature indicate that changes in updraft characteristics at 8 km2 resolution. In cases where both the NALMA and the LIS detected flashes at 1. All EDR reports from below 20,000 ft. the same time and place it was found that only LMA flashes with 30 or more sources routinely 2. All EDR reports that were determined to be registered on the LIS (Proch 2010). GLM proxy from either ascending or descending legs of a data will likely differ from native LMA data in flight (flagged and removed if total ascent or overall flash count and rate due to the more descent was greater than 5000 ft over 15 stringent source per flash criteria and can differ in minutes), and EDRs which indicate a change spatial location as a result of coarser 8 km2 in altitude of greater than 333 ft min-1. resolution of the GLM proxy. In addition to NALMA VHF data, this study also The vast majority of flights that met the above examines data from the Earth Networks Total criteria were associated with flights operated by Lightning Network (ENTLN) in order to compare Delta Airlines.

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