PICTURE of the MONTH a High-Resolution Lightning Map Of
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VOLUME 142 MONTHLY WEATHER REVIEW JULY 2014 PICTURE OF THE MONTH A High-Resolution Lightning Map of the State of Colorado BRANDON J. VOGT University of Colorado Colorado Springs, Colorado Springs, Colorado STEPHEN J. HODANISH NOAA/National Weather Service, Pueblo, Colorado (Manuscript received 18 October 2013, in final form 4 February 2014) ABSTRACT For the state of Colorado, 10 years (2003–12) of 1 April–31 October cloud-to-ground (CG) lightning stroke data are mapped at 500-m spatial resolution over a 10-m spatial resolution U.S. Geological Survey (USGS) digital elevation model (DEM). Spatially, the 12.5 million strokes that are analyzed represent ground contacts, but translate to density values that are about twice the number of ground contacts. Visual interpretation of the mapped data reveals the general lightning climatology of the state, while geospatial analyses that quantify lightning activity by elevation identify certain topographic influences of Colorado’s physical landscape. Elevations lower than 1829 m (6000 ft) and above 3200 m (10 500 ft) show a positive relationship between lightning activity and elevation, while the variegated topography that lies between these two elevations is characterized by a fluc- tuating relationship. Though many topographic controls are elucidated through the mappings and analyses, the major finding of this paper is the sharp increase in stroke density observed above 3200 m (10 500 ft). Topography’s role in this rapid surge in stroke density, which peaks in the highest mountain summits, is not well known, and until now, was not well documented in the refereed literature at such high resolution from a long-duration dataset. 1. Introduction 2011; Barker Schaaf et al. 1988) and can support and re- fine empirically derived conclusions that result from In map form, lightning climatologies reveal variations shorter-duration datasets and/or from a more coarse in lightning activity across an area. This cartographic spatial analysis. information benefits a wide range of entities including A primary motivating factor for this work is that there those that maintain networks and infrastructures vul- is no comprehensive cloud-to-ground (CG) lightning nerable to lightning and those with interests in hydrology climatology for the state of Colorado in the formal lit- and wildland fire management. In Colorado and other erature. The most formal comprehensive work that an- mountainous regions with abundant summertime out- alyzes CG lightning in Colorado was published by Lopez door recreation opportunities, a high-resolution lightning and Holle (1986). The authors examine a single year climatology map is particularly relevant from a lightning (1983) of CG lightning data collected over the state’s hazards perspective. In meteorology and regional cli- northeast region where they found the lightning activity matological studies, the ability to document lighting to be mainly a warm season event, with a large majority attachment patterns at fine scales across a variegated of the flashes occurring June through September. Other landscape contributes a deeper understanding of thun- results of the work by Lopez and Holle (1986) show that derstorm behavior (Cummins 2012; Hodanish and Wolyn lightning activity ranges from extremely intense with some days experiencing 5-min flash rates in excess of 100, to thunderstorms producing less than 10 flashes in Corresponding author address: Brandon J. Vogt, Geography and Environmental Studies, University of Colorado Colorado Springs, a 5-min period. Geographically, Lopez and Holle (1986) 1420 Austin Bluffs Parkway, Colorado Springs, CO 80918. found that maximum flash concentrations were aligned E-mail: [email protected] north–south along the eastern slopes of the Colorado DOI: 10.1175/MWR-D-13-00334.1 Ó 2014 American Meteorological Society 2353 Unauthenticated | Downloaded 09/29/21 08:27 AM UTC 2354 MONTHLY WEATHER REVIEW VOLUME 142 Front Range, and then southeast to east along the north (DEM) terrain surface roughness in a lightning study, facing slopes of the Palmer Divide. our approach is made possible through the use of a suite In another investigation, Reap (1986) analyzed two of geospatial tools that are managed in a geographic in- years (1983 and 1984) of June through September formation system (GIS). A GIS is a mapping technology lightning data and other remotely sensed meteorological that enables users to interact with and analyze maps and variables over the western United States, including other sources of data. In this study, we used GIS and its Colorado, using a grid size of 47 3 47 km2.Ahigh sophisticated analytical methods to visualize the relation- correlation was found between terrain elevation and ship between lightning activity and elevation and to the hour of maximum frequency of lightning: the higher quantify this relationship in 152 m (500 ft) elevation the elevation, the earlier the onset of lightning activity. classes. The paper first discusses the data and the data The author concluded that a fairly stable seasonal dis- processing. This is followed by a brief discussion of the tribution and a generally homogeneous geographical lightning climatology of Colorado. Next, the focus shifts distribution of lightning activity exists over the western to a demonstration of the utility of GIS in quantitative United States, generally west of 1038W longitude. The lightning climatology studies through the characteriza- consistent spatial and temporal distribution observed tion of lightning activity by elevation. was likely the result of the strong control exerted by the underlying topographical features characteristic to that 2. Data part of the United States. Building on the successes of the works of Lopez and Lightning detection in the state of Colorado has been Holle (1986), Reap (1986), and others who have exam- ongoing since the 1970s. The Bureau of Land Manage- ined components of lightning activity in some or all of ment (BLM) was first to use such a system in order to the Colorado landscape (e.g., Orville and Huffines 2001; provide early warning of fire starts and to direct re- Zajac and Rutledge 2001; Boccippio et al. 2001; Barker sources to potential fire initiation locations (Krider et al. Schaaf et al. 1988), a lightning climatology paper by 1980). Throughout the 1980s, the BLM system was up- Hodanish and Wolyn (2011) comprehensively analyzed graded (Reap 1986), and in 1989 it was combined with lightning patterns over Colorado. This informal paper other regional lightning detection systems across the (e.g., not peer reviewed) analyzed 17 yr of CG flash data United States (Orville et al.1983; Mach et al. 1986)to on a 1-km spatial resolution map. The authors found form the U.S. National Lightning Detection Network that mountainous regions that had access to a continued (NLDN; Orville 2008, 1991). The NLDN itself has gone source of low-level moisture had the largest concentra- through numerous upgrades and expansions through the tions of CG lightning. These areas included the mountain– 1990s and 2000s (Cummins et al. 1998, 2006), and is now plains interface and the southern exposure of the known as the North American Lightning Detection San Juan Mountains. Mountainous terrain that did not Network, operated by Vaisala (Orville et al. 2002). The have access to a continued source of available moisture NLDN detects CG lightning flashes and strokes over experienced significantly less CG activity. It was also North America with a stroke detection efficiency of hypothesized that a certain meteorological regime, 60%–80% (during 2003–12), a flash detection efficiency known as the Denver Cyclone vorticity zone (DCVZ; of 90%–95%, and a spatial accuracy of 250 m (Cummins Szoke et al. 1984), plays a key role in both enhancing and and Murphy 2009). limiting the lightning activity across the greater north- Most negative CG flashes consist of multiple strokes, and east Colorado region. The authors also noted that large about half of these flashes have multiple ground contact valley regions surrounded by mountains show notice- points (GCP; Stall et al. 2009; Cummins and Murphy able minimums in lightning activity. 2009). On average, a negative CG flash will have from 1.45 The purpose of the current paper is to demonstrate to 1.7 GCP per flash (Valine and Krider 2002). These in- that a set of geospatial analytical tools can be used to dividual GCPs can be separated over a range of tens to build upon the Hodanish and Wolyn (2011) findings hundreds of meters. In this paper, stroke data are ana- and ultimately create a high-resolution statewide light- lyzed. Spatially, strokes characterize the GCP distribution, ning climatology. Advantages of our approach lie in the but represent about twice the number of actual GCPs. opportunity to visually explore vast datasets in map Recognizing the significant 2002–03 NLDN improve- form, a research format that facilitates visual thinking ments in stroke detection efficiency (Cummins and (MacEachren et al. 2004), and in the ability to generate Murphy 2009), the dataset examined in our study ex- quantitative results over discrete geographical regions. cludes the years prior to 2003. The dataset examined in Similar in methodology to the work of Cummins (2012) this paper covers the 10-yr period 2003–12. The 2002–03 who incorporated measures of digital elevation model NLDN upgrades are evident when comparing stroke Unauthenticated | Downloaded 09/29/21 08:27 AM UTC JULY 2014 P I C T U R E O F T H E M O N T H 2355 FIG. 1. Stroke density by elevation histograms for two sets of years: (top) 1996–2002 and (bottom) 2003–12. By comparing the two histograms, the 2002–03 upgrades to the NLDN are clearly discernible. Most notably, the later years (2003–12) show a marked increase in stroke density in the higher elevations, above about 3200 m (10 500 ft). density by elevation class between two periods (1996– 2011). The resulting number of strokes examined over the 2002 and 2003–12) over the state of Colorado (Fig.