2006 19th International Lightning Detection Conference 24-25 April Tucson, Arizona, USA 1st International Lightning Meteorology Conference 26-27 April Tucson, Arizona, USA CLIMATOLOGY OF POSITIVE POLARITY FLASHES AND MULTIPLICITY AND THEIR RELATION TO NATURAL WILDFIRE IGNITIONS Beth L. Hall and Timothy J. Brown Desert Research Institute Reno, Nevada, USA 1. INTRODUCTION From 1995 through 2004, 42% of all wildfires recorded on US Department of Interior (DOI) and US Department of Agriculture (DOA) land were natural caused (i.e., lightning started). There are three factors of a cloud-to-ground (CG) lightning strike that are theorized to have a role in the potential for a wildfire ignition: 1) polarity of the strike; 2) multiplicity of the strike; 3) the existence of a long continuing current (LCC). It has been suggested that given since positive flashes are hotter than negative flashes, perhaps positive polarity strikes are more likely to ignite wildfires than negative polarity strikes (Latham and Williams 2001). The multiplicity of a strike is the number of return strokes per strike. The more return strokes that are focused on fuel, the more likely a wildfire will ignite due to the persistent attention of repeated lightning strokes (Flannigan and Wotton 1990). Finally, in about 30% of return strokes, a sustained current of low amplitude (i.e., a long continuing current (LCC)) is observed to flow in the channel to ground immediately following the current peak for a period varying from milliseconds to hundreds of milliseconds. It has also been suggested the neither the polarity nor the multiplicity of the lightning increases the probability of ignition, but the presence of an LCC due to its extended time that the lightning strike is continuously in contact with the fuels (Fuquay et al 1972; Latham and Williams 2001). In order to determine which of these factors, if any, have the strongest relationship to natural wildfire ignitions, a better understanding of the spatial and temporal climatology of polarity, multiplicity, and LCCs are needed. Both polarity and multiplicity have been found to be a function of latitude, coastal location, and temperature (Seity et al 2001). This study examined monthly patterns of the percentage of positive polarity strikes along with the average multiplicity of strikes across the US. This highlights not only coastal and latitudinal differences of the lightning characteristics, but also seasonal differences. Unfortunately, the lightning data used in this analysis does not include information on the existence of a LCC for each strike, so both the climatological and fire-related analysis presented here is only for lightning strike polarity and multiplicity. After the climatology of these lightning factors are presented, the spatial and temporal proximity of nearby positive strikes and the strikes having the greatest multiplicity are reported to determine if polarity or multiplicity are significant to the probability of a natural wildfire ignition. The hypothesis is if the polarity and multiplicity are significant factors for natural wildfire ignition, then they should be occurring close to an ignition location at a higher proportion that what is seen in that region climatologically. 2. DATA Cloud-to-ground (CG) lightning flash data and wildfires that occurred on federal lands within the continental US from 1995-2004 were examined in this study. Lightning data for this analysis was acquired from the National Lightning Detection Network (NLDN ). Location of each cloud-toground strike is reported to the nearest tenthousandth of a degree latitude and longitude with a time resolution of a thousandth of a second (1 millisecond). NLDN was upgraded and improved in 1995. Since this upgrade, the detection efficiency ranges from 80-90% for those CG flashes have a peak current > 5kA (Cummins et al 1998a). There is a location accuracy of approximately 0.5 km and a temporal accuracy of near 5 microseconds (Cummins et al 1998b). Due to the possible detection of non-cg flashes when the polarity is positive, several thresholds have been used as a cutoff to isolate the most probable positive CG flashes. This threshold ranges from 5 ka to 15 ka (Kempf and Krider 2003; Lang et al 2004; Cummins et al 1998b). For this analysis a threshold of 10 ka was used to count positive CG flashes. The percentage of positive strikes for each 0.1x0.1 degree grid cell is examined for the climatological
analysis along with the number of nearby positive strikes to each fire start. Flannigan and Wotton (1991) found that multiplicity, particularly from negative polarity flashes, was the most important predictor for naturally ignited wildfires. A weak connection between the multiplicity of the strike and the existence of a LCC has been suggested in Uman (1969), Shindo and Uman (1989), and Flannigan and Wotton (1991). In this study, the average multiplicity for each 0.1x0.1 degree grid cell is computed for the climatological analysis and the highest multiplicity value closest to each fire start is examined. Natural wildfire data was acquired from the Program for Climate, Ecosystem, and Fire Applications (CEFA; http://cefa.dri.edu) fire database that consists of wildfire records on public lands of the US Department of Interior (DOI) (Bureau of Land Management, Fish and Wildlife Services, Bureau of Indian Affairs, and National Park Service) and US Department of Agriculture (DOA) (Forest Service) from the US DOI Form-1202 and the US DOA FS Report 5100-29, respectively (Brown et al 2002). Fires were recorded in (or converted to) degrees latitude and longitude to the nearest one hundredth of a degree. The temporal resolution is the local calendar day and time when the fire was discovered. Both general and specific causes are listed for each fire record. Only fires with the general cause of natural were included in this study. Figure 1. Percentage of wildfires on federal land during 1995-2004 assigned a natural cause. White areas are primarily locations of nonfederally managed land. When only natural wildfires are considered, the time of year is a strong factor in the number of ignitions. Figure 2 shows that July and August are the most active months for natural wildfire ignitions; however, this varies by region. For example, Florida has its highest number of starts in the early spring, and southern California has most of its ignitions in early to late autumn. 3. GENERAL CLIMATOLOGY OF NATURAL WILDFIRES Figure 1 shows the percentage of naturally caused from 1995-2004 on federal managed lands across the U.S. based on a.25 degree grid. Areas of higher percentages of natural caused fires are primarily the western U.S. and Florida. Figure 2. Total number of natural fires on federal land in the continental US by month for 1995-2004. Comparing natural wildfires to polarity and multiplicity for the nation as a whole, the highest percentage of positive polarity strikes occur during
the colder months of November through February, but the highest average multiplicity occurs in July and August. Orville (1990) found that mean peak current (either positive or negative) varies by almost a factor of two from 25 ka in the New England region to 40-45 ka in northern Florida. However, he also found that the regions of the greatest percent of positive polarity strikes occur throughout British Columbia and the Midwest U.S. (Orville et al 2002). The former is a very northern, coastal location, and the latter is a mid-latitude, interior location. Figure 3 shows the monthly distribution of the percentage of positive polarity CG flashes for January, April, July, and October. In January, there are few positive strikes over most of the northwestern and north-central portions of the U.S., while the highest percentage of positive strikes occurs along the Appalachian Mountains, northwestern Ohio and the Missouri-Illinois border. In April, a high percentage of positive CG flashes occur to the east of the Rocky Mountains (e.g., eastern Colorado, western Kansas, Nebraska, and the Dakotas). However, the highest percentage of positive CG flashes during this spring month is in northern Minnesota and the New England states. For the summer month of July (when most natural, federal land wildfires occur), most of the US has a positive CG flash percentage of < 10%. The only regions that show a slightly higher percentage of positive CG flashes are Nebraska, eastern Dakotas, western Minnesota, and non-continuous regions in the Pacific Northwest. In October, the Carolinas have the highest percentage of positive polarity flashes along with some scattered regions in the central U.S. that were also dominant in April and July. Figure 3 suggests that the percentage of positive polarity flashes does not appear to be purely a function of latitude or land-water interfaces. Because of the monthly differences in not only the percentage but also location of the high percentage of positive polarity strikes, perhaps temperature of the land or atmosphere could be a driving factor if strongly influencing convection and thunderstorm development. Elevation does not seem to play a big role in the percentage of positive strikes; however, there does appear to be a higher percentage of positive flashes on the leeward side of the Rocky and Appalachian mountain ranges. Figure 3. Percentage of positive polarity flashes for the months of January, April, July, and October. Figure 4 shows the average multiplicity of strikes for the same months as shown in Figure 3. For most months, the average multiplicity appears to be relatively uniform across the U.S. with a slight increase in multiplicity in the southern states. July is the only month that shows a strong increase in average multiplicity, with the highest averages occurring in the southern states and the lower Midwest region. This north-south gradient in average multiplicity suggests that multiplicity might be a function of latitude. Also note the gradient of average multiplicity from the Gulf of Mexico onto land in July. This implies that multiplicity might be a function of surface roughness or larger daily temperature ranges distinguishing characteristics between land and water this time of year. Figure 4. Average multiplicity by month for January, April, July, and October.
4. FLASH POLARITY AND MULTIPLICITY RELATED TO NATURAL WILDFIRES In order to understand the possible role that polarity and multiplicity have on lightning ignited fires, natural wildfires that occurred on U.S. federal lands from 1995 through 2004 were related to lightning occurrence. These wildfires had to have lightning within 2 km of the discovery location and within 5 hours leading up to the hour of discovery. There were over 35,000 fires that met the criteria (Figure 5). Lightning strike data within a 2 km and 4 km distance from the fire were examined to determine (1) the total number of positive CG flashes, (2) the percentage of positive CG flashes among all CG flashes within the radius of concern, and (3) the highest multiplicity within these two radial distances. Though some fires have a higher number of positive strikes nearby, the proportion of positive strikes to negative strikes may be unchanged. Therefore, distinguishing between the total number of positive flashes and the percentage provides information on the quantity and ratio of positive strikes. number of positive flashes are represented as the percentage of total flashes recorded within a particular radius, positive strikes still tend to comprise less than 6% of the flashes (Figure 3). This percentage is typical for most regions of the U.S. (Figure 3), regardless of the occurrence of an ignition. In terms of probability, therefore, ignition locations appear to have the same proportion of positive flashes as locations without ignitions suggesting a weak relationship between positive flashes and ignition beyond mere climatology. In fact, note that nearly 5000 fires (approximately 1/7) had less than 2 positive flashes within 2 km of the ignition location. This high fraction of fires indicates that positive flashes can not be the single most important factor for wildfire ignition. Figure 6. Number of natural wildfires with ranges of total positive CG flashes within a 2 km (a) and 4 km (b) distance from the fire. Note the varying bin ranges in each plot. Figure 5. Location of natural wildfires on federal land from 1995 through 2004 that had lightning occurrence within a 2 km radius of the ignition and within 5 hours leading up to and including the hour of wildfire discovery. Figures 6 and 7 show the number of wildfires that had particular ranges of positive CG flashes and percentage of positive CG flashes within a 2 km (sub-figures a ) and 4 km (sub-figures b ) from the wildfires. Most fires either had less than 2 or between 25 and 100 positive CG flashes within 2 km of the wildfire (Figure 6a). When the analysis radius was extended to 4 km, the majority of fires had over 50 positive CG flashes within 4 km of the wildfire (Figure 6b). If the Figure 7. The number of natural wildfires for ranges of percentage positive flashes within a 2 km (a) and 4 km (b) distance from the wildfire. Note the varying bin ranges in each plot. The highest multiplicity found within a 2 km and 4 km distance from natural wildfire locations is shown in Figure 8. The number of fires increases with multiplicity. When comparing these results to Figure 4, particularly in the western U.S., these multiplicity values are in the same range as observed climatologically. This would suggest that multiplicity might also be a weak to negligible factor for wildfire ignition.
Figure 8. The number of natural wildfires for ranges of the highest multiplicity value within 2 km (a) and 4 km (b) radial distances from the ignition location. 5. SUMMARY The examination of polarity and multiplicity using over 35,000 wildfires (mostly in the western U.S.) showed that the number and percentage of positive flashes along with the highest multiplicity occurring within a 2 km and 4 km radius of the fire was not greater than climatological occurrence. This would suggest that other factors involved with the potential for ignition, such as fuel availability and fuel moisture, are more important than lightning polarity or multiplicity. Of course, lightning itself is an important component of wildfire both globally and nationally. It would be interesting to apply a similar analysis utilizing LCC information to see if wildfires might have a stronger relationship to LCCs compared to polarity or multiplicity. Since naturally ignited wildfires make up such a substantial fraction of all wildfires in the U.S., it would be beneficial to determine more definitively the components, if any, of CG flashes that play the largest role in ignitions. Climatology of lightning indicates that positive CG flashes make up a relatively small percentage of all lightning strikes. In July, the western U.S. has a slightly higher percentage of positive flashes than the eastern U.S. There does not appear to be a strong latitudinal or coastal relationship to positive versus negative flashes. Multiplicity, on the other hand, does show potential coastal and latitudinal relationships where higher multiplicity is typically found in the southern and southeastern U.S. There is also a coastal influence where multiplicity tends to increase on the land-side of the coast. REFERENCES federal wildland fire occurrence data. Report for the National Wildfire Coordinating Group, CEFA Report 02-04, December 2002, 30 pp. Cummins, K. L., M. J. Murphy, E. A. Bardo, W. L. Hiscox, R. B. Pyle, and A. E. Pifer, 1998a: A combined TOA/MDF technology upgrade of the U.S. National Lightning Detection Network. J. Geophys. Res., 103, 9035-9044. Cummins, K. L., E. P. Krider, and M. D. Malone, 1998b: The U. S. National Lightning Detection Network and Applications of cloud-to-ground lightning data by electric power utilities. Trans. On Electromag. Comp. 40(4), 465-480. Flannigan, M. D. and B. M. Wotton, 1990: Lightning-ignited forest fires in northwestern Ontario. Can. J. For. Res., 21, 277-287. Fuquay, D. M., et al 1972: Lightning discharges that caused forest fires. J. Geophy. Res., 77, 2156-2158. Kempf, N. M. and E. P. Krider, 2003: Cloud-toground lightning and surface rainfall during the Great Flood of 1993. Mon. Wea. Rev., 131, 1140-1148.,, K. C. Wiens, 2004: Origins of positive cloud-to-ground lightning flashes in the stratiform region of a mesoscale convective system. Geophys. Res. Lett. 31, L10105. Latham, D. and E. Williams, 2001: Lightning and forest fires. Chapter 11, Forest fires: Behavior and ecological effects. Academic Press, 375-418. Orville, R. E., 1990: Peak-current variations of lightning return strokes as a function of latitude, Nature, 343, 149-151. Orville, R. E., and G. R. Huffines, W. R. Burrows, R. L. Holle, and K. L. Cummins, 2002: The North American Lightning Detection Network (NALDN) First results: 1998-2000, Mon. Wea. Rev., 130, 2098-2109. Seity, Y., S. Soula, and H. Sauvageot, 2001: Lightning and precipitation relationships in coastal thunderstorms. J. Geophys. Res., 106 (D19), 22801-22816. Shindo, T. and M. A. Uman 1989: Continuing current in negative cloud-to-ground lightning. J. Geophys. Res. 94, 5189-5198. Uman, M. A., 1969: Lightning. McGraw-Hill, New York. Brown, T. J., B. L. Hall, C. R. Mohrle, and H. J. Reinbold, 2002: Coarse assessment of