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Taking the Guesswork out of Lightning-Caused Wildfire Marjie Brown US Forest Service, [email protected]

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Taking the Guesswork out of Lightning-Caused Wildfire Marjie Brown US Forest Service, Marjie@Marjiebrown.Com University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln JFSP Briefs U.S. Joint Fire Science Program 2008 Taking the Guesswork Out of Lightning-caused Wildfire Marjie Brown US Forest Service, [email protected] Follow this and additional works at: http://digitalcommons.unl.edu/jfspbriefs Part of the Forest Biology Commons, Forest Management Commons, Other Forestry and Forest Sciences Commons, and the Wood Science and Pulp, Paper Technology Commons Brown, Marjie, "Taking the Guesswork Out of Lightning-caused Wildfire" (2008). JFSP Briefs. 23. http://digitalcommons.unl.edu/jfspbriefs/23 This Article is brought to you for free and open access by the U.S. Joint Fire Science Program at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in JFSP Briefs by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Lightning and fi re smoke. Taking the Guesswork Out of Lightning-caused Wildfi re Summary Lightning is a natural source of wildfi re ignitions and causes a substantial portion of large wildfi res across the globe. Simple predictions of lightning activity don’t accurately determine fi re ignition potential because fuel conditions must be considered in addition to the fact that most lightning is accompanied by signifi cant rain. Fire operations managers need improved tools for prediction of widespread dry thunderstorms, which are those that occur without signifi cant rainfall reaching the ground. It is these dry storms that generate lightning most likely to result in multiple fi re ignitions, often in remote areas. In previous work the researchers developed a formula that estimates the potential for cloud- to-ground lightning when dry thunderstorms are expected. This new study demonstrated the value of the formula as a predictive tool for estimating the likelihood of dry thunderstorms across much of the western U.S. This expanded utility was accomplished by integrating the formula with the predictive capacities of the Pacifi c Northwest MM5 weather forecast model. In testing during the summers of 2004 and 2005 the majority of lightning-caused fi res occurred where the predicted risk of dry thunderstorms was greater than 75%. These results indicate that this predictive tool can be very useful for identifying days when conditions are right for wildfi re outbreaks due to lightning. This forecast tool is currently available 24 hours a day for the Pacifi c Northwest region at http://www.fs.fed.us/pnw/airfi re. More work is already underway to expand coverage and improve usability, further supporting managers as they plan for the many potential fi res that can be started by lightning from dry thunderstorms. Fire Science Brief Issue 12 September 2008 Page 1 www.fi rescience.gov Key Findings • New formula/model combination demonstrates value as a predictive tool for estimating the likelihood of dry thunderstorms across much of the western U.S. • During testing, the majority of lightning-caused fi res occurred where the predicted risk of dry thunderstorms was greater than 75%. • Results indicate that the tool can be useful for identifying days when conditions are right for wildfi re outbreaks due to lightning. The lightning prediction game it’s a recipe for fi res when lightning reaches the ground, rain Lightning from dry thunderstorms causes most doesn’t, and fuels are ready to ignite. wildfi res in the western United States. Predicting this Birth of a bolt phenomenon has been a long standing puzzle for scientists. When George Carlin said, “Electricity is just organized Predicting when and where lightning may strike, and lightning,” he wasn’t far off. It actually works both ways whether or not subsequent wildfi res are likely, has been a in that lightning is just electricity organizing itself. This game of chance for land managers. Where is it most likely organization occurs because typically the bottoms of storm to strike? Will there be a few strikes or hundreds? When clouds carry a negative charge which creates a localized will it begin, and when can we rest easy again? Step right up positive charge on the ground. When the voltage builds to to the table and roll the dice. a level where the air can’t insulate it anymore, a shot of Many things affect the likelihood that lightning will electrons with a negative charge zigzags down from the make a direct hit on any specifi c point within a given cloud base seeking the path of least resistance in the same time span. The width of a cloud base, the height of the way water fi nds the easiest route downhill. As it approaches base above the ground, the conductivity of the soil and the area of positive charge at the earth’s precipitation all affect the chances. Thunderstorms are surface there is a “fl ashover’ when always taking place on the planet. It has been estimated the positive charge shoots up to meet from satellite observations that lightning occurs the incoming electrons. The circuit FUN LIGHTNING approximately 50–100 times per second on a global basis. is complete and a huge discharge FACT: Even One of its many functions is to distribute and dissipate though the follows instantly, starting at the ground individual the 450 megawatt electrical charge that exists between the and moving up to the cloud. This electrons move ground and the upper atmosphere. That’s enough power to discharge, called the “return stroke,” downward, the sustain a medium size city. At any given moment thousands is responsible for the visible fl ash. It’s discharge as a whole moves up. of thunderstorms are in progress releasing this charge in only a few centimeters in diameter and small doses, sparing us from the electrifying experience as hot as the surface of the sun. of all that power zapping the planet in one shot. Lightning Fires start within the few detection networks suggest that bolts blast the ground some milliseconds that this current is fl owing. Once that fi rst 25 to 30 million times per year. stroke establishes a route between a cloud and the ground Thunderstorms are a double edged sword for land the path is clear for more return strokes. The whole process managers and fi refi ghters in the western U.S. If a storm can be repeated many times. If there is only one return brings signifi cant precipitation the moisture may help stroke a lighting strike lasts about 20 milliseconds. It can extinguish any existing blazes and hinder the ignition last as long as 500 milliseconds if 10 or 15 return strokes of new ones. But if the incoming weather is a “dry occur. The more return strokes there are, the louder the thunderstorm” where precipitation typically evaporates thunder, the brighter the fl ash and the greater the risk of fi re before reaching the ground, nature may be lighting some if conditions are right. The game is underway and the odds big matches—potentially hundreds of them. No one really are anyone’s guess. knows exactly where, and there won’t be much moisture Fire operations managers use a variety of fi re potential to deter the fl ames. Dry thunderstorms crop up frequently assessments in their efforts to anticipate the likelihood of during summer afternoons in the arid, mountainous West, wildfi re and to plan for resource allocation. Several planning where humidity is often so low that rain falling from tools exist to help narrow the guesswork when evaluating thunderstorms evaporates shortly after being released from the potential effects of weather and fuel conditions on fi re a cloud. This evaporating rain is called virga and can be starts. These include daily weather forecasts and tools seen from desert canyons to mountain tops. Curtains of rain designed specifi cally for measuring fi re risk such as the billow down from the base of spectacular storm clouds then National Fire Danger Rating System (NFDRS) and the vanish into thin air. Towering tempests fl icker with electrical Haines Index, which indicates the potential for wildfi re charge, sending bolts of lightning racing toward the earth. growth by measuring the stability and dryness of the air This atmospheric theatre can be awe inspiring to watch, but above a fi re once it’s underway. Fire Science Brief Issue 12 September 2008 Page 2 www.fi rescience.gov But these tools aren’t designed to address the specifi c moisture content. These two measurements allowed them question of whether or not a potential thunderstorm to discriminate between dry and wet lightning days. In will be “wet” or “dry.” Looking at previous lightning 1999, they used these indicators, derived from data around strike locations alone isn’t useful for estimating risk of Spokane (WA), to develop a simple index from historical fi re because of the complicated matrix of defi nitions data. When they tested the index during the 2000 fi re season and conditions surrounding the phenomenon. There they were able to effectively identify days with greater risk is controversy about the defi nition of “dry lightning.” for dry convection and the resulting potential for lightning Lightning can be said to be “dry”—or lacking signifi cant caused fi res. This was the fi rst formula that could actually concurrent rain—under several different storm conditions. discriminate between ”wet” and “dry” lightning events, and “Dry” lightning occurs when thunderstorms form at the fi rst step in giving managers the upper hand. relatively high altitudes with rainfall evaporating before it That study laid the foundation for this project that reaches the ground. Lightning is considered “dry” when it further improves prediction of dry thunderstorms by fl ashes outside the rain shaft of a “wet” thunderstorm; or is expanding the geographic area where the tool can be “dry” if it’s spawned by a fast moving storm that, because of successfully applied and giving it 24 hour predictive its speed, doesn’t dump a lot of rain in one location.
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