Pil­lar of fire

Popular Science - - FEATURES - By Kyle Dick­man pho­to­graph by The Voorhes

In 2011, a New Mex­ico wild­fire went from nor­mal to nu­clear, kick­ing up a 45,000-foot col­umn of tor­nadic winds and burn­ing de­bris. Three lo­cal sci­en­tists set out to learn why.

A WOR­RIED HOME­OWNER NAMED MARK Winkel stood on his porch and pointed his tele­scope at a wild­fire rip­ping through the for­est sev­eral miles from his home out­side Los Alamos, New Mex­ico. The blaze had started 12 hours ear­lier when a strong gust knocked an aspen into a power line in Las Con­chas, a hik­ing trail along a 13-mile-wide caldera called the Valles Grande. Al­ready it had torched about 7,000 acres, an im­pres­sive rate of spread, but pre­dictable given the heavy winds and the on­set of fire sea­son, which would last un­til July’s mon­soons fi­nally sat­u­rated the tin­der­box.

Hav­ing faced three big wild­fires in the span of 20 years, lo­cals in this part of parched and drought-stricken New Mex­ico knew enough to con­sider this one dan­ger­ous. But it was now 1:30 a.m., an hour when most fires, faced with cool air, calm for the night. As Winkel pointed his scope up one of the eight canyons that ra­di­ate like spokes from the caldera, he saw some­thing un­ex­pected: a yel­low-or­ange wall march­ing down the south­ern face of the Je­mez Moun­tains that sur­round the Valles Grande caldera.

Wild­fires don’t typ­i­cally burn down­hill. They climb up­ward, their flames dry­ing and ig­nit­ing the fresh veg­e­ta­tion above. This one was rac­ing downs­lope, at night, di­rectly at Winkel. Wor­ried, he scram­bled up­hill for a bet­ter view. Near the top, a hot wind struck his chest, and he watched to the north­west as the blaze’s front rolled like bar­rels in 35-foot-high flames. He had never seen this ef­fect be­fore— few peo­ple have. Winkel was wit­ness­ing a blowup, an in­tense and sud­den force, sec­ond in power to a nu­clear ex­plo­sion, able to boil stream wa­ter, melt dirt, and crack boul­ders. This one would spawn a hor­rific 45,000-foot fur­nace of smoke and soot, spin up 400-foot-high fire tor­na­does, gen­er­ate pow­er­ful up­draft­ing and down­draft­ing winds, cre­ate light­ning high in the plume, and send em­bers fly­ing al­most 25 miles away.

Fire-be­hav­ior ex­perts had pre­dicted Las Con­chas would ex­pand to 12,000 acres overnight. In­stead, by the time the sun rose, it had ren­dered 43,000 acres to white ash. Now the con­fla­gra­tion was ad­vanc­ing on the towns of White Rock and Los Alamos. Afraid it would burn homes, au­thor­i­ties or­dered roughly 18,000 res­i­dents to flee. Some ex­perts went on TV to ex­press con­cern that the ap­proach­ing flames might reach the Los Alamos Na­tional Lab­o­ra­tory, which houses tons of nu­clear waste. Fire­fight­ers from state, lo­cal, and fed­eral agen­cies de­scended on the area to stop or at least slow the blaze. It took weeks for them to halt its progress.

To­day, the eastern caldera re­mains a scorched moon­scape. Six miles from the Los Alamos lab, as High­way 4 climbs steeply into the caldera, the an­cient pon­derosa for­est sud­denly gives way to a sprawl­ing ex­panse of dead trees, a land­scape so bar­ren that it takes luck to find a stick thin­ner than 4 inches. Even pines just be­yond the inferno’s reach were baked to death, their nee­dles kiln-dry. In the end, Las Con­chas proved one of the most vi­o­lent blowups in re­cent his­tory. But what trig­gered it?

EX­TREME WEATHER DRIVES EX­TREME FIRE. A STRONG AND RE­LENT­LESS wind in a dry area can stoke even smol­der­ing trash into a run­away inferno. The Las Con­chas Fire ig­nited in a dense for­est dur­ing the worst drought in mil­len­nia, with winds blow­ing up to 40 miles per hour, 20 feet off the ground, driv­ing the flames for­ward.

Ex­treme wild­fires, in turn, cre­ate their own weather. As in­tense heat lofts smoke into the air, it forges a con­vec­tive col­umn that gen­er­ates pow­er­ful up­drafts. It car­ries fuel-rich hy­dro­car­bons, a byprod­uct of burn­ing veg­e­ta­tion, that can ig­nite like gaso­line va­por. The heat also pro­pels mois­ture that con­denses into py­rocu­mu­lus clouds. These anvil-shaped thun­der­heads perch atop smoke col­umns and spawn ex­treme tur­bu­lence, down­draft­ing winds, and even hail that, rather than cool­ing flames, stokes them by churn­ing out even more er­ratic winds.

For 22 years, Rod Linn has stud­ied wild­fires, build­ing com­pu­ta­tional tools to un­ravel the mys­ter­ies of their be­hav­ior. Linn heads up fire and at­mo­spheric re­search at Los Alamos Na­tional Lab­o­ra­tory.

“Some fields have gaps of un­der­stand­ing that are this big,” says Linn, pinch­ing thumb to fore­fin­ger. It’s a mid-April af­ter­noon, and we are sit­ting in the lab’s ex­pan­sive re­search li­brary. “With fire,” he says, stretch­ing his hands far apart, “the gaps are this big.”

Each year, up to 100,000 wild­fires burn be­tween 1 mil­lion and 11 mil­lion acres across the United States, much of it in the drought-af­flicted West and South­west (though a higher per­cent­age of fires burns in the East and South­east). They claim dozens of lives—both civil­ians and fire­fight­ers—and de­stroy sev­eral bil­lion dol­lars’ worth of prop­erty. The fed­eral govern­ment spends up to $2.2 bil­lion an­nu­ally fight­ing them—roughly four times what it did 25 years ago. Ex­perts cite three rea­sons for the in­flat­ing trend: a warm­ing and dry­ing cli­mate that turns forests into kin­dling; the ill-ad­vised prac­tice of ex­tin­guish­ing nat­u­ral for­est-thin­ning fires, leav­ing more to ig­nite later; and 140 mil­lion Amer­i­cans who now live in vul­ner­a­ble places. That fig­ure was close to zero some 150 years ago, and it grows each year as ci­ties and towns sprawl farther into wild­lands. “Fires have prob­a­bly al­ways blown up,” says Linn. “But the con­se­quences weren’t dire when it hap­pened on a sparsely pop­u­lated land­scape and fire­fight­ers didn’t have to fight them.”

It some­times seems like wild­fires are get­ting more fre­quent. They’re not. But they are get­ting more in­tense, giv­ing rise to an age where megafires raze towns, claim lives, and bring cam­era crews run­ning. Cal­i­for­nia’s 2013 Rim Fire burned 402 square miles. Ari­zona’s 2013 Yar­nell Hill Fire de­stroyed a town and killed 19 elite fire­fight­ers by trap­ping them in a canyon. These con­fla­gra­tions are out­liers, mon­sters that color Amer­ica’s fear of flames. But they all started the same way, as tiny, in­signif­i­cant blazes that, for rea­sons science has yet to de­scribe, tran­si­tioned in a few dev­as­tat­ing mo­ments into sav­age blowups.

About four months af­ter the Las Con­chas blaze, Linn and some sci­en­tists vis­it­ing from around the South­west toured the site. What they saw had all the tells of an ex­ceed-

in­gly rare phe­nom­e­non known as a col­umn col­lapse; ex­perts have long be­lieved this event oc­curs when a tower of smoke and soot gets so heavy, it falls back to earth, cre­at­ing a wind so pow­er­ful that it blows on the sur­round­ing fire like a bel­lows.

Linn was in­trigued. What­ever the ex­act cause of the blowup, he now had an ex­cuse to study, in his own back­yard, one of the rarest events in wild­fire science. And he had ac­cess to what al­most no other sci­en­tist did: su­per­com­put­ing power, cour­tesy of Los Alamos Na­tional Lab­o­ra­tory. He quickly as­sem­bled a team of spe­cial­ists and landed a lab-spon­sored, three-year re­search grant. It was the same type of grant that had launched Linn’s re­search more than two decades ear­lier. Back then, his task was cri­sis fore­cast­ing. He used su­per­com­put­ers to try to pre­dict when and where the next great inferno would burn. Now he would ex­plore the forces that gen­er­ate them.

ONE FRI­DAY IN APRIL, I MEET LINN’S TWO TOP re­searchers on the Las Con­chas project: Jesse Can­field, a slow-talk­ing Chicago-born fluid dy­nam­i­cist who wears thread­bare Carhartts and hik­ing boots, and Jeremy Sauer, a fast-talk­ing geo­physi­cist and Mon­tana na­tive. We gather around a con­fer­ence table in the li­brary. Across the street, past a bank of lock­ers where lab em­ploy­ees stow their cell­phones to pre­vent hack­ing, a fire-sta­tion ra­dio pops with non­emer­gency traf­fic.

Sauer stands at a white­board, doo­dling a pic­ture of the Je­mez Moun­tains with flames rac­ing down from its peaks. As the fire burns, he ex­plains, it re­leases en­ergy that pushes soot, smoke, and hy­dro­car­bon gases into the at­mos­phere. Take away its fuel— like if it en­coun­ters wa­ter or rocks—and its en­ergy sud­denly dis­ap­pears. All that stuff that rose, now heav­ier than the air around it, can now fall back on it­self, cre­at­ing a force­ful wind that ex­plodes the inferno.

The tech­ni­cal term for such a wind is a den­sity cur­rent, heavy air plow­ing into lighter air. “In this case, it was a layer of dense air that gen­er­ated a wind as it flowed down­hill,” Sauer says. Can­field had pored through the wild­fire science lit­er­a­ture, turn­ing up other blowups, in­clud­ing one in 1871 that burned a mil­lion acres in a few days. Den­sity cur­rents most likely caused it. But ex­perts had blamed only one fire on a pos­si­ble col­umn col­lapse. That was the 1990 Dude Fire in Payson, Ari­zona, that killed six fire­fight­ers. Like Las Con­chas, it had raced downs­lope.

To test the hy­poth­e­sis that a col­lapse had stoked the Las Con­chas Fire, the re­searchers headed to the su­per­com­put­ers. For six months, Can­field coded a sim­u­la­tion of the blaze, map­ping the ter­rain and the smoke col­umn. They wanted to know, the­o­ret­i­cally, the high­est wind that a col­laps­ing col­umn would pro­duce. In his sim­u­la­tion, you can see an ink-pur­ple cloud ris­ing above Las Con­chas to­pog­ra­phy, but with no fire be­neath it, the cloud falls. When the col­umn strikes land, it’s like a det­o­nated build­ing bil­low­ing dust, the force of the den­sity wind rush­ing out in ev­ery di­rec­tion, gen­er­at­ing sur­face winds of up to 131 feet per sec­ond, plenty strong to trig­ger a blowup.

“It was proof of con­cept,” Can­field says.

Trou­ble was, the re­sults were mis­lead­ing. And Can­field knew it. He pulls out a note­book filled with math sym­bols. “Nerd hi­ero­glyph­ics,” he calls them. The cal­cu­la­tions show that at peak in­ten­sity, Las Con­chas sent an es­ti­mated 2.3 tons of soot into the at­mos­phere ev­ery sec­ond. “That’s equiv­a­lent to the weight of about 9,000 Honda Ac­cords launched into the sky in two and a half hours,” Sauer says, mar­veling at the enor­mity of the fire’s power.

But those Hon­das ac­tu­ally dis­prove the hy­poth­e­sis. In or­der for all that weight to fall at once, the plume would have to lose heat faster than it shed weight. Physics dic­tates that process is a near im­pos­si­bil­ity. So for this col­umn to col­lapse so sud­denly, the fire would have to have flicked off like a light switch. But Las Con­chas did not go out like that. Not be­fore and not af­ter the blowup. In fact, it burned on for five more weeks.

Can­field’s sim­u­la­tion not only dis­proved that a col­umn col­lapse trig­gered the Las Con­chas blowup, the les­son likely holds true for the 1990 Dude Fire—and any other blowup blamed on col­umn col­lapse. “Fire­fight­ers have long be­lieved that col­umn col­lapses ex­ist be­cause that’s what eye­wit­ness ac­counts tell them,” Linn says. “But eye­wit­ness ac­counts are no­to­ri­ously un­re­li­able. The science doesn’t ver­ify what peo­ple see on the ground.”

So the team’s pri­mary—and most com­pelling—sus­pect turned out to be a boogey­man. Af­ter a year of work, it would’ve been un­der­stand­able if Linn’s team was dis­ap­pointed. They weren’t. “That’s science,” Can­field says with a shrug. “You gen­er­ate a hy­poth­e­sis, then you set out to dis­prove it.” With one of fire­fight­ing’s most per­sis­tent myths dis­patched, they moved on.

Ex­treme wild­fires cre­ate their own weather, forg­ing con­vec­tive col­umns that can rise 45,000 feet, spawn­ing tur­bu­lence, wind, and even hail.

BY NOW THE TEAM HAD IDEN­TI­FIED A FEW OF LAS CON­CHAS’ MAIN char­ac­ter­is­tics. Among them, some­thing Sauer had spot­ted from the front stoop of his house: a pair of coun­ter­ro­tat­ing vor­texes that churned in­ward along the en­tire axis of the 45,000-foot-tall plume of smoke and ash and fire. “I was blown away; see­ing that col­umn was to wit­ness all my the­o­ret­i­cal science in real life,” Sauer says.

It’s Satur­day morn­ing, and I’ve joined him on that stoop as he looks west from the small ranch house to­ward the burned trees lin­ing the Valles Grande, the caldera about 12 miles away where the plume once tow­ered. The vor­texes he saw had cre­ated a vac­uum that very likely sucked up flames and gen­er­ated 400-foot-high tor­na­does of fire. The same “mas­sive ver­ti­cal ve­loc­ity,” as Sauer put it, had ripped pine cones from branches, ig­nited them, and shot them into the air, where winds car­ried them as far as 2 miles away, ig­nit­ing smaller blazes ahead of the main fire. Lighter de­bris, like pine nee­dles, rose to the col­umn’s full height and rained out miles farther. One wit­ness re­ported ash fall­ing 25 miles to the west.

At the col­umn’s top, mois­ture sucked up from trees and brush con­densed into wa­ter and ice. When these plum­meted to the sur­face, they cre­ated a mas­sive down­draft push­ing the col­umn to­ward the ground. “That’s what makes peo­ple think they saw a col­umn col­lapse,” says Sauer. “But re­ally it’s just those high winds blow­ing down­ward.”

Sauer goes on about coun­ter­ro­tat­ing vor­texes like some­one in a midlife cri­sis talk­ing vin­tage Rolexes. We en­ter his home of­fice: a dark room with three com­puter mon­i­tors, and Fris­bees stacked along­side dense physics books. Sauer knew the vor­texes prob­a­bly didn’t cause the blowup. He’d also seen them ear­lier, at day­light, and the fire ex­ploded be­tween 10 p.m. and 3 a.m. “We started ask­ing our­selves,” he says, “what kind of me­te­o­ro­log­i­cal phe­nom­ena pro­duces en­hanced winds at night?”

Early in their re­search, the team con­sid­ered a num­ber of sus­pects but set them aside to pur­sue the col­umn-col­lapse hy­poth­e­sis. Now they re­turned to the Weather Re­search and Fore­cast­ing Model. This uses an at­mo­spheric data set at mesoscale—weather sys­tems rang­ing from 3 to 60 miles, or roughly the size of sea breezes and ocean squalls—and helps me­te­o­rol­o­gists cre­ate daily fore­casts. Us­ing the lab’s su­per­com­put­ers, Sauer and the team plugged in the wind and lo­cal-tem­per­a­ture mea­sure­ments from nearby weather tow­ers for June 26 and 27, along with to­pog­ra­phy, such as moun­tains and canyons, and scaled the fore­cast to the fire’s size.

What the com­puter spit out re­sem­bled a topo­graph­i­cal map, but for the at­mos­phere, with pres­sure, wind speed, and di­rec­tion plot­ted at sev­eral el­e­va­tions. For the night of June 26, Sauer no­ticed a cu­ri­ous pat­tern. Be­tween 400 and 600 feet above the fire, in a bound­ary be­tween two at­mo­spheric lay­ers, a sine wave rep­re­sent­ing os­cil­lat­ing pres­sure be­gan at the crest of the Je­mez and rolled and broke in the ex­act di­rec­tion in which Las Con­chas had blown up: a sig­nal for a strong wind blow­ing down­canyon.

As he looked at the chart, Sauer rec­og­nized that the pass be­tween the Valles Grande and Fri­jo­los Canyon was a text­book ex­am­ple of the to­pog­ra­phy that can shape moun­tain waves. These winds form on the lee­ward side of a peak when low and dense air squeezes be­tween the peak and an­other air mass above it. As pres­sure builds, air speeds up—like wa­ter in a crimped hose— and be­comes a heavy wind. Highly tur­bu­lent air above moun­tains has been known to cause air­plane crashes. But al­most no re­search has been done on the sur­face-level cur­rents that moun­tain waves cre­ate.

Could one have cre­ated the den­sity cur­rent that blew up Las Con­chas? Sauer thought it pos­si­ble. So he dug in. He spent a year de­vel­op­ing a sim­u­la­tion of air mov­ing from the caldera down the canyons. He pulls up the re­sult­ing an­i­ma­tion. It looks like a rain­bow bred with a lava lamp. “Check out right here,” he says, point­ing to a river of dig­i­tized air squeez­ing through a moun­tain pass.

Where his fin­ger rests, a wind cur­rent shoots down the Je­mez moun­tain slopes. Near the fire, the air roils like froth at the end of a break­ing ocean wave. It was just as Mark Winkel, the only known eye­wit­ness to the blowup, had re­ported see­ing that night: air rolling like burn­ing bar­rels.

Fi­nally, all of the pieces fit snugly into place. All ex­cept one. Their model put out wind mea­sure­ments as strong as 114.8 feet per sec­ond. But the night of the blowup, lo­cal weather tow­ers mea­sured winds gust­ing only as high as 26 feet per sec­ond—far too low to be a moun­tain wave. “We re­ally had thought this was the mech­a­nism,” Sauer says. He clicks away from the an­i­ma­tion. “But now we knew it had to be some­thing else.”

ONE AF­TER­NOON I JOIN CAN­FIELD, SAUER, and Linn at Los Alamos’ only de­cent bar, Bath­tub Row, a mi­cro­brew­ery busy with sci­en­tists who have ac­cepted life be­side the moon­scape left by the Las Con­chas Fire. Beer helps.

From some­where in the Je­mez, a cool wind sinks down on us. “In the sum­mer­time,

The cul­prit, it turns out, may have been slyer and qui­eter than any­one ex­pected.

ev­ery night around 9 p.m., we can hear the wind scream­ing through the trees,” Can­field says. “Any­body who lives here will tell you about it.”

Af­ter de­ter­min­ing that the blowup wasn’t caused by col­umn col­lapse, coun­ter­ro­tat­ing vor­texes, or a moun­tain wave, the team re­turned yet again to step one, and this time looked in­ward. They in­ves­ti­gated an ef­fect that they’d ob­served on evenings just like this, talk­ing fire and drink­ing beer on their back porches. One night, Kee­ley Costi­gan, an at­mo­spheric chemist in the lab, over­heard them vent­ing their frus­tra­tion. She had been study­ing the molec­u­lar makeup of smoke par­ti­cles in the Las Con­chas Fire. It turned out that she’d been pulling data from a 150-foot re­search tower that stands in an un­burned canyon just north of the blowup. The team had not in­cluded it in their mesoscale mod­el­ing.

In­trigued, they pulled the tower’s data. Once again, a com­pelling pat­tern emerged. On three of the five nights prior to June 26, pre­cisely be­tween 10 p.m. and 3 a.m., a down­canyon breeze had blown at a rate they rec­og­nized with amaze­ment: roughly 26 feet per sec­ond. “The fin­ger­print matched,” Sauer says.

In the nine hours pre­ced­ing the blowup, 40-mile-per-hour gales had stretched the Las Con­chas Fire into a thin and nar­row foot­print about 6 miles long by 1 mile wide. Af­ter the sun dipped be­hind the caldera’s 11,000-foot Re­dondo Peak, the tem­per­a­ture cooled, the winds dropped, and the fire—as ex­pected—be­gan to calm.

But then, as the night­time air cooled and be­came denser, it be­gan to pool inside the caldera like wa­ter fill­ing a 13-mile bath­tub. At around 10 p.m., this pool of dense, oxy­gen-rich air spilled over, gen­er­at­ing 26-foot-per-sec­ond winds that sloshed down the canyons. They struck the flames per­pen­dic­u­lar to the fire’s path. And just like that, what had been a 6-mile-long sim­mer­ing south­ern flank woke in a dragon’s breath.

The cul­prit, it turns out, may have been slyer and qui­eter than any­one ex­pected. By the time the team fig­ured this out, their grant fund­ing had nearly run out. They couldn’t pur­sue any other sus­pects, but to them, the bath­tub hy­poth­e­sis re­mains the most plau­si­ble. It’s also, pos­si­bly, the most use­ful. “Most fire blowups are prob­a­bly best ex­plained not by the rare or un­pre­dictable,” Sauer says, “but by the rel­a­tively com­mon ef­fect of lo­cal­ized me­te­o­rol­ogy.”

That means fu­ture erup­tions might be pre­dictable. And that could in­form how we fight wild­fires. “We can’t pre­dict to the hour or the minute when a fire will blow up or even if it will blow up,” Linn says. “But know­ing lo­cal weather pat­terns could tell fire­fight­ers that when a fire is burn­ing at a cer­tain time in a cer­tain place, a blowup is pos­si­ble.”

So fire­fight­ers—and the me­te­o­rol­o­gists who some­times work along­side them—can de­ploy more-so­phis­ti­cated mod­els to pre­dict whether a wild­fire will go nu­clear, en­dan­ger­ing lives and prop­erty.

Linn drinks from his beer. Be­hind him, the set­ting sun casts num­ber­less torched trees in sil­hou­ette. If Las Con­chas has a sil­ver lin­ing, it’s a grim one. The risk of an­other blowup strik­ing Los Alamos is thin. There’s noth­ing left to burn.

Up in Smoke

The wild­fire torched more than 150,000 acres, cre­ated strong winds, and spewed a mas­sive plume— seen here from 60 miles away— packed with smoke, de­bris, and burn­ing hy­dro­car­bons.

The Fire De­tec­tives

The sci­en­tists who in­ves­ti­gated the Las Con­chas inferno, de­bunk­ing a decades-old fire myth: Rod Linn, Jesse Can­field, and Jeremy Sauer.

Fire in the Sky

On June 26, fire and smoke gained a choke­hold on Los Alamos. Fire-be­hav­ior ex­perts ex­pected the blaze to calm with the cool night air. In­stead,af­ter mid­night, it ex­ploded into a rolling inferno.

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