How to Start a Hur­ri­cane

What makes one small eddy fiz­zle out, and another turn into the planet’s most de­struc­tive storm?

Air & Space Smithsonian - - Front Page - BY MARK BE­TAN­COURT

The long, sci­en­tific search for the trig­ger.

Two cen­turies later, the ma­ture hur­ri­cane is a rel­a­tively well un­der­stood phe­nom­e­non. For one thing, it’s one of the most sta­ble struc­tures in the at­mos­phere. A big trop­i­cal storm feeds on a con­tin­u­ous pos­i­tive feed­back loop, a heat en­gine that moves wa­ter and en­ergy from the warm oceans into the at­mos­phere and back in an eas­ily ob­serv­able cy­cle. Pre­vail­ing wind pat­terns make a storm’s track rel­a­tively easy to pre­dict, and with ev­ery im­prove­ment in ob­ser­va­tion tech­nol­ogy and com­put­ing

JOHN FAR­RAR, A HAR­VARD SCI­EN­TIST who in 1815 wit­nessed one of the rare hur­ri­canes to hit New Eng­land, wrote about the storm with a sim­plic­ity and won­der that re­veal just how lit­tle was known about the phe­nom­e­non at the time. “I have not been able to find the cen­tre or the lim­its of this tem­pest,” he wrote. “It was very vi­o­lent at places sep­a­rated by a con­sid­er­able in­ter­val from each other, while the in­ter­me­di­ate re­gion suf­fered much less…. In these cases it ap­pears to have been a mov­ing vor­tex, and not the rush­ing for­ward of the great body of the at­mos­phere.”

power, pre­dic­tions are only get­ting bet­ter. Hur­ri­cane me­te­o­rol­ogy to­day is a far cry from the wide-eyed ob­ser­va­tions of John Far­rar.

The thing is, sci­en­tists still don’t know ex­actly how hur­ri­canes form. They don’t un­der­stand why, no mat­ter how ripe the con­di­tions, hur­ri­cane for­ma­tion is ac­tu­ally very rare. Only about 20 per­cent of the dis­tur­bances that look like they might spawn hur­ri­canes do. Ed Zipser, a me­te­o­rol­o­gist at the Univer­sity of Utah who has been prob­ing and dis­sect­ing trop­i­cal cy­clones for more than 50 years, puts it best: “A hur­ri­cane is an ab­so­lutely, beau­ti­fully smooth-run­ning en­gine with a very tem­per­a­men­tal starter mo­tor.”

That starter mo­tor, what me­te­o­rol­o­gists call cy­clo­ge­n­e­sis, is the dif­fer­ence be­tween a storm that be­comes a full-blown hur­ri­cane and one that fiz­zles. While find­ing the cat­a­lyst for cy­clo­ge­n­e­sis is one of the big­gest chal­lenges in mod­ern at­mo­spheric sci­ence, sci­en­tists at least have a ba­sic un­der­stand­ing of its re­quire­ments.

Warm ocean wa­ter is vi­tal—at least 80 de­grees Fahren­heit. Colder wa­ter would rob the storm of en­ergy even as it tried to get go­ing. En­ergy is the key to all weather, and it moves and changes form most spec­tac­u­larly in large storms. The very first step in the life of a hur­ri­cane is amaz­ingly sim­ple, and

hap­pens on a scale too small to see with the naked eye. Wa­ter at the sur­face evap­o­rates, con­denses, and falls, ul­ti­mately, as rain—the fa­mil­iar wa­ter cy­cle we learned in el­e­men­tary school (see “Evap­o­ra­tion– Con­den­sa­tion–pre­cip­i­ta­tion,” be­low).

Over the trop­i­cal ocean, this cy­cle hap­pens across vast ar­eas si­mul­ta­ne­ously—thun­der­storms grow­ing up and ex­haust­ing them­selves in lit­tle puffs of con­vec­tion like bub­bles ris­ing from the bot­tom of a pot of boil­ing wa­ter.

At larger scales, other forces come into play. Some patches of ocean de­velop more thun­der­storms than oth­ers, and in those places the air con­tains more wa­ter va­por, which traps more of the heat ra­di­at­ing from the sur­face—the green­house ef­fect. This cre­ates a pos­i­tive feed­back loop that ex­cites more evap­o­ra­tion and con­vec­tion, which in turn cre­ates a gen­eral up­draft of warm air that’s more en­er­getic than the spotty, tem­po­rary ris­ing at in­di­vid­ual lo­ca­tions.

It used to be that only sailors or is­lan­ders would ob­serve these con­vec­tive gath­er­ings. In the mid- 19th cen­tury, with the ad­vent of weather ob­serv­ing sta­tions around the At­lantic and the con­tem­po­rary in­ven­tion of the tele­graph, data col­lected over vast ar­eas could be trans­mit­ted and com­bined in near-real time, en­abling sci­en­tists to see pat­terns in pre-hur­ri­cane weather and track storms as they moved. Once they saw the big pic­ture, me­te­o­rol­o­gists be­gan notic­ing that storms on the verge of be­com­ing trop­i­cal cy­clones un­dergo a dis­tinct change: they start to be­come or­ga­nized. All the lit­tle thun­der­storms are no longer iso­lated. They’re now work­ing to­gether as

“A hur­ri­cane is an ab­so­lutely, beau­ti­fully smooth-run­ning en­gine with a very tem­per­a­men­tal starter mo­tor.”

one storm, feed­ing them­selves through a col­lec­tive cir­cu­la­tion that roars to life like an en­gine.

In a sense, we owe much of our knowl­edge of a hur­ri­cane’s struc­ture to the per­sonal bravado of a U.S. Army Air Forces pi­lot named Joseph Duck­worth. Up un­til World War II, hur­ri­cane data was lim­ited to what­ever could be ob­served at fixed weather sta­tions and by ships that hap­pened to be in a storm’s path, and sci­en­tists didn’t know much about the in­ter­nal struc­ture of storms. Duck­worth was known as a skilled in­stru­ment flier, and in July 1943 he was train­ing Bri­tish pi­lots at Bryan Field in Texas when a small but in­tense hur­ri­cane be­gan brew­ing off the Gulf Coast. Duck­worth had heard the Brits com­plain­ing about the AT-6 Texan they were train­ing in, so to show them what the air­plane could do with a skilled pi­lot, he took off and flew straight into the eye of the hur­ri­cane and re­turned with­out a scratch. The field’s weather of­fi­cer asked Duck­worth to fly him into the storm as well, so he did. In the process, Duck­worth founded the now es­sen­tial prac­tice of study­ing hur­ri­canes from within.

A year later, the Great At­lantic Hur­ri­cane of 1944 pounded the East­ern Seaboard, and mil­i­tary tech­ni­cians op­er­at­ing the still-fledg­ling tech­nol­ogy of radar be­came the first to see the ghostly im­age of a hur­ri­cane’s spi­ral­ing arms on their screens. It wasn’t long be­fore me­te­o­rol­o­gists were us­ing radar to track and study storms, even putting minia­ture radar in­stru­ments on the air­planes that now flew reg­u­larly into the eyes of hur­ri­canes.

But the real coup came in 1960, when the TIROS weather satel­lite was launched and hu­mankind got its first wide-an­gle pic­ture of weather on Earth. Satel­lites are the ul­ti­mate tool for meet­ing the first chal­lenge of hur­ri­cane study: ob­serv­ing them at their full scale. They’re also key to study­ing cy­clo­ge­n­e­sis. A hur­ri­cane hit­ting Florida may have trav­eled thou­sands of miles, and with re­peated im­ages taken from space, me­te­o­rol­o­gists can sim­ply rewind the movie and look for pat­terns that tell of the hur­ri­cane’s ori­gins.

Scrub­bing through any such time­line, you’ll see that the ma­jor­ity of hur­ri­canes that hit the United States orig­i­nate in a broad area stretch­ing from the Lee­ward Is­lands to Africa. This phe­nom­e­non was first de­scribed in 1940 by me­te­o­rol­o­gist Gor­don Dunn, who was an­a­lyz­ing wind flow pat­terns over the trop­ics when he no­ticed a steady pa­rade of low-pres­sure sys­tems that move across the At­lantic from Africa in pe­ri­odic waves. These “African east­erly waves” tended to be as­so­ci­ated with in­clement weather, and Dunn ob­served that they oc­ca­sion­ally de­vel­oped into hur­ri­canes. In fact, 60 per­cent of trop­i­cal cy­clones in the north At­lantic are as­so­ci­ated with east­erly

waves, which are caused by dips in the east-blow­ing African jet­stream. They pro­vide just the cat­a­lyst At­lantic storms need to tip the ther­mo­dy­namic scales and kick off a hur­ri­cane. Dunn had pinpointed the nurs­ery where some of the most in­tense At­lantic storms are born.

Satel­lite imagery shows that Hur­ri­cane Katrina be­gan as an east­erly wave, though it didn’t turn into a hur­ri­cane un­til it had al­most reached Florida. Hugo (1989), An­drew (1992), Floyd (1999), Is­abel (2003), Ivan (2004), Frances (2004), and Ike (2008) all started as waves. But not all waves be­come hur­ri­canes, even when the con­di­tions are per­fect. Says Kerry Emanuel of the Mas­sachusetts In­sti­tute of Tech­nol­ogy, one of the lead­ing the­o­rists study­ing hur­ri­cane for­ma­tion, “Most of them just hap­pily die in the At­lantic.” There’s some­thing else to the ig­ni­tion of a hur­ri­cane’s en­gine (see “A Self-sus­tain­ing Heat En­gine,” p. 29), and me­te­o­rol­o­gists are throw­ing ev­ery tech­nol­ogy they have at this mys­tery.

Dur­ing the past three hur­ri­cane sea­sons, a large and di­verse group of sci­en­tists have searched for an­swers to cy­clo­ge­n­e­sis and other mys­ter­ies as part of an am­bi­tious NASA mis­sion called Hur­ri­cane and Se­vere Storm Sen­tinel, or HS3. The stars of the project were two un­manned Global Hawk drones, launched from NASA’S Wal­lops Flight Fa­cil­ity in Virginia to where no storm chaser had gone be­fore. One of the re­motely pi­loted air­craft flew above the hur­ri­canes, where it could mea­sure rain­fall, hu­mid­ity, wind speed, and other fac­tors through­out the en­tire 46,000-foot height of the storm. The other flew around the out­side of each storm, us­ing Li­dar and a kind of tem­per­a­ture gauge called a scan­ning high-res­o­lu­tion in­ter­fer­om­e­ter sounder to ob­serve con­di­tions that help or hurt the storm’s for­ma­tion and in­ten­si­fi­ca­tion. In ad­di­tion to the on­board in­stru­ments, the over-the-storm air­craft de­ployed drop­son­des, small in­stru­ment pack­ages that drift down un­der para­chutes, tak­ing pres­sure, tem­per­a­ture, hu­mid­ity, and wind mea­sure­ments through­out a storm’s ver­ti­cal cross sec­tion.

The Global Hawks fly higher than most air­craft pi­loted by hu­mans, but—cru­cially to the study of fickle trop­i­cal storms—they can also fly longer. While crewed flights typ­i­cally stay “on sta­tion” at a storm for only four to six hours, the Global Hawks can last for 18, so sci­en­tists get a much more com­pre­hen­sive pic­ture of spin-up pro­cesses all over the At­lantic.

Last Septem­ber, for ex­am­ple, one of the Global

Hawks (the other was grounded due to me­chan­i­cal prob­lems) flew a pair of two-day flights over Hur­ri­cane Edouard as the storm was strength­en­ing. Drop­sonde data showed low-level warm­ing and dry­ing, in­dica­tive of air sink­ing as the hur­ri­cane’s eye formed. Data like this ul­ti­mately feed into mod­els that give the sci­en­tists a more de­tailed pic­ture of what’s hap­pen­ing.

Smaller and cheaper drones have also been brought in on the case. “The Global Hawks are nice,” says Neal Dorst, a me­te­o­rol­o­gist with the At­lantic Oceano­graphic and Me­te­o­ro­log­i­cal Lab­o­ra­tory’s Hur­ri­cane Re­search Di­vi­sion, but they can’t fly into a storm at low al­ti­tude with­out risk­ing go­ing down in the heavy, er­ratic winds near the ocean sur­face. A new drone called the Coy­ote, fielded in co­op­er­a­tion with the Of­fice of Naval Re­search, is much smaller (it could fit on a desk­top) and so cheap it’s es­sen­tially ex­pend­able, mak­ing it cost-ef­fec­tive to fly it in the few thou­sand feet that make up the crit­i­cal re­gion where the warm ocean meets the storm.

“This is the area where en­ergy is com­ing from the ocean into the at­mos­phere,” says Dorst. “We need the in­for­ma­tion about how the en­ergy is be­ing trans­ferred, and we re­ally don’t have a clue as to what some of the pa­ram­e­ter val­ues are. [The Coy­ote] will get us ac­tual num­bers.” De­ployed from

( a P-3 Orion Hur­ri­cane Hunter air­craft much the way drop­son­des are, two Coy­otes made suc­cess­ful flights into Hur­ri­cane Edouard last year. Hav­ing proven the drone’s abil­ity to col­lect low-level data, the sci­en­tists are plan­ning more Coy­ote flights dur­ing this year’s sum­mer storm sea­son.

While this data is be­ing di­gested, hur­ri­cane sci­en­tists have a few guesses about the key to cy­clo­ge­n­e­sis. Some think that more light­ning in a cer­tain area of a de­vel­op­ing storm may sig­nal that the trans­for­ma­tion is about to hap­pen, oth­ers that huge rock­ets of

con­vec­tion called hot tow­ers are the place to look.

Also hack­ing away at the prob­lem of cy­clo­ge­n­e­sis are the­o­rists like Emanuel, who in­creas­ingly are look­ing at ther­mo­dy­nam­ics—the roles that heat and en­ergy play in the move­ments of weather. It’s an area that un­til re­cently had been largely glossed over. “We’re both­er­ing to learn pieces of physics that our cul­ture avoided for decades,” Emanuel says, adding that he ex­pects big break­throughs in our un­der- stand­ing of hur­ri­canes within the next few years.

Even so, the search for cy­clo­ge­n­e­sis will face a new chal­lenge: The rules that gov­ern hur­ri­canes are chang­ing. Emanuel is one of the prom­i­nent me­te­o­rol­o­gists work­ing on the ques­tion of how global cli­mate change will af­fect hur­ri­cane for­ma­tion and be­hav­ior. Much of the change will come with ris­ing sea sur­face tem­per­a­tures, which will pro­vide more wind and more rain—fuel for hur­ri­canes. “You’ll start to see hur­ri­canes with wind speeds that you have not seen be­fore, be­cause it wasn’t phys­i­cally pos­si­ble,” says Emanuel.

While warm­ing will in­crease the size and in­ten­sity of storms, Emanuel the­o­rizes that it also will make it more dif­fi­cult for them to form, as it will dry parts of the at­mos­phere. The re­sult: Fewer storms make

While warm­ing will in­crease the size and in­ten­sity of storms, Emanuel the­o­rizes that it will also make it more dif­fi­cult for them to form.

it through to the spin-up phase. But once they get go­ing, they’ll be harder to stop. Ac­cord­ing to Emanuel, we’ll likely see more Cat­e­gory 5 hur­ri­canes—storms with sus­tained winds of at least 156 mph—than we do now, even with fewer storms spin­ning up in any given sea­son. Some sci­en­tists have even sug­gested we’ll need a Cat­e­gory 6, for storms with winds of 174 mph and above.

Emanuel’s col­league James Kossin has shown that hur­ri­canes are reach­ing their peak fur­ther from the equa­tor, pos­si­bly as a re­sult of a gen­eral ex­pan­sion of the trop­i­cal zones. As a re­sult, cities that have never ex­pe­ri­enced hur­ri­canes may start to be reg­u­larly as­saulted. To make mat­ters worse, there’s some in­di­ca­tion in the mod­els that storms will be larger than they are to­day (although it’s not yet clear why). So when a storm makes land­fall, more of the coast­line will be in­un­dated, not just be­cause of rain but also be­cause of the storm surge cre­ated by wind trav­el­ing over hun­dreds of miles of ocean. Ris­ing sea level will make the surge risk even greater.

Over­all, the fu­ture of hur­ri­canes looks pretty scary. Su­per­storm Sandy wasn’t nec­es­sar­ily a re­sult of cli­mate change, but it may have been the first of many storms to hit a whole coast­line of un­pre­pared cities. Given that cli­mate change cre­ates a mov­ing tar­get in terms of com­puter model­ing, with many of the phys­i­cal fac­tors that de­ter­mine storm for­ma­tion about to change over the next cen­tury, it won’t be easy for sci­en­tists to pin down what ex­actly we should ex­pect. Time—and an army of drones, drop­son­des, dar­ing pi­lots, and dogged ques­tion­ers—will tell.

In the mean­time, sci­en­tists have still not worked out very ba­sic char­ac­ter­is­tics of hur­ri­cane for­ma­tion. Think­ing I’d missed some­thing ob­vi­ous dur­ing our con­ver­sa­tion, I asked Emanuel why the rain bands in­side hur­ri­canes form the way they do, with such seem­ingly reg­u­lar al­ter­na­tion of heavy rain and rel­a­tively clear skies. He laughed.

“Wouldn’t we like to know?”

TRMM satel­lite data (above) shows Katrina’s struc­ture. Go­ing low: NOAA me­te­o­rol­o­gist Joe Cione (right) holds a Coy­ote drone sent to study the crit­i­cal zone where ocean meets air.

Drone’s-eye view: From 60,000 feet, a NASA Global Hawk ob­served Trop­i­cal Storm Frank swirling over the Pacific in Au­gust 2010. Op­po­site: Dur­ing Hur­ri­cane Katrina in 2005, the Aqua satel­lite took the At­lantic Ocean’s tem­per­a­ture; yel­low, or­ange, or red...

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