The game-changer

Fu­elled by pol­lu­tants, clouds can trig­ger ex­treme weather events

Down to Earth - - COVER STORY - Shree­shan Venkatesh

FOR MORE than a month, Satna district of Mad­hya Pradesh re­ceived de­fi­cient rain­fall. And then, just in two days, 400 mm of in­tense down­pour trig­gered se­vere floods in the en­tire district. In a nor­mal year, the district would have re­ceived this much rain span­ning over two months. In fact, “ex­treme” spells of rain—de­fined as rain­fall above 204.5 mm in 24 hours by the In­dia Me­te­o­ro­log­i­cal De­part­ment ( imd)— have flooded sev­eral places across 18 states in June and July. Though deaths due to light­ning are com- mon dur­ing mon­soon months, it was un­prece­dent­edly high for four days. Be­tween July 31 and Au­gust 3, at least 56 peo­ple died from light­ning in dif­fer­ent parts of Odisha. At least 400 peo­ple have been killed by light­ning in eight states since the mon­soon sea­son be­gan in June. The sum­mer mon­soon this year has also been un­usu­ally strong in China. It brought in­tense rain­fall and caused flash floods, killing 400 peo­ple and dis­plac­ing 100,000 in July. In early Au­gust, se­vere thun­der­storms wreaked havoc in parts of the US.

Sim­i­lar in­ci­dences of in­tense storms, cloud-

bursts and flood­ing are be­com­ing the norm world­wide. But sci­en­tists find it dif­fi­cult to pre­dict their on­set de­spite ad­vance­ment in weather mod­els and satel­lite tech­nol­ogy. A con­sen­sus has been build­ing up that these un­cer­tain­ties in pre­dic­tive ca­pa­bil­ity could be due to a miss­ing link—clouds.

“Clouds are the car­ri­ers of weather, so to speak. They en­able hy­dro­log­i­cal cy­cles and are re­spon­si­ble for al­most all pre­cip­i­ta­tion,” says Thara Prab­hakaran, chief project sci­en­tist of Physics and Dy­nam­ics of Trop­i­cal Clouds pro­gramme at the In­dian In­sti­tute of Trop­i­cal Me­te­o­rol­ogy in Pune. They also act as the me­di­a­tor be­tween global cli­matic sys­tems and their lo­cal man­i­fes­ta­tion. Yet, sci­en­tists know lit­tle about them, and barely un­der­stand how they af­fect weather, let alone cli­mate cy­cles.

As ex­treme and freak weather events be­come fre­quent with ris­ing tem­per­a­tures, sci­en­tists are scur­ry­ing to crack the mys­ter­ies be­hind clouds. So far, most sci­en­tists have ze­roed in on one fac- tor that plays a cru­cial role in the for­ma­tion and evo­lu­tion of clouds and thereby dic­tates its in­flu­ence on weather—it is the aerosol.

Hid­ing be­hind a com­pli­cated-sound­ing name, aerosols are mi­cro­scopic or­ganic and in­or­ganic par­ti­cles that are con­stantly be­ing re­leased into the at­mos­phere. These could be nat­u­ral, such as dust, vol­canic ash and vapours emit­ted by plants, or hu­man-made, such as agri­cul­tural dust, ve­hic­u­lar ex­haust, emis­sions from mines and soot from ther­mal power plants. These mi­cro­scopic pol­lu­tants act as the sites where wa­ter vapour con­denses to form cloud droplets. “The in­flu­ence of clouds on weather ap­pears to start at this mi­nut­est level,” says Prab­hakaran.

The in­flu­ence of clouds on weather, be it pre­cip­i­ta­tion or re­flec­tiv­ity, ap­pears to start at the level of aerosols, which act as the nu­clei of cloud droplets

For in­stance, the type of aerosols and their abun­dance in the at­mos­phere dic­tate the be­hav­iour of cloud—whether it will re­sult in pre­cip­i­ta­tion, bounce back so­lar ra­di­a­tion into space or trap the ra­di­ated heat. Lo­cal me­te­o­ro­log­i­cal fac­tors fur­ther in­flu­ence cloud be­hav­iour.

Pol­luted air sup­presses rain­fall

Greater the num­ber of aerosols, larger is the num­ber of cloud droplets, says Daniel Rosen­feld, pro­fes­sor at the In­sti­tute of Earth Sciences, He­brew Univer­sity in Jerusalem.

But more cloud droplets do not nec­es­sar­ily mean higher pre­cip­i­ta­tion. As cloud wa­ter gets dis­trib­uted among too many aerosols, they re­sult in a large num­ber of smaller droplets, which are slower to co­a­lesce into big­ger droplets that can fall as pre­cip­i­ta­tion, he adds. In other words, pol- luted air of­ten sup­presses rain­fall.

A study, pub­lished in WaterRe­sources Re­search in 2008, cor­rob­o­rates Rosen­feld’s ob­ser­va­tion. To un­der­stand how dust and pre­cip­i­ta­tion are re­lated, re­searchers from Univer­sity of Virginia, usa, and nasa God­dard In­sti­tute for Space Stud­ies an­a­lysed aerosol in­dices, wind di­rec­tions and rain­fall data of the Sa­he­lian stretch in Cen­tral and North Africa for the pe­riod of 1996-2005. Their anal­y­sis shows that dust from the re­gion was sup­press­ing rain­fall. The fine dust par­ti­cles in the re­gion re­sulted in a large num­ber of seed­ing sites of cloud droplets, which pre­vented the for­ma­tion of droplets of the size re­quired for rain­fall, the study notes.

While this is usu­ally true for shal­low clouds, Rosen­feld says pol­luted clouds are ca­pa­ble of caus­ing greater havoc.

Role in light­ning, cloud­burst

Pol­luted clouds at times grow to greater heights due to con­vec­tion, says Rosen­feld. At this level, the be­hav­iour of cloud droplets changes. They at­tain suf­fi­cient size to start co­a­lesc­ing into big­ger droplets that can fall as pre­cip­i­ta­tion.

Un­for­tu­nately, these clouds, known as cu­mu­lonim­bus, are re­spon­si­ble for ex­treme events such as tor­na­does, hail­storms, se­vere dust storms, squalls and cloud bursts, says M Mo­ha­p­a­tra, head of the Re­gional Spe­cialised Me­te­o­ro­log­i­cal Cen­tre at In­dia Me­te­o­ro­log­i­cal De­part­ment in Delhi.

If a cu­mu­lonim­bus cloud, with a large num­ber of aerosols, grows be­yond the freez­ing level, it re­sults in a large num­ber of free ice pel­lets in the up­per parts of clouds. The con­tin­u­ous and rapid ver­ti­cal flows of wa­ter and ice pel­lets within clouds cre­ate an enor­mous amount of static en­ergy, which in turn re­sults in light­ning and hail­stones. Sev­eral stud­ies have shown that vol­canic erup­tions are usu­ally fol­lowed by bouts of light­ning be­cause of high lev­els of aerosol load­ing in the clouds.

This ob­ser­va­tion can be ap­plied to un­der­stand why light­ning-re­lated deaths are com­mon in cer­tain places in In­dia. Light­ning is the lead­ing weather-re­lated cause of death in the coun­try. Since 2000, over 30,000 peo­ple have died due to light­ning strikes. Al­though no event-spe­cific stud­ies have been con­ducted to con­firm this link, a com­par­i­son of the re­gions chron­i­cally af­fected by light­ning with an aerosol emis­sion map of In­dia shows a strong cor­re­la­tion (see ‘Strik­ing re­al­ity’, p30). Aerosol load­ing over the main­land is high­est along the Gangetic plains in the north­ern and east­ern parts of the coun­try, in Cen­tral In­dia and in the Dec­can Plateau. These re­gions are home to most pol­lut­ing in­dus­tries. Peo­ple here also burn or­ganic and farm waste on a large scale. And these are also the re­gions that re­port max­i­mum num­ber of light­ning deaths.

Be­sides, tall clouds serve as a ver­ti­cal path­way for en­ergy flow. “This is what we feel as tur­bu­lence while trav­el­ling in an air­craft that moves through a cloud,” says Mo­ha­p­a­tra. The wa­ter droplets and ice within clouds con­stantly and rapidly move up and down and un­dergo a lot of col­li­sion, mak­ing them ex­tremely volatile and dan­ger­ous. Aerosols can have an in­vig­o­rat­ing ef­fect on this volatil­ity. A trig­ger is all that is re­quired for the en­tire wa­ter con­tent of the cloud to be un­leashed at once, he adds. This is the rea­son tall clouds of­ten re­sult in cloud­bursts as they move along the Hi­malayan slopes.

Per­fect recipe for tor­na­does

Still ex­treme pre­cip­i­ta­tion is just one of the pos­si­ble side-ef­fects of cu­mu­lonim­bus clouds. They can gen­er­ate high wind speeds. An abun­dance of aerosol par­ti­cles in such a cloud sys­tem can ex­tend their size and life­span by de­lay­ing rains and caus­ing ex­treme storms when the rains fi­nally ar­rive. This re­sults in ex­treme events such as tor­na­does, dust storms and squalls, says Mo­ha­p­a­tra.

A re­cent study by re­searchers from the Univer­sity of Texas at Austin in col­lab­o­ra­tion with the Univer­sity of Colorado Boul­der and nasa’s Jet Propul­sion Lab­o­ra­tory val­i­dates Mo­ha­p­a­tra’s cau­tion. The re­searchers an­a­lysed geo­sta­tion­ary satel­lite data of 2,430 cloud sys­tems and con­cluded that aerosol con­cen­tra­tion could in­crease the life­span of storm sys­tems by as much as three to 24 hours, de­pend­ing on lo­cal me­te­o­ro­log­i­cal con­di­tions. “High con­cen­tra­tions of aerosol par­ti­cles cause a re­duc­tion in droplet size, which could de­lay pre­cip­i­ta­tion, and in the case of storm clouds we have seen that it con­trib­utes to the ex­ten­sion of the life and in­ten­sity of the storm,” says Sudip Chakraborty, lead au­thor of the pa­per that was pub­lished in the Pro­ceed­ings of Na­tional Acad­emy of Sciences in June this year.

While there is no dearth of hy­pothe­ses that say cu­mu­lonim­bus clouds mostly re­sult in ex­treme weather events, the study led by Chakraborty is the only study that shows the role of aerosols in al­ter­ing the cloud’s prop­er­ties.

The main rea­son for this is the dearth of data re­gard­ing cloud prop­er­ties, says Sag­nik Dey, as­sis­tant pro­fes­sor at the Cen­tre for At­mo­spheric Sciences, iit- Delhi. As data qual­ity and quan­tity both im­prove with the avail­abil­ity of more so­phis­ti­cated and de­tailed satel­lite im­agery, re­searchers are tak­ing more in­ter­est in study­ing how chang­ing cloud prop­er­ties are re­sult­ing in freak and un­usual weather events.

An abun­dance of aerosol par­ti­cles in a storm cloud sys­tem can ex­tend its size and life­span by de­lay­ing rains and caus­ing ex­treme storms when the rains fi­nally ar­rive

Sources: Na­tional Crime Records Bureau and NASA Earth Ob­ser­va­tory Satel­lite * Av­er­age aerosol data is for pre-mon­soon and mon­soon months dur­ing 2010-2015

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