Clouds, pol­lu­tion and the sum­mer mon­soon

Pol­lu­tion can cause fun­da­men­tal changes in cloud be­hav­iour and pre­cip­i­ta­tion pat­terns

Down to Earth - - COVER STORY -

THE IN­DIAN sum­mer mon­soon sea­son be­gins when the land sur­face be­comes hot enough to drive a pow­er­ful ris­ing mo­tion of air in the at­mos­phere, pro­duc­ing heavy pre­cip­i­ta­tion. Cooler, hu­mid air over the Ara­bian Sea flows in­land to com­pen­sate for the ris­ing air. Air in this com­pen­sat­ing cir­cu­la­tion en­coun­ters the sur­face heat­ing and also rises, per­pet­u­at­ing the cy­cle. This sim­ple ther­mo­dy­namic sys­tem is as­so­ci­ated with a cloud re­sponse that varies in scale from mi­crons to thou­sands of kilo­me­tres, sim­u­lated by some of the most pow­er­ful su­per­com­put­ers ever made, and ob­served by a mas­sive net­work of weather sta­tions and dozens of ad­vanced satel­lites.

At the small­est scales, an in­crease in tiny par­ti­cles in the at­mos­phere can shade the land sur­face while ab­sorb­ing sun­light aloft, caus­ing a re­duc­tion in the heat that reaches the sur­face. This weak­ens the ris­ing mo­tion of air that drives the pre­cip­i­ta­tion re­sponse to the mon­soon. Clouds that do form in these pol­luted en­vi­ron­ments are less likely to rain and more likely to per­sist be­cause the droplets are smaller. These longer­lived clouds fur­ther cool the sur­face and weaken the cir­cu­la­tion.

The ex­tent to which pol­lu­tion af­fects clouds, pre­cip­i­ta­tion, and, there­fore, agri­cul­ture in In­dia is still an open ques­tion and an ac­tive area of re­search. This work is car­ried out by dozens of in­sti­tu­tions glob­ally, each us­ing ad­vanced mod­els and ex­tremely pow­er­ful com­put­ers to sim­u­late the bil­lions of mi­cro­scopic pro­cesses oc­cur­ring ev­ery sec­ond.

At larger scales, the mi­cro­scopic pro­cesses ob­tained in cloud mod­els show how chang- es in pol­lu­tion lead to changes in the types of clouds. Weather ob­servers across In­dia have for years pro­vided ob­ser­va­tions to show a sig­nif­i­cant re­duc­tion in the cloud types as­so­ci­ated with a strong mon­soon, ac­com­pa­ny­ing an in­crease in pol­lu­tion. The num­ber, qual­ity and con­sis­tency of these ob­ser­va­tions have con­trib­uted to one of the long­est cloud records ever pro­duced. This record is the re­sult of decades of con­tin­u­ous ob­ser­va­tions with no change in ob­serv­ing pro­ce­dures, and years of metic­u­lous data pro­cess­ing, fil­tra­tion and sta­tis­ti­cal anal­y­sis car­ried out by re­search groups glob­ally.

At even larger scales, the amount of heat and mois­ture moved by the In­dian mon­soon is sig­nif­i­cant enough to change the large-scale plan­e­tary waves that drive weather glob­ally. Other trop­i­cal cir­cu­la­tions have been shown to have sig­nif­i­cant ef­fects as well. A global net­work of weather sta­tions as well as cloud-ob­serv­ing satel­lites have shown that the In­dian mon­soon alone is sig­nif­i­cantly cor­re­lated with changes in cloudi­ness over six con­ti­nents.

The range of cloud re­sponse to the In­dian mon­soon shows how the cloud record is a pow­er­ful tool to study cli­mate change. Re­cent stud­ies have shown that clouds as­so­ci­ated with the mid­lat­i­tude storm track (routes of stormy plan­e­tary winds seen in the mid­dle lat­i­tudes) have, on an av­er­age, been dis­placed pole­ward. The mean lo­ca­tion of the storm track is sen­si­tive to the tem­per­a­ture gra­di­ent be­tween the equa­tor and the poles, and a pole­ward shift would be con­sis­tent with a re­duc­tion in this gra­di­ent, which has in­deed hap­pened be­cause of the dis­pro­por­tion­ate

warm­ing of the po­lar re­gions. This is an omi­nous sign of global warm­ing shown us­ing two com­pletely in­de­pen­dent data sources, one based on satel­lite cloud ob­ser­va­tions, the other from sur­face weather sta­tions.

Lower-level clouds are sen­si­tive to largescale cir­cu­la­tions and lo­cal changes in tem­per­a­ture and pol­lu­tion. Apart from the above ex­am­ple re­gard­ing pol­lu­tion pos­si­bly af­fect­ing the mon­soon, low clouds over the trop­i­cal oceans are sen­si­tive to the tem­per­a­ture of the sea sur­face. A warm­ing ocean is more likely to re­duce the at­mo­spheric sta­bil­ity that sup­ports the ex­is­tence of large-scale strat­i­form cloud decks (which are low-hang­ing clouds that grow hor­i­zon­tally on a semi-uni­form base). It also drives a de­cline in cloud cover, an in­crease in sun­light reach­ing the ocean sur­face and fur­ther warm­ing of the ocean in an ex­am­ple of a pos­i­tive feed­back cy­cle. How­ever, an in­crease in pol­lu­tion could have a com­pet­ing ef­fect, in­creas­ing the life­time of clouds.

Pro­foundly com­pli­cated

The sin­gle phe­nom­e­non of the In­dian mon­soon well rep­re­sents the com­plex­ity of the phys­i­cal sys­tems as­so­ci­ated with clouds and the chal­lenges faced by the com­mu­ni­ties ob­serv­ing and mod­el­ling clouds. Each com­mu­nity con­trib­utes to a dif­fer­ent, es­sen­tial un­der­stand­ing of the sys­tem.

Cloud mod­els pro­vide a pic­ture of the in­ter­nal work­ings of clouds and how cloud droplets in­ter­act with one an­other, which can­not be seen by ob­servers at the sur­face or from satel­lites. Air­craft data are also used to test cloud­mod­elling re­sults, but are cur­rently in short sup­ply. Sur­face ob­servers and long-lived satel­lites con­trib­ute to a record that is long enough to dis­tin­guish trends in cloud cover from changes that are driven by cycli­cal vari­abil­ity in the sys­tem. Satel­lites of­fer a valu­able, but lower-res­o­lu­tion view of sys­tems like the In­dian mon­soon over the en­tire planet, many in places where ob­ser­va­tions from the sur­face are lack­ing.

Each branch within the cloud cli­ma­tol­ogy com­mu­nity re­lies on the other to ad­vance the science. Sim­u­lated pro­cesses need to be com­pared to real-world ob­ser­va­tions to ver­ify re­sults. Ob­served phe­nom­ena re­quire not only phys­i­cal ex­pla­na­tions tested in cloud mod­els, but in­de­pen­dent ver­i­fi­ca­tion be­tween ob­serv­ing plat­forms.

The above ex­am­ples out­line how the cloud record can be used to study at­mo­spheric changes on scales rang­ing from mi­cro­scopic to global, from the sur­face to the strato­sphere. The most dif­fi­cult work go­ing for­ward will be in im­prov­ing our un­der­stand­ing of how clouds will in­ter­act with a chang­ing at­mos­phere and whether changes in cloud cover will act to ex­ac­er­bate or off­set global warm­ing. It is es­sen­tial that we con­tinue to main­tain the cur­rent ob­serv­ing plat­forms in or­der to bet­ter di­ag­nose changes in the at­mos­phere, while si­mul­ta­ne­ously, we must con­tinue to in­vest in im­proved cloud mod­el­ling to bet­ter un­der­stand the causes and ef­fects of cloud changes.

RYAN EAST­MAN Staff sci­en­tist, de­part­ment of at­mo­spheric sciences, Univer­sity of Wash­ing­ton, USA


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