Enig­matic clouds

Clouds hold the key to pre­dict­ing mon­soon and cli­mate change, but there is very lit­tle we can say with cer­tainty about them

Down to Earth - - COVER STORY -

UN­DER­STAND­ING CLOUDS, that cover 60 per cent of the planet, is es­sen­tial to un­der­stand­ing weather. For one, they pro­duce rain, snow, hail and light­ning and, there­fore, hold clues to the mi­cro­phys­i­cal pro­cesses in the at­mos­phere; two, they mod­u­late the plan­e­tary en­ergy bud­get (the amount of so­lar en­ergy cir­cu­lat­ing in the planet) and the hy­dro­log­i­cal cy­cle; and three, they ex­plain cloud-ra­di­a­tion feed­back, or the dy­nam­ics of clouds’ in­ter­ac­tion with so­lar ra­di­a­tion, which is nec­es­sary to pre­dict cli­mate change.

Ad­di­tion­ally, un­der­stand­ing them could help en­hance ar­ti­fi­cial pre­cip­i­ta­tion through cloud seed­ing. Our ca­pac­ity to pre­dict light­ning, which kills thou­sands ev­ery year, too would im­prove if we un­der­stood clouds bet­ter.

These float­ing en­ti­ties can be stud­ied well only by us­ing re­mote sens­ing meth­ods like satel­lites, radars and air­craft. So far, our un­der­stand­ing of clouds is very rudi­men­tary.

We know that they are a mass of very tiny droplets of wa­ter, one-hun­dredth of a mil­lime­tre in di­am­e­ter, or ice crys­tals float­ing in the at­mos­phere, and are formed when wa­ter vapour con­denses on a con­den­sa­tion nu­clei, or an aerosol. As the droplets col­lide, some grow larger and start to fall. If the cloud is dense enough to form droplets greater than one tenth of mil­lime­tre in di­am­e­ter, the droplets reach the ground as rain.

All clouds do not pre­cip­i­tate, and even in the ones that do, the pre­cip­i­ta­tion ef­fi­ciency varies. Some clouds give a driz­zle, oth­ers tor­ren­tial rain. In nu­mer­i­cal weather pre­dic­tion mod­els (these are math­e­mat­i­cal mod­els which take into con­sid­er­a­tion dy­nam­ics and physics of the at­mos­phere) to pre­dict rain, rep­re­sen­ta­tion of clouds and the pre­cip­i­ta­tion pro­cesses is in­ad­e­quate. We are not cer­tain about the num­ber of aerosols in a cloud or what a change in one vari­able might do to oth­ers. Most of the er­rors in pre­dict­ing rain­fall come from this in­ad­e­quate rep­re­sen­ta­tion.

Even in cou­pled cli­mate mod­els, which take into ac­count fac­tors re­lat­ing to oceanic pro­cesses as well as at­mo­spheric cir­cu­la­tions, im­prove­ments are needed to sim­u­late mon­soon con­di­tions. This type of model re­quires a sim­u­la­tion of three di­men­sional cloud dis­tri­bu­tion and its ef­fect on the at­mo­spheric ra­dia­tive bud­get.

En­ergy man­agers

Clouds mod­u­late plan­e­tary en­ergy bud­get as they in­ter­act with both so­lar ra­di­a­tion com­ing from the sun and in­frared ra­di­a­tion emit­ted by the earth and its at­mos­phere. The largest un­cer­tain­ties as­so­ci­ated with cli­mate change pro­jec­tions are re­lated to clouds.

Clouds nor­mally scat­ter so­lar ra­di­a­tion, but ab­sorb in­frared ra­di­a­tion. Low clouds scat­ter more so­lar ra­di­a­tion than they ab­sorb in­frared ra­di­a­tion, and thus have a cool­ing ef­fect. How­ever, high clouds have a warm­ing ef­fect be­cause of high ab­sorp­tion of in­frared ra­di­a­tion. With global warm­ing, some pre­dic­tion mod­els say we are likely to have more low clouds, while oth­ers pre­dict more high clouds. If we wit­ness an in­crease in low clouds, we should ex­pect re­duc­tion in global warm­ing.

Clouds, thus, have an im­por­tant role in long-term cli­matic changes ei­ther di­rectly through their im­pact on the ra­dia­tive fluxes, or

in­di­rectly through their in­ter­ac­tion with other vari­ables such as at­mo­spheric tem­per­a­ture, pres­sure, sur­face tem­per­a­ture and hu­mid­ity. Un­der­stand­ing the cloud-ra­di­a­tion in­ter­ac­tion is, there­fore, es­sen­tial to ac­cu­rately sim­u­late cli­mate change pat­terns. A dif­fer­ence in cloud dis­tri­bu­tion can af­fect the sim­u­la­tion of sur­face en­ergy bud­get and, thus, sur­face tem­per­a­tures. How­ever, quan­ti­fy­ing the ra­dia­tive im­pacts of clouds is quite dif­fi­cult. For in­stance, wa­ter droplet dis­tri­bu­tion de­cides how re­flec­tive a cloud will be, but this dis­tri­bu­tion is dif­fi­cult to mea­sure.

Ini­tia­tives to de­code cloud

In 2009, the Union Min­istry of Earth Sciences launched a pro­gramme to un­der­stand clouds and to im­prove their rep­re­sen­ta­tion in weather and cli­mate mod­els. The Cloud-Aerosol In­ter­ac­tion and En­hance­ment of Pre­cip­i­ta­tion is a na­tional pro­gramme tar­geted to un­der­stand the in­ter­ac­tion be­tween aerosols and clouds and the mi­cro­phys­i­cal prop­er­ties of clouds. Un­der this pro­gramme, many ob­ser­va­tional ex­er­cises have been con­ducted, in­clud­ing air­craft ob­ser­va­tions. In the next three years, more ex­haus­tive ob­ser­va­tional cam­paigns will be con­ducted to un­der­stand the pre­cip­i­ta­tion en­hance­ment pro­cesses (ar­ti­fi­cial rain­mak­ing).

The es­tab­lish­ment of a high-al­ti­tude cloud ob­ser­va­tory at Ma­ha­balesh­war by the In­dian In­sti­tute of Trop­i­cal Me­te­o­rol­ogy, Pune, in 2014 was an­other mile­stone in weather stud­ies in In­dia. Since the ob­ser­va­tory is at a height of about 1.2 km, it works as a nat­u­ral lab­o­ra­tory. Mon­soon clouds form­ing in the val­ley pass through the var­i­ous in­stru­ments kept at the ob­ser­va­tory, al­low­ing mi­cro­phys­i­cal ob­ser­va­tions.

These ini­tia­tives have added to our un­der­stand­ing of clouds, but there is still a long way be­fore we can claim to have de­coded the in­ter­nal mech­a­nisms of clouds.

Clouds can be stud­ied well only by us­ing re­mote sens­ing meth­ods like satel­lites, radars and air­craft. So far, our un­der­stand­ing of clouds is very rudi­men­tary

MRAJEEVAN Weather sci­en­tist and sec­re­tary, Min­istry of Earth Sciences, Gov­ern­ment of In­dia

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