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1. IntroductionCloud Aerosol Interaction and Precipitation

Enhancement Experiment (CAIPEEX) is an Indiannational experiment with a focus on the researchand application aspects of processes in thepremonsoon and monsoon clouds. The two mainobjectives of the CAIPEEX program were a) toaddress the physics and dynamics of aerosol-cloud-precipitation interactions and b) to formulatea scientific basis for rain formation and rainenhancement using the recent cloud seedingtechnologies. Indian Inst itute of TropicalMeteorology conducted a cloud seeding experimentin the 70's during which no definite conclusionscould be made on the efficacy of seeding. CAIPEEXwas formulated with the understanding thatmonsoon rainfall is heterogeneous in space andtime and in spite of existing rain bearing clouds,several locations in India experience droughtconditions during the monsoon season. TheCAIPEEX program benefitted from the recenttechnological advances of in situ observationalcapabilities to measure cloud and aerosolmicrophysics. The main research emphasis onaerosols in India has been on its direct effect andCAIPEEX is first experiment to investigate theindirect effect of aerosols in monsoon clouds. Thebasis for cloud droplet and rain drop formation inmonsoon clouds and how aerosol pollutioninfluences these processes has been addressedin CAIPEEX.

ABSTRACT

Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX), an IndianNational Experiment was carried out during 2009-2015 in three phases with a focus on the aerosol-cloud-precipitation interactions in the premonsoon and monsoon environment and to derive usefulguidelines for cloud seeding purpose. In situ observations of aerosol, cloud microphysics andassociated thermodynamics and dynamics were carried out with this campaign. There have beenseveral research papers in refereed journals and data that was collected during the campaign hascontributed significantly to the understanding of cloud microphysics in different environmentsover the Indian region. These observations are used at process level understanding and developmentof process parameterization. Vertical profiling of aerosol and cloud microphysical properties wascarried out for the first time over India during CAIPEEX and revealed insightful understanding onthe warm rain process and its link with subcloud aerosols, rain drop formation, ice nucleation, etc.

Keywords : Aerosols, cloud microphysics, rain drop formation, ice nucleation.

2. ObservationsAirborne observations of cloud microphysical

parameters, aerosol size distribution, cloudcondensation nuclei (CCN) concentrations andenvironmental parameters such as temperature,humidity and winds were carried out with the helpof a twin engine Piper Cheyene pressurized aircraftin 2009, which was exploratory experiment for tworeasons. This was to survey the aerosol, CCN andcloud droplet characteristics over different parts ofIndia and to identify a suitable location to conductcloud seeding experiment (Kulkarni et al., 2012 fordetails). During the Phase II of CAIPEEX (during2010-2011), airborne observations were conductedwith Hyderabad as the base station and generallycontinental clouds were observed. Total of 670hours of observations were carried out undervarying aerosol pollution conditions over these twophases of the experiment (Phase I in 2009 andPhase II in 2010-2011). CAIPEEX 2011 alsoincluded an Integrated Ground ObservationalCampaign (IGOC), which has made ground to cloudlayer observations possible in an integrated way,addressing the thermodynamical and microphysicalaspects of convection over the lee side of WesternGhat. The information on CAIPEEX and data andavailabili ty is at the website http://www.tropmet.res.in/~caipeex/

The main focus of CAIPEEX so far was on thescientific aspects of cloud-aerosol-precipitationinteractions and the objective with the cloud seeding

Cloud Aerosol Interaction andPrecipitation EnhancementExperiment (CAIPEEX) :Recent Findings

Thara PrabhakaranIndian Institute of

Tropical Meteorology, PuneE-mail: thara@tropmet.res.in

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could be addressed only with the help of collocatedC-band radar. We have collected a fewobservations which have already been discussedin the report provided at the website http://www.tropmet.res.in/~lip/Publication/Scientific-Reports/SR-15.pdf . Formulation of guidelines willrequire more number of observations with aircraftand collocated C-band radar.

The understanding of cloud and precipitationformation is essential for accurate weather andclimate modeling, where representation of cloudsintroduce one of the biggest uncertainties. RecentIPCC AR5 report states that "Cloud and aerosolproperties vary at scales significantly smaller thanthose resolved in climate models, and cloud-scaleprocesses respond to aerosol in nuanced ways atthese scales. Until subgrid-scaleparameterisations of clouds and aerosol-cloudinteractions are able to address these issues,model estimates of aerosol-cloud interactionsand their radiative effects will carry largeuncertainties". Data collected in CAIPEEX fieldcampaign address these aspects.

3. Aircraft and InstrumentsDetails of the aircraft and instruments used in

Phase I of CAIPEEX have already been describedin Kulkarni et al., (2012, CS). The Aero Commander690A (South African Weather Service SAWS) wasused as the CAIPEEX-II research aircraft in 2010and 2011. The instruments were chosen (Fig.1) tohave most accurate measurements of cloud

particles that have a range of sizes, shapes, andconcentrations. It was noted that water and icecoexisted in several of the observations made indeep cumulus clouds and more instruments wereadded in 2011 campaign to have detailed andreproducible observations. Cloud hydrometeors ofdifferent size and shapes were measured in situby aircraft is invaluable for cloud microphysicsstudies.

Cloud droplets (<50 m) were measured witha forward scattering probe (FSSP was used inPhase II and CDP in Phase I and III). Optical arrayprobes are used for larger particle sizes whichprovides images for size and shape, which need tobe processed later to derive various parameters ofinterest. These are CIP, PIP and 2D-C probeshaving different resolution and ranges as illustratedin the Figure. PMS Forward Scatter SpectrometerProbe (FSSP-100; 3-47 m), Cloud AerosolSpectrometer (CAS; 0.5-50 m), the Cloud ImagingProbe (CIP; 25-1550 m), Precipitation Imaging Probe(PIP), Liquid Water Content (LWC) hot-wire probe,the King LWC hot-wire probe, two dimensional stereoprobe (2D-S; 10-1280 m) were used in the campaign.Passive Cavity Aerosol Spectrometer Probe (PCASP;0.1- 3 m) and CCN coun ter was us ed tocharacterize the subcloud aerosol size distributionand CCN concentration. The nucleation modeaerosol size and concentration were measured by ahigh flow differential mobility analyzer (DMA; 0.02to 0.47 m) in 2011 campaign.

Fig.1 Summary of various instruments used for aerosol, cloud droplet, rain drops, ice particle sizedistributions (DMA; Differential Mobility analyser ,PCASP; Passive Cavity Aerosol SpectrometerProbe , FSSP; Forward scattering spectrometer probe, CAS; Cloud Aerosol spectrometer, CDP;Cloud Droplet Probe, CIP; Cloud Imaging Probe, 2D-S; 2D stereo cloud probe, PIP; PrecipitationImaging Probe) from 4 phases of CAIPEEX.

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The single engine Ayres Turbo Thrush aircraftwas used as a seeder aircraft. It could dispersesalt micropowder and had flare racks that carry 20hygroscopic or glaciogenic flares. The aircraft wasalso instrumented with a PCASP, FSSP, CCN,CPC, LWC and AIMMS probe during 2011CAIPEEX.

Beechcraft King Air B-200 aircraft was hired inthe Phase III with the cloud physics and aerosolinstrumentation as illustrated in Figure 1, in additionto the LTC, Total Water content probe (TWC),AIMMS and several instruments such as CloudCombination Probe (CCP), Chemistry of aerosoland precipitation particles, CRDS gas analyserswere installed by IITM.

4. Droplet Size Distribution inPremonsoon and Monsoon Clouds

Fig.2 gives an example of aerosol and clouddroplet size distribut ion observation duringCAIPEEX. It may be noted that the observations

in the size ranges from 0.01 to 10000 µm requiresdifferent instruments as illustrated earlier. The sizedistribution from various instruments also has anoverlap, which indeed demonstrates the quality andreliability of these observations.

The indirect effect of aerosols was notaddressed in any of the observational campaignsprior to CAIPEEX. In polluted conditions, with moreaerosol particles, droplet size is much smaller andspectrum is narrower as collision-coalescence issuppressed as compared to the cleaner conditions.

Fig.2 Aerosol size distribution (a) outside cloud andcloud droplet - precipitation and ice particlesize distribution (b) measured in a congestuscloud. The combined size distribution is fromvarious probes. There is very goodagreement between the different probes.

Fig.3 The variation of cloud droplet numberconcentration in the premonsoon,transition and monsoon periods(indicated with boundary layer watervapor). The observations correspondingto hazy conditions are indicated in thefigure. The droplet size distribution in thepremonsoon (PRE1, PRE2) and monsooncloud (MON1, MON2, MON3) showedgradual change from the single mode tomult iple modes in the condi tions duringthe monsoon (MON2, MON1) with highaerosol. (from Prabha et al., 2012 JGR).

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Droplet size distributions (DSDs) in highlypolluted premonsoon clouds are substantiallynarrower than DSDs (Fig.3) in less pollutedmonsoon clouds. High DSD dispersion isattr ibuted to the existence of small c louddroplets with diameters less than 10 µm, foundat al l levels. The presence of small dropletslead to the bimodal spectra (Fig.4), which wasmore pronounced with the polluted monsoonclouds. Observational ev idence for inc loudactivation was presented. It was found that suchmult iple modes are attributed to inc loudactivation/evaporation of particles/c louddroplets (Prabha et al., 2011, JAS). Thesecondary modes in the droplet sizedistribution as i l lustrated in the Figure 5.Incloud act ivation may be the presence ofinterstitial aerosol particles in cloud updraftsand there is high l iquid water in the polluted

clouds close to its top. This study emphasizedthe need to include aerosol processing in cloudmodels, where aerosol activation is designedonly at cloud base.

Morwal et al (2012; JASTP) illustrated the clouddroplet size distribution effective radius overgeographically different locations in India. The dropsize distribution was wider at elevated layers andthe effective radius increased with height. Nair etal., (2012, AR) i llustrated various cloudmicrophysical parameters in the premonsoon,transition to monsoon and active monsoonconditions.

Padmakumari et al., (2012, IJRS) comparedthe space-borne l idar (CALIPSO) and radar(CloudSat) together with the aircraftobservations (Fig.5). Similar features of aerosollayering and cloud parameters are observedby both aircraft and CALIPSO. The CloudSat

Fig.4 Variation of droplet size distribution in the downdrafts (left) and updrafts (right) from Cloud DropletProbe (CDP) and the Cloud Imaging Probe images of cloud droplets, graupel and supercooleddrops in the monsoon cloud. The legend is labeled with Time, Height, droplet number concentration,liquid water content, effective radius, vertical velocity, quasistationary supersaturation and adiabaticfraction. The mixed phase nature of these clouds was illustrated for the first time with in situobservations. (derived Prabha et al., 2011, JAS).

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profiles of liquid water content, droplet numberconcentration, and effective radii areunderestimated when compared with thecorresponding aircraft profi les. It is alsoil lustrated that there is possible dust aerosolsacting as ice nuclei.5. Process Parameterization

A simple parameterization for cloud dropleteffective radius, useful in the numerical models wasproposed from CAIPEEX observations of 2009(Nair et al., 2012, AR). A linear relationship betweenthe cloud droplet number concentration andadiabatic fraction (a measure of mixing between thecloud and its environment) was suggested as asimple formulation that will be useful for large scalemodels to predict cloud droplet numberconcentration, when cloud liquid water content onlyis diagnosed. Filter samples collected on theCAIPEEX aircraft are analysed in a thermalgradient diffusion chamber (TGDC) (developed atPune University) designed to process the ice nuclei(IN) sample. IN activated by deposition mode overice supersaturation of 6-24% and at a temperaturerange of -18.5 to -13.5 oC were found to be lessthan 5 L-1. These samples correspond to firstobservations at several vertical levels including thecloud layer and over several geographically distinctlocations in India during the monsoon season.These observations illustrate that the Meyers INparameterization currently used in the numericalmodels overestimate IN concentration at all

Fig.5 CloudSat-and aircraft-derived cloud microphysical parameters such as (a) droplet numberconcentration, (b) liquid water content, and (c) droplet effective radii. (The bars represent spatialvariation along specific level. Different symbols are given for CloudSat, each symbol representingretrievals at different neighbouring lat-long coordinates close to the aircraft profiled area along thesatellite track.) (Derived from Padmakumari et al., IJRS, 2012).

supersaturations by an order. The simulationsindicate that this can have a significant impact onthe spatial distribution of mixed phase and ice cloudspredictions.

6. Rain Drop Formation in MonsoonClouds

The effective radius in developing cloudincreases with height and it becomes more than acritical effective radius when the coll isioncoalescence becomes very effective. The heightof this critical effective radius (12-14 µm) depends

Fig .6a

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Fig.6a&b Vertical variation of effective radius(a; vertical l ine is effective radiuysthreshold for warm rain initiation 12 µm)shows that warm rain initiated at lowerheights over the Bay of Bengal comparedto moderately polluted peninsular landregion and very highly polluted IndoGangetic Plains (IGP), Higher CCNincreases the warm rain depth (b) andpresence of Giant CCN (GCCN; indicatedby size of symbols, color is massnormalized reflectivity) reduces the warmrain depth. Each point in the plot is anobservation from CAIPEEX. (derived fromKonwar et al., 2012 JGR).

linearly on the aerosol concentration at the cloudbase as illustrated from CAIPEEX and severalother field campaigns around the world (Freud etal., 2011). Increase in warm rain depth with aerosolnumber concentration (Figure 6) in several cloudobservations during CAIPEEX has been illustratedby Konwar et al., (2012 JGR).

It has been further illustrated from the 2D and3D bin microphysics simulations (Khain et al., 2013,JGR) and CAIPEEX observations that the heightof first raindrop formation in monsoon cloudsdepends linearly on the droplet numberconcentration at cloud base. First raindrops formin slightly diluted cloudy volumes at the tops ofbubble cores. First rain formation is determinedlargely by the basic condensation and collisionmechanisms in ascending adiabatic volumes.These results also indicated that we can predictthe height of the formation of first rain dropsconsidering the processes of nucleation, diffusiongrowth and collisions. This idea has lead to the

Fig .6b

formulation of an autoconversion parameterizationusing CAIPEEX observations.

7. Cloud Microphysics in the Premonsoonand Monsoon Clouds and Dispersion

In polluted condi tions, with more aerosolpart icles, droplet size is much smaller andspectrum is narrower as collision-coalescenceis suppressed, compared to the cleanerconditions. A simple parameterization for clouddroplet effective radius, useful in the numericalmodels was proposed from CAIPEEXobservations of 2009 (Nair et al., 2012, AR).The aerosol effect on cloud microphysicalproperties and droplet dispersioncharacteristics were addressed by (Pandithuraiet al., 2012 JGR; Prabha et al., 2012b JGR). Itis shown that the entrainment effect candrastically influence the droplet dispersion andis heterogeneous with height, whi le raindropformation occurred in the slightly mixed cloudparcels (Prabha et al 2012b, JGR), indicatingthat entrainment effects did not play a role inthe raindrop/large drop formation.

The vertical variation of spectral width and meanradius in different aerosol conditions duringmonsoon is illustrated in Fig. 7. Mean radiusincreases rapidly above cloud base in the cleanmonsoon. Raindrops form at a lower level in cleanmonsoon, while at elevated layer in the high aerosolconditions (indicated with vertical pointing arrow).Mixing at cloud edges reduces the droplet size andspectral width, which is illustrated with the adiabaticfraction (Fig.7). Adiabatic fraction is ratio of the liquidwater content measured in the cloud and theadiabatic estimate of liquid water content. The lowadiabatic content cloud parcels exhibit reduceddroplet mean radius and spectral width. Highadiabatic fraction parcels have maximum spectralwidth and mean radius, indicating large droplets/drops in adiabatic parcels, also the region for raindrop formation. The results also indicate that theevaporation of cloud droplets through entrainmentand mixing mechanism is very important and maylead to near linear variation of mean radius withheight. Meanwhile in adiabatic regions, the dropletdiffusional growth and collision coalescenceprevails, which lead to a more non linear variationof mean radius with height. As evident, the collisioncoalescence is more active in the clean monsooncloud at lower elevations. In the case of pollutedmonsoon cloud, collision coalescence becomeactive at higher levels in the cloud.

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the foothills of the Himalayas is investigated byPadmakumari et al, (2013 QJRMS). Elevatedaerosol layers up to 4 km (Fig. 9) with varyingconcentrations were observed which are attributedto sources from local anthropogenic activities andbiomass burning, dust from local and long-rangetransport. The aerosol size distributions indicatedincreases in both fine and coarse mode aerosolsin elevated layers. Single scattering albedo profilesdepicted the presence of both absorbing and non-absorbing type aerosols on different days. Cloudsobserved above the elevated aerosol layersshowed higher droplet concentrations exceeding1000 cm?3 with small effective radii <6 ?m.

The origin of these elevated aerosol layers wereinvestigated by Prabha et al (2012a, JGR).Horizontal flights across the Himalayan slopes wereused to investigate the mechanism that favorstransport across the valley to Tibetan Plateau.Turbulence measurements from a horizontal flightpath are used to illustrate the scale interactions inthe vertically sheared flow below the high-levelsubtropical westerly jet. Large eddies that scale 10-12 km near the slopes (Fig. 10) could bring pollutionfrom the valley to the Tibetan Plateau through acirculation adhering to the slopes. There is asubsidence region away from the slopes couldcontribute to the build-up of pollution in elevatedlayers over the Plains. The vertical velocity and

8. Dispersion EffectPandithurai et al (2012, JGR) showed that the

aerosol indirect effect may be offset by thedispersion effect up to 39 % (Fig.8) and this willhave implications to climate/weather model resultswhere dispersion effect is not incorporated.

9. Elevated Aerosol Layers andMechanism of their Formation

The fine mode (0.1-3.0 ?m) aerosol verticalprofiles up to 6 km at different regions showeddifferent vertical structures mostly influenced by theatmospheric boundary layer (ABL) depth as wellas the origin of air mass trajectories and thepresence of clouds (Padmakumari et al., 2013, AE).Elevated aerosol layers are observed during pre-monsoon and during monsoon. During monsoon,aerosol number concentration showed strongvertical gradient. The coarse mode (>3 ?m) aerosolvertical profiles also showed elevated layers athigher altitudes due to the incursion of dry air ladenwith dust. The spatial distribution shows significantvariation at the elevated layers as compared to thatin the boundary layer during pre-monsoon, whilehigh variability is observed in the boundary layerduring monsoon.

Spatial and vertical distribution of aerosols nearthe foothills of the Himalayas during May 2009 near

Fig.7 Vertical variation of mean radius and spectral width of cloud droplet size distribution in three differentaerosol environments in monsoon clouds (derived from Prabha et al., 2012, JGR).

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Fig.8 Variation of cloud droplet number concentration (a), Effective radius (b) relative dispersion (ratio ofspectral width and mean radius) with the aerosol number concentration and effective radius ratio ()for cloud liquid water per droplet. (Derived from Pandithurai et al., 2012 JGR).

Fig.9 Vertical variation of aerosol number concentration from aircraft observations near the foothills ofHimalayas (Padmakumari et al., 2013, QJRMS).

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temperature spectra from research flight datashowed clear indications of (-5/3) slope in themesoscale range. A conceptual understanding ofthe flow in the region close to the foot hills and itsrole in the distribution of aerosol and cloudcondensation nuclei was illustrated in this study.

CAIPEEX observations together with WRF-Chem model simulations was used to investigatethe dust emission from Thar desert and its role inthe elevated aerosol layers and warming andsubsequent influence on the cloud microphysicsof propagating systems from BoB during monsoononset of 2009 (Dipu et al., 2012, AE). It is observedthat aerosol loading increased over the centralIndian region in spite of the increase in surfacerainfall, attributed to increase in elevated layeraerosols (2-5 km level above surface). The originand influence of elevated aerosol layer have beeninvestigated with the help of WRF-Chemsimulations by conducting sensitivity experimentsfor dust emissions based on erodible fraction over

the Thar Desert region attributing to enhanced dustemissions by a factor of 1.25. This resulted in anincreased radiative heating in the elevated layers,which leads to an increase in the ice mixing ratioand ice water content associated with the monsoononset (Fig.11). It is illustrated that even natural dustemissions (without changes in anthropogenicemissions) may also influence the spatial andtemporal distribution of cloud and precipitation andthe hydrological cycle.

Special research flights carried out on 19 and20 October, 2011 in the Bay of Bengal in the vicinityof a depression illustrated the aerosol-cloudinteractions over marine environment (Deshpandeet al., 2013 AR). The concentration of aitken/accumulation mode particles was high at 500 mabove sea surface level over the ocean after thepassage of the depression. They attributed thesource of these particles and their subsequentgrowth at about 200 km from coastline to oxidationof dimethyl sulfide (DMS) because of upwelling of

Fig.10 Horizontal distribution of vertical velocity at 6.6 km above mean sea level (a) CAIPEEX flighttracks are shown with thick line, also indicates regions of (i) gravity waves across slopes alongCAIPEEX flight track , (ii) gravity waves away from slopes, (iii) strong updrafts on the slopes in thepockets, (iv) downdraft area on the summit, (v) narrow convective updrafts and wide downdraftson the Plateau. The vertical velocity spectra from the CAIPEEX aircraft is compared with highresolution model (b). (c) shows a conceptual understanding derived from CAIPEEX observationsand numerical model for the aerosol spatial distribution and transport across Indo Ganges Valley.(Derived from Prabha et al., 2012a JGR).

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Fig .11 Difference in Cloud water mixing ratio (a), ice water mixing ratio (b) and accumulated precipitationfrom (c) from a sensitivity experiment by increasing Dust emission from Thar Desert (d). Theexperiment was conducted for the dry to wet transition during end of May 2009. It shows thatnatural emissions itself can contribute to a radiative-dynamically driven differences in theprecipitation and cloud fields. (derived from Dipu et al., 2012, AE).

the deep ocean water during the depression andanthropogenic aerosols transported from inland. Noevidence of increasing particle concentration andgrowth has been observed at about 60 km fromcoastline towards southeast.

Particle size distribution in cloud free zone (CFZ)and cloud shadow zone (CSZ) and the radiativeforcing associated with them were investigated byKonwar et al., (2015, JGR). It was shown thatextinction coefficient decreases with increasingdistances from the cloud edges. Large deviationsof effective radius of particles from their meanvalues are observed in highly humid conditions;however, there was positive correlation with relativehumidity indicating swelling of aerosols in humidifiedenvironment (Fig.12). Mean radiative surfaceforcing (SF) and top of the atmosphere (TOA)forcing in the cloud free zone (CFZ) and cloudshadow zone (CSZ) indicated that mean surfacecooling may be increased by 4% due to theaerosols in the near cloud regions and mean TOAforcing was increased by nearly 20% in the Cloudshadow region.

10. Vertical Profiling of Black CarbonVertical variation of black carbon (BC) play an

important role in the direct and indirect effect byacting as an absorbing aerosol or as cloudcondensation or ice nuclei which help formation ofclouds. CAIPEEX observations of BC conductedover two locations were used to investigate theaerosol induced radiative forcing for central Indiato the north and the pristine ocean to the south toillustrate the north south asymmetry in the heating(Manoj et al., 2011, CD). In that study, a seminal Fig .12 a & b

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role played by the absorbing aerosols in thetransition from break to active spells was illustratedthrough modification of the north-south temperaturegradient at lower levels.

Safai et al., (2012 STE) illustrated that the BCobservations onboard the aircraft from Bangaloreand Hyderabad. BC mass loadings decreasedmonotonically from 103 to 104 ng/m3 at the surfaceto ~102 ng/m3 at an altitude of about 7 km; althoughlayers at intermediate levels containinganomalously high BC loadings were frequentlyencountered that were attributed mainly to theconvective transport from surface sourcesaccompanied by changes in the local boundarylayer and atmospheric stability. The presence ofBC in cloud in the regions where clouds areobserved have important implications for cloudmicrophysics and subsequent rainfall mechanismover this region. Apart from this, the effects onhuman health are equally important.

Aircraft observations on vertical profiles of BlackCarbon (BC) observations conducted duringCAIPEEX I in monsoon 2009 over Guwahati, in theBrahmaputra River Valley (BRV) region, NE Indiawas illustrated by Rahul et al., (2013, SR). Theynoted that surface/near surface loading of BC dueto anthropogenic processes caused a heating of 2K/day. The large-scale Walker and Hadleyatmospheric circulations associated with the Indiansummer monsoon helped in the formation of asecond layer of BC in the upper atmosphere.

Elevated layer of BC generates an upperatmospheric heating of 2 K/day with the lofting ofBC aerosols.

Simultaneous aircraft observations of BlackCarbon (BC) mass concentrations and cloudmicrophysical parameters were used by Panikeret al., (2014, AE) to illustrate possible 'semi directeffect' or the cloud burning effect (Fig.13). Elevatedpollution layers of BC (concentration exceeding 1?g m -3) observed resulted in instantaneous verticalheating rate induced by BC in cloud layers to be2.65 K day-1, leading to a significant reduction inthe measured cloud liquid water content (LWC) overthe site. BC stimulated heating was found to bereducing the cloud fraction (CFR) and may becontributing to a "cloud burning effect (Semi direct

Fig .12 Combined aerosol size distribution fromPassive Cavity Aerosol SpectrometerProbe (PCASP; 0.1 to 3 ?m) and CloudAerosol Spectrometer (CAS 0.5-50 ?m)(derived from Konwar et al., 2015, JGR).

Fig .13 Flight tracks from Varanasi (9-30th Sept2014) and from Juhu Airport during theCAIPEEX Phase III campaign.

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effect)" over the region. The estimatedinstantaneous BC induced radiative forcing in thecloud regime was +12.7 to + 45.1Wm-2 during theexperimental period. This large warming andreduction in cloudiness can decrease theprecipitation over this vulnerable region.

11. Rainfall Characteristics over theWest Coast and over Western Ghat

The mechanism responsible for high rainfall overthe Indian west coast region has been investigatedby studying dynamical, thermodynamical andmicrophysical processes over the region for themonsoon season by Maheshkumar et al., (2013CD). The moist adiabatic and multi-level inversionstratifications were esisting during the high and lowrainfall spells, respectively. In the moist adiabaticstratification regime, shallow and deep convectiveclouds coexist. The low updrafts (as indicated byCAIPEEX observations) provided sufficient timerequired for warm rain processes to produce rainfallfrom shallow clouds. The updrafts at the highspectrum end go above freezing level to generateice particles produced due to mixed-phase rainfallprocess from deep convective clouds.

Suppression of rainfall over the rainfall over therain shadow region was discussed by Konwar etal., (2010 ACPD) from CAIPEEX observations.Low rainfall over the rain-shadow region of WesternGhats is investigated by Narkhedkar et al., (2015,CD). They noted that convergence at 850 hPa anddivergence at 400 hPa level is due to dynamicalforcing and generates high frequency of occurrenceof congestus clouds. These congestus clouds arecontinental and have high cloud bases and havelarge number of small droplets. The high cloudbases reduce the depth of warm region in the cloudleading to an inefficient warm rain process. Thethermodynamic structure showed dry surface leveland warm and moist middle troposphere withtongues of dry air and multilevel inversions whichhave been attributed to advection of aerosol-richdry air. They illustrated the integrated impact ofthese factors is to produce low rainfall over the rain-shadow region of the north peninsular India.

CAIPEEX Aircraft observations over the WGrevealed that forced updrafts foster rapidcondensational growth of cloud droplets, triggeringcoalescence process within few hundred metersof cloud depth (Konwar et al., 2013, JGR), givingthe idea that the clouds are dynamically forced. Biand mono-modal drop size distribution are observedduring light and heavy rainfall respectively. Withshallow storm heights, small raindrops mainly

contribute to both types of rainfall. The DSDs areparameterized and their radar reflectivity factor-rainfall intensity relationships were evaluated,suggesting the dominance of collision-coalescenceprocesses.

12. CAIPEEX Phase IIIThe CAIPEEX Phase III was planned to take

place in two parts. Part I in 2014 with an emphasison convective clouds and their vertical structureduring the course of the monsoon season andinvestigating the convection invigoration byaerosols. Beechcraft B200 aircraft was hired withseveral state of the art aerosol and cloud physicsinstruments. In 2014, the Phase III of CAIPEEX wasconducted in the Ganga Basin with Varanasi asthe base. The main objective of the experiment wasto investigate the hypothesis that pollution mayinvigorate clouds. The ground based observationalnetwork in this region by various research groupsadds value to the airborne observations to bring itto a regional campaign. There was a specialemphasis in the program to make chemistrymeasurements in clouds, which could not be thefocus of earlier CAIPEEX I and II campaigns.

The Phase III of CAIPEEX started in May 2014with a set up of ground based and airborneobservations started on 9th Sept 2014. The aircraftobservations together with the ground basedobservations were conducted during 2014. 49Hours of airborne observations were conductedfrom Varanasi during the Phase III. Ground basedobservations were conducted from the ruralcampus of Banaras Hindu University. The PhaseIII also carried out several aerosol and precipitationchemistry measurements on board the aircraft suchas Particle in Liquid Sampler (PILS), Photo AcousticExtinct iometer (PAX) for black carbonmeasurements, Cavity Ring Down Spectrometer(CRDS) for CO2, CH4, CO and H2O.

IITM has a High Altitude Cloud PhysicsLaboratory (HACPL) at Mahabaleswar, where insitu observations of clouds are conducted withground based observations and with X-band andKa-band radars. 26 Hours of airborne observationswere carried out in 2014 in coordination with theHACPL and Radar observations. Phase III data iscurrently undergoing corrections.

It was also proposed to conduct the cloudseeding experiment in 2015-2016 as part II of PhaseIII experiment. However, to collect morerandomized samples so that scientifically basedguidelines for the cloud seeding can be derived,there is requirement for a C-band radar. CAIPEEX

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Phase III experiment is continuing in Varanasi withdetailed airborne observations and ground basedobservations investigating aerosol-cloudinteraction.

AcknowledgementsCAIPEEX project and IITM are fully funded by

Ministry of Earth Sciences, Government of India,New Delhi. This document is dedicated to thecontributions and effort of various CAIPEEX teammembers who worked hard to make this experimentpossible with their extensive field efforts team workand dedication.

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Kulkarni. 2013 In situ measurements of aerosolvertical and spatial distributions over continentalIndia during the major drought year 2009."Atmospheric Environment.

Khain, A., T. V. Prabha, N. Benmoshe, G.Pandithurai, and M. Ovchinnikov (2013), Themechanism of first raindrops formation in deepconvective clouds, J. Geophys. Res. Atmos., 118,9123-9140

Dipu. S., T. V. Prabha, G. Pandithurai, J. Dudhia,G. Pfister, K. Rajesh, B.N. Goswami,2013, Impactof elevated aerosol layer on the cloudmacrophysical properties prior to monsoon onset,Atmospheric Environment, Volume 70, May 2013,Pages 454-467

Padmakumari B., Maheskumar R.S., HarikishanG., Kulkarni J.R., Goswami B.N. Comparative studyof aircraft- and satellite-derived aerosol and cloudmicrophysical parameters during CAIPEEX-2009over the Indian region International Journal ofRemote Sensing, 34, January 2013, DOI:10.1080/01431161.2012.705442, 358-373

Padmakumari B., Maheskumar R.S., HarikishanG., Morwal S.B., Prabha T.V., Kulkarni J.R. In situmeasurements of aerosol vertical and spatialdistributions over continental India during the majordrought year 2009 Atmospheric Environment, 80,August 2013, DOI:10.1016/j.atmosenv.2013.07.064,107-121

Padmakumari B., Maheskumar R.S., Morwal S.B.,Harikishan G., Konwar M., Kulkarni J.R., GoswamiB.N. Aircraft observations of elevated pollutionlayers near the foothills of the Himalayas duringCAIPEEX-2009 Quarterly Journal of RoyalMeteorological Society, 139, April 2013,DOI:10.1002/qj.1989, 625-638

Prabha, Thara V.; Patade, S.; Pandithurai, G.;Khain, A.; Axisa, D.; Pradeep, Kumar P.;Maheshkumar, R.S.; Kulkarni, J.R.; Goswami,B.N., Spectral width of premonsoon and monsoonclouds over Indo-Gangetic valley, J. Geophys.Res., Vol. 117, No. D20, D20205, October 2012,doi:10.1029/2011JD016837, 1-15

Pandithurai, G.; Dipu S.; Prabha, Thara V.;Maheshkumar, R.S.; Kulkarni J.R.; Goswami B.N.,Aerosol effect on droplet spectral dispersion inwarm continental cumuli, J. Geophys. Res., Vol.117, No. D16, D16202 , 17 August 2012

S.B. Morwal, R.S. Maheskumar, B. Padma Kumari, J.R. Kulkarni., B.N.Goswami ,Cloudmicrophysical properties over Indian MonsoonRegions during CAIPEEX-2009 , Journal of

18

Atmospheric and Solar-Terrestrial Physics, , June2012, DOI:10.1016/j.jastp.2012.04.01076-85.

Konwar M., Maheskumar R.S., Kulkarni J.R.,Freud E., Goswami B.N., Rosenfeld D., Aerosolcontrol on depth of warm rain in convective clouds,Journal of Geophysical Research, , 117, July 2012,D13204, DOI:10.1029/2012JD01785, 1-10.

Gayatri Urankar, Prabha Thara. V, Pandithurai G,Pallavi P, Achuthavarrier D, Goswami B.N.,Aerosol and cloud feedbacks on Surface EnergyBalance over selected regions of Indiansubcontinent , J. Geophys. Res., ,117, February2012, DOI:10.1029/2011JD016363, D04210.

Prabha T. V., Anandkumar Karipot, D. Axisa, B.Padma Kumari, R.S. Maheskumar, M. Konwar, J.R.Kulkarni, and B.N. Goswami, Scale interactionsnear the foothills of Himalayas during CAIPEEX, ,J. Geophys. Res., ,117, D10203,DOI:10.1029/2011JD016754 .

Safai P.D, M.P. Raju, R.S. Maheskumar, J.R.Kulkarni, P.S.P. Rao, P.C.S. Devara, Verticalprofiles of black carbon aerosols over the urbanlocations in South India, Science of the TotalEnvironment , 431 (2012) 323-331.

Nair S., Sanjay J., Pandithurai G., Maheskumar,Kulkrni J.R., On the parameterization of clouddroplet effective radius using CAIPEEX aircraftobservations for warm clouds in India (2012),Atmos. Res., Vol.108, 2012, doi:10.1016/j.atmosres.2012.02.002, 104-114

Kulkarni J.R., Maheshkumar R.S., Morwal S.B.,Padma kumari B., Konwar M., Deshpande C.G.,Joshi R.R., Bhalwankar R.V., Pandithurai G., SafaiP.D., Narkhedkar S.G., Dani K.K., Nath A., NairSathy, Sapre V.V., Puranik P.V., KandalgaonkarS.S., Mujumdar V.R., Khaladkar R.M., VijaykumarR., Prabha T.V., Goswami B.N., The Cloud AerosolInteraction and Precipitation Enhancement

Experiment (CAIPEEX): overview and preliminaryresults (2012), Curr. Sci., Vol.102, 2012, 413-425

Prabha T.V., Khain A., Maheshkumar R.S.,Pandithurai G., Kulkarni J.R., Goswami B.N.(2011),Microphysics of Premonsoon and MonsoonClouds as Seen from In Situ Measurements duringthe Cloud Aerosol Interaction and PrecipitationEnhancement Experiment (CAIPEEX), J. Atm.Sc., Vol.68 , 2011, DOI: 10.1175/2011JAS3707.1,1882-1901

Kulkarni J.R., Maheshkumar R.S., Morwal S.B.,Padma kumari B., Konwar M., Deshpande C.G.,Joshi R.R., Bhalwankar R.V., Pandithurai G., SafaiP.D., Narkhedkar S.G., Dani K.K., Nath A., NairSathy, Sapre V.V., Puranik P.V., KandalgaonkarS.S., Mujumdar V.R., Khaladkar R.M., VijaykumarR., Prabha T.V., Goswami B.N., The Cloud AerosolInteraction and Precipitation EnhancementExperiment (CAIPEEX): overview and preliminaryresults (2012), Curr. Sci., Vol.102, 2012, 413-425

Freud E., Rosenfeld D., Kulkarni J.R. (2011),Resolving both entrainment-mixing and number ofactivated CCN in deep conective clouds, Atmos.Chem. Phy., Vol. 11, 2011,DOI:10.5194/acp-11-12887-2011, 12887-12900

Manoj M.G., Devara P.C.S., Safai P.D., GoswamiB.N. (2011), Absorbing aerosols facilitate transitionof Indian monsoon breaks to active spells, Clim.Dyn., Vol. 37, December 2011, DOI 10.1007/s00382-010-0971-3, 2181-2198

Konwar M., Maheskumar R.S., Kulkarni J.R.,Freud E., Goswami B.N., Rosenfeld, (2010),Suppression of warm rain by aerosols in rain-shadow areas of India (2010), Atmos. Chem.Phys. Discuss, Vol. 10, 2010, doi:10.5194/acpd-10-17009-2010, 17009-17027