Aeolian Research 17 (2015) 15–31

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Aeolian Research

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Meteorological, atmospheric and climatic perturbations during majordust storms over Indo-Gangetic Basin

http://dx.doi.org/10.1016/j.aeolia.2015.01.0061875-9637/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Fax: +91 0542 2368390.E-mail address: abhay_s@rediffmail.com (A.K. Singh).

Sarvan Kumar a, Sanjay Kumar a,b, D.G. Kaskaoutis c, Ramesh P. Singh d, Rajeev K. Singh e, Amit K. Mishra f,Manoj K. Srivastava e, Abhay K. Singh a,⇑a Atmospheric Research Lab., Department of Physics, Banaras Hindu University, Varanasi 221005, Indiab School of Electrical and Electronic Engineering (EEE), Nanyang Technological University (NTU), Singapore 639815, Singaporec Department of Physics, School of Natural Sciences, Shiv Nadar University, Dadri 203207, Uttar Pradesh, Indiad Earth and Environmental Sciences, Schmid College of Science and Technology, Chapman University, One University Drive, Orange, CA 92866, USAe Department of Geophysics, Banaras Hindu University, Varanasi 221005, Indiaf Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 June 2014Revised 16 January 2015Accepted 16 January 2015

Keywords:Dust stormsOptical propertiesAERONETCALIPSORadiative forcingIGB

During the pre-monsoon season (April–June), the Indo-Gangetic Basin (IGB) suffers from frequent andintense dust storms originated from the arid and desert regions of southwest Asia (Iran, Afghanistan),Arabia and Thar desert blanketing IGB and Himalayan foothills. The present study examines the columnarand vertical aerosol characteristics and estimates the shortwave (0.25–4.0 lm) aerosol radiative forcing(ARF) and atmospheric heating rates over Kanpur, central IGB, during three intense dust-storm events inthe pre-monsoon season of 2010. MODIS images, meteorological and AERONET observations clearly showthat all the dust storms either originated from the Thar desert or transported over, under favorablemeteorological conditions (low pressure and strong surface winds) affecting nearly the whole IGB andmodifying the aerosol loading and characteristics (Ångström exponent, single scattering albedo, size dis-tribution and refractive index). CALIPSO observations reveal the presence of high-altitude (up to 3–5 km)dust plumes that strongly modify the vertical aerosol profile and are transported over Himalayan foothillswith serious climate implications (atmospheric warming, enhanced melting of glaciers). Shortwave ARFcalculations over Kanpur using SBDART model show large negative forcing values at the surface (�93.27,�101.60 and �66.71 W m�2) during the intense dusty days, associated with planetary (top of atmo-sphere) cooling (�18.16, �40.95, �29.58 W m�2) and significant atmospheric heating (75.11, 60.65,37.13 W m�2), which is translated to average heating rates of 1.57, 1.41 and 0.78 K day�1, respectivelyin the lower atmosphere (below �3.5 km). The ARF estimates are in satisfactory agreement with the AER-ONET ARF retrievals over Kanpur.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Dust aerosols play an important role in the climate system,monsoon and hydrological cycle due to their influence in theEarth-atmosphere radiative budget (Ramanathan et al., 2001;Tegen et al., 2004; Bollasina et al., 2008). The main source ofsuspended dust is the arid and desert areas over the globe(Prospero et al., 2002; Ginoux et al., 2012; Crosbie et al., 2014),while depending on the wind field dust is transported from onecontinent to another (Liu et al., 2012; Nastos, 2012). Middle East,Arabia, southwest Asia, and Thar desert are the main sources for

dust which are transported over the IGB depending upon the inten-sity of westerly winds and meteorological conditions (Dey et al.,2004; Prasad and Singh, 2007; Prasad et al., 2007; Sharma et al.,2012; Gharai et al., 2013; Aher et al., 2014; Kaskaoutis et al., inpress-a), western Himalayan range (Hegde et al., 2007; Guleriaet al., 2011; Srivastava et al., 2011a; Kumar et al., 2014) andbeyond to the far east up to the Everest region and Darjeeling(Duchi et al., 2011; Chatterjee et al., 2012), hundreds to thousandsof kilometers away from the dust source. These dust storms modifythe atmospheric composition, meteorological parameters and aer-osol optical properties, as well as the radiation balance, heatingrates and atmospheric stability over Indian sub-continent(Bhattacharjee et al., 2007; Gautam et al., 2009a, 2010, 2013;Kaskaoutis et al., 2012, 2013).

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Dust storms are common over the north-western parts of Indiaduring the pre-monsoon (April–June) season (Prasad and Singh,2007; Das et al., in press-a), with more than 10 events per year,whose influence is progressively reducing towards the downwindareas (eastern IGB), where only 5 events, on an average, can beconsidered as dust storms as per the criteria of World Meteorolog-ical Organization (visibility < 1000 m) (Middleton, 1986). Except oflocal–regional factors, like soil moisture, surface winds, convection,the dust activity over the Indian sub-continent was found to beassociated with synoptic meteorology and weather systems (Daset al., in press-b; Kaskaoutis et al., in press-b) as well as with theintensity of the monsoonal circulation and rainfall (Rahul et al.,2008; Vinoj et al., in press). Study of dust activity over northernIndia has significant importance during the pre-monsoon season,because of the aerosol mixture with the background pollution lev-els that enhances the aerosol loading (Kumar et al., 2012) and asso-ciated effects in radiative forcing, atmospheric heating, monsooncirculation and rainfall distribution (Gautam et al., 2009b, 2011;Srivastava et al., 2010; Giles et al., 2011; Das et al., 2013; Dumkaet al., 2014). In this respect, Dey et al. (2004) and El-Askary et al.(2006) have focused on certain dust storms in the IGB region, whilePrasad and Singh (2007) studied dust storms during a long-termperiod (2001–2005) using ground and satellite data. Aerosol radia-tive forcing estimates during dust-storm events in the IGB are dis-cussed in Singh et al. (2005), Pandithurai et al. (2008), Sharma et al.(2012) and Singh and Beegum (2013) and over the Gangetic–Hima-layan region by Hegde et al. (2007), Srivastava et al. (2011a),Kumar et al. (2014) among others. The real-time monitoring andforecasting of dust events, the source regions and their pathwaysare important factors in reducing the exposure time and dose ofthese events to the large population (�900 million; Kumar et al.,2012) living in the IGB. The ground-based sunphotometer mea-surements provide accurate optical characterization of the dustaerosols, while satellite observations give extra information aboutthe spatial and vertical distribution of the dust storms along theirpaths (Shao et al., 2011).

In the present study, we have considered three major dust-storm events that occurred during the 2010 pre-monsoon seasonover the IGB (20–21 April, 26–28 May and 2–3 June, 2010) to studytheir influence on aerosol optical properties, vertical profiles, aero-sol radiative forcing (ARF) and heating rates in the atmosphere.NCEP/NCAR reanalysis maps and anomalies are used to examinedifferences in the atmospheric circulation over south Asia associ-ated with these three events. The aerosol properties and ARF areanalyzed over Kanpur, located in central IGB, using AERONET dataand the Santa Barbara DISORT Atmospheric Radiative Transfer(SBDART) model. Moreover, MODIS observations are used to studythe modification in aerosol loading along the transport of dustplume, while CALIPSO profiles provide the vertical distribution ofdust that helps in examining its influence on atmospheric heating.

2. Instrumentation and data analysis

2.1. Ground-based sunphotometry

The aerosol characteristics during the dust-storm events wereobtained from the CIMEL sun/sky radiometer measurements atKanpur AERONET site (26.51� N, 80.23� E, Elev. 123 m). The pro-cessed aerosol data are available in two versions (1 and 2) andthree categories: cloud contaminated (level 1.0), cloud screened(level 1.5) and quality assured (level 2.0) (Smirnov et al., 2000).We have used the AERONET version 2, level 2 data for AOD500,Ångström exponent (a, AE440–870nm) and water vapor (WV) con-tent, and the level 1.5 data for the aerosol size distribution (ASD),single scattering albedo (SSA) and refractive index (RI). The spec-

tral AOD has a great accuracy in the order of �0.02, while higheruncertainties are revealed for SSA, ASD, refractive index, whichare usually below 10–15% for AOD440 > 0.4 (Dubovik and King,2000; Dubovik et al., 2000, 2002). During these three event periods,AOD at Kanpur was much higher than 0.4, thus reducing the uncer-tainties in SSA, refractive index, ASD via the almucantar retrievals.

2.2. Satellite measurements

2.2.1. Terra MODIS observationsIn addition to AERONET data, Level 3 (1� � 1�) Collection 5

C005.1 Terra-MODIS daily observations (MOD08_D3.051, http://modis-atmos.gsfc.nasa.gov/) of AOD550 have been used over theIGB during the three dust events. MODIS has high spatial resolution(250 m at nadir), wide swath (2330 km) and large spectral range(36 channels between 0.412 and 14.2 lm) able to provide contin-uous observations of the Earth’s atmosphere in 1–2 days (Kinget al., 2003). The aerosol properties are retrieved in the wavelengthrange 0.4–2.1 lm using two separate algorithms over ocean (best)and land (corrected). An uncertainty of ±(0.05 ± 0.15 * AOD) inMODIS AOD is expected over land (Levy et al., 2007). In the presentstudy, the MODIS AOD550 values were obtained over IGB (22.5–32.0� N and 68.0–88.0� E) aiming to examine the west-to-eastgradient of aerosol loading during the three dust events in pre-monsoon of 2010.

2.2.2. CALIPSO observationsLidar systems onboard satellites have been increasingly used in

the recent years for monitoring the dust storms, their transport andvertical extent (Liu et al., 2012), since the dust-aerosol vertical pro-files play a crucial role in radiative forcing and atmospheric heatingrate (Lemaitre et al., 2010). The Cloud-Aerosol Lidar and InfraredPathfinder Satellite Observations (CALIPSO) satellite was launchedon April 28, 2006 to study the vertical distributions, optical andphysical properties of aerosols and clouds at 532 and 1064 nm aswell as their impact on the Earth’s radiation budget and climate.Equipped with a depolarization channel at 532 nm, discriminationbetween water and ice clouds is possible, as well as the identifica-tion of the non-spherical aerosols (Winker et al., 2007). CALIPSOlevel 1 data include lidar calibrated and geo-located profiles withassociated browse imagery, while the level 2 includes cloud-layerproducts with horizontal resolution of 1/3 km, 1 km and 5 km, anaerosol layer product at 5 km resolution and an aerosol profileproduct with horizontal resolution of 40 km and vertical resolutionof 120 m. More details of the Cloud-Aerosol Lidar with OrthogonalPolarization (CALIOP) data product can be find elsewhere (http://www-calipso.larc.nasa.gov/products/CALIPSO_DPC_Rev2x4.pdf,Mishra and Shibata, 2012 and references therein). In the presentstudy, we provide images (CALIOP level 2 version 3.01) for verticalprofiles of total attenuated backscatter at 532 nm and aerosol sub-types from both nighttime/daytime observations during thedust-storm days. We have also analyzed numerical values for back-scatter coefficient profiles at two wavelengths (at 532 nm and1064 nm) and two polarization planes (perpendicular and parallel)at 532 nm (60 m vertical resolution), color ratio (CR) and particledepolarization ratio (PDR) in order to compare the profiles betweenthe dusty days and seasonal (pre-monsoon) means. For this scope,we used the CALIOP level 2 version 3.01 data products (cloudscreened) from 1 April 2010 to 30 June 2010, which consist of atotal of 77 daytime and nighttime overpasses over the IGB. Allthe profiles were screened for the artifact signals using the method-ology described in Winker et al. (2013) and Mishra et al. (2014). TheCR is a parameter that determines the size of the particles and isdefined as the ratio of the backscatter coefficient at 1064 nm to thatat 532 nm. The PDR is the ratio of the perpendicular backscattercoefficient to that of parallel backscatter coefficient at 532 nm

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 17

and defines the particle’s shape. In general, higher values of CR(>0.6) and PDR (>0.2) indicates the presence of large and non-spherical particles (mostly desert dust) whereas smaller valuesindicate the abundance of smaller and spherical particles (Omaret al., 2009; Mishra and Shibata, 2012).

2.3. NCEP/NCAR reanalysis and air-mass back trajectories

For the identification of the main atmospheric circulation asso-ciated with the three dust events over IGB, daily composites andanomalies from the mean climatology (1981–2010) of mean sealevel pressure (MSLP), geopotential heights at 850 and 700 hPaand vector winds (850 hPa, only daily composites) were obtainedfrom the National Center for Environmental Prediction/NationalCenter for Atmospheric Research (NCEP/NCAR) Reanalysis project(Kalnay et al., 1996). The data set corresponds to a spatial resolu-tion of 2.5� � 2.5� over the region 45–100� E and 5–45� N. TheMSLP patterns help to examine the influence of the near-surfacepressure systems to the dust emission over the Thar desert andother arid regions in southwest Asia. The 850 and 700 hPa geopo-tential heights have been considered to investigate long-range dusttransport at lower (�1500 m) and mid (�3500 m) troposphere(Gautam et al., 2010). Analysis of meteorological parametersduring the three dust events reveals differences in atmospheric cir-culation patterns that influence the aerosol characteristics andtransport of dust over IGB.

Moreover, 5-day HYSPLIT air-mass back trajectories (Draxlerand Rolph, 2003) were computed at 500, 1000 and 4000 m aboveKanpur during the central days of the three dust events in orderto reveal the dust sources, transport mechanisms and pathwaysand/or possible changes between the air masses during the dustevents. The NCEP/NCAR reanalysis (available 1948–present) mete-orological data were used for the trajectory simulations and for thevertical velocity field.

3. Methodology for ARF estimates

The aerosol radiative forcing (ARF) at the top of the atmosphere(TOA) and at the surface is defined as the difference in the net radi-ation fluxes (down minus up) (solar plus long wave; in W m�2)with and without aerosol at the TOA and surface, respectively. Inthis study, changes in the net radiative flux over Kanpur are com-puted only in the solar spectrum (0.25–4.0 lm) using the SBDARTmodel (Ricchiazzi et al., 1998). The model is based on reliablephysical models of radiation transfer and is being widely used forthe radiative transfer calculations. The model was run at 1-h inter-val for a 24-h period and the daily-averaged ARF values at surfaceand TOA were estimated on days before and during the three dust-storm events during pre-monsoon of 2010 (April 17–24, May 25–30 and May 31–June 4, 2010). The difference between the TOAand surface forcing corresponds to atmospheric forcing (heating)by the aerosol light-absorption (Das and Jayaraman, 2011). Theimportant aerosol input parameters for estimating the ARF viaSBDART include the spectral AOD, SSA and asymmetry factor,which were obtained from the Kanpur AERONET station in orderto attain greater accuracy in the ARF estimations. Other parametersused in the model that affect the radiative forcing are astronomicalfactors (solar geometry, sun elevation, azimuth, etc.), a modeledatmosphere for the vertical profiles of pressure, temperature,humidity and the surface albedo. Based upon the regional climatol-ogy and the prevailing weather conditions (Summer), we selectedthe mid-latitude summer atmospheric profile (Prasad et al., 2007)along with an average integrated column water vapor of1.76 � 10�2 g mm�2 for the first dust storm and 2.92 � 10�2 -g mm�2 for the second and third storms, respectively. Furthermore,

the daily values for the total column ozone concentrations wereobtained from the Ozone Monitoring Instrument (OMI) onboardNASA’s Aura satellite and a mean value was used for each dustevent. The surface albedo value (0.15) was obtained from the AuraOMI version 3 reflectivity data through the Giovanni visualizationonline data system. The overall uncertainty in the estimated radi-ative forcing due to uncertainties in the input parameters isexpected to be in the range of �10–15% (Prasad et al., 2007). Fur-thermore, the atmospheric heating rate can be estimated followingthe formula by Liou (2002) as:

@T@t¼ g

CP

DFAtmos

DPð1Þ

where oT/ot is the heating rate (K day�1), g is the acceleration due togravity, DFAtmos is the atmospheric forcing, CP is the specific heatcapacity of air at constant pressure and DP is the atmospheric pres-sure difference between top and bottom boundary of each layer.Here we have taken DP as 300 hPa for the layer up to �3.5 km (cor-responds to 1000–700 hPa pressure layer).

4. Results and discussion

4.1. Meteorological conditions during the dust storms

Fig. 1 summarizes the NCEP/NCAR meteorology maps duringcertain days of the three dust events, i.e. 20 April, 26 May and 2June 2010. Despite the fact that all the dust storms seem to be orig-inated or even enhanced in severity over the Thar desert, the mete-orology fields during these events seem to have substantialdifferences, thus leading to variations in the dust-erosion regions(Fig. 2a), altitudinal variation of the dust plumes (Figs. 3 and 4),columnar AOD and WV content (Figs. 5 and 9), optical–physicalproperties of aerosols (Figs. 6–8), radiative impact and atmosphericheating rates (Fig. 10).

The intense dust event on 20 April, which led to significant dustemissions over NW India and strong influence in the Himalayanfoothills (Kumar et al., 2014), is associated with a deep(�1000 hPa) low-pressure system centered over NW India andPakistan (Fig. 1a), which seems to be expanded covering nearlythe whole north-central India, Pakistan, northern Arabian Seaand SE Arabia on 26 May 2010 (Fig. 1b). In contrast, the thermallow seems to be weakened and less expanded over northern Indiaand Pakistan on 2 June, while it is enhanced over Arabian Peninsula(Fig. 1c). These atmospheric circulation patterns were found to beassociated with numerous aerosol episodes over IGB (Kaskaoutiset al., in press-a) due to favorable meteorological conditionsresponsible for the transport of dust from west to east over theIGB. The anomaly in MSLP from the mean climatology (1981–2010) seems to be similar for the events of 20 April (Fig. 1d) and26 May (Fig. 1e) suggesting a deeper, than usual, surface low (�6to �10 hPa) over NW India and Pakistan. This negative anomalycombined with the positive one over the Caspian Sea enhancesthe pressure gradient and, as a consequence, the Levar wind blow-ing in eastern Iran-Afghanistan-Pakistan borders (Rashki et al.,2013; Kaskaoutis et al., in press-b). The end result is additionaldust emission over southern Iran–Pakistan, which contributes tothe dust erosion from the Thar desert and, therefore, constitutesan additional source for dust plumes over the IGB. Dust emissionsin southeastern Iran and along the Makran mountains that contrib-ute to the dust episode on 20 May were simulated via WRF-Chemmodel giving support to the present discussions (Kumar et al.,2014). In contrast, 2 June is characterized by much lower andrather positive pressure anomalies over Indian sub-continent andadjoining oceanic regions (Fig. 1f) that do not favor dust emissionsand transportation from SW Asia.

Fig. 1. NCEP/NCAR reanalysis composite and anomaly from mean climatology (1981–2010) maps for mean sea level pressure, geopotential heights at 850 and 700 hPa andvector wind (850 hPa) during the three dust events.

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The 850 hPa geopotential height (GH) fields (Fig. 1g–i) andanomalies (Fig. 1j–l) present large similarities with those of MSLPfor the three cases examined leading to intense winds (8–13 m s�1) over NW India and southern Pakistan (Fig. 1s–u), thusfavoring the dust exposure. The surface winds (not shown) are ofthe same magnitude, being well above the threshold wind velocityfor dust emissions (Kumar et al., 2014). In contrast, on 2 June, the

GHs are much higher over Indian landmass (Fig. 1i), and the rela-tive low GH (�1450) is limited over central Pakistan, while a sec-ondary one is situated over eastern IGB. The combination of thelow GH in SE Arabia with the relative high over the SW coastalIndia (Fig. 1i) enhances the wind speed (14–16 m s�1) over SWand central Arabian Sea (Fig. 1u) thus favoring the moisture trans-port, along with dust from Thar, over IGB (higher amount of WV

Fig. 2. (a) TERRA MODIS images (band combination 1-4-3) showing dust-storm events over IGB on April 20, May 26 and June 2, 2010. The dust appears in pale beige color,while clouds are appeared in white (http://rapidfire.sci.gsfc.nasa.gov); the red arrows show the pathways of the dust plumes and yellow circle represents the location ofKanpur; (b) 5-day air-mass back-trajectories ending at Kanpur during the three dust-storm events; (c) longitudinal variation of the AOD550 via Terra MODIS observations overIGB (22.5–32.0� N, 68.0–88.0� E) during the three dust-storm events in pre-monsoon 2010 (see manuscript for details). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 19

content for this dust episode, Fig. 9). The atmospheric circulation at700 hPa pressure level is the one that strongly differentiates themeteorology fields associated with these dust events. On 20 April,an intense trough is located in central Asia (northern Afghanistan,

Turkmenistan) (Fig. 1m), which is associated with the deep cyclo-nic circulation at the surface (Fig. 1a). The anomaly from the meanclimatology (Fig. 1p) shows that this trough is strong enough andextends further to the south, i.e. in coastal north Arabian Sea. As

Fig. 2 (continued)

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a consequence, the deep depression in surface (Fig. 1a) is mostlyattributed to the mid-troposphere trough (Fig. 1m) and not somuch to the surface heating, as seems to be the case on 26 May.On 26 May, the atmospheric circulation at 700 hPa is characterizedby the Arabian ridge and the eastern Indian trough (Fig. 1n) leadingto a north-westerly flow over the IGB. This atmospheric circulationpattern resembles the common atmospheric situation during duststorms over IGB in late pre-monsoon and monsoon seasons(Kaskaoutis et al., in press-a). In contrast, higher GH (Fig. 1o), asso-ciated with positive anomalies (Fig. 1r) over central-south India arefound on 2 June, similarly to those found at 850 hPa leading to cal-mer winds over the Indian landmass (Fig. 1u).

In synopsis, the dust event on 20 April is characterized by thepresence of a trough at 700 hPa and an associated cyclone in thesurface centered over Thar desert. This deep depression enhancesthe surface winds and is the ignition force for the dust outbreakover Thar desert. Strong winds over SE Iran and southern Pakistanenhance the dust erosion from these regions as well. The dustevent on 26 May is mostly associated with the well-organizedIndian low near the surface and at 850 hPa centered over NW Indiaand extended to central-eastern India, Pakistan and northern Ara-bian Sea. This system induces a westerly/north-westerly flow overthe IGB favoring the dust transport from the neighboring Thar des-ert and, to a much lesser extent, from SW Asia and Arabia. The dustevent on 2 June is also associated with the thermal low over Thar;however, the intensity of the SW flow over the Arabian Sea associ-ated with the Indian monsoon brings humid air masses over Indianlandmass causing a mixture of dust, marine and anthropogenicaerosols.

4.2. Satellite monitoring of the dust storms over IGB

During the pre-monsoon season of 2010, three major dust-storm events were detected over western and central IGB fromthe MODIS Rapid Response website (http://earthdata.nasa.gov/

data/near-real-time-data/rapid-response) and NASA Earth Obser-vatory tool (http://earthobservatory.nasa.gov/NaturalHazards/).Real-time (�13:30 LST) Aqua MODIS images centered over westernIGB on certain days of the three dust events are shown in Fig. 2a.MODIS observations detect thick dust plumes (pale color) coveringmainly the Thar desert (source of these events), which are initiallymoving to northeast and then towards southeast, along the mainaxis of the Ganges valley following a clockwise movement withprogressively attenuation in the plume thickness. The movementof the dust plumes is strongly influenced by the regional meteorol-ogy and local winds (see Fig. 1), while the initial northeastwardtransport seems to affect the southern slopes of the western Hima-layas depositing dust particles over the glaciers (Gautam et al.,2013). The dust outbreaks are intense enough, especially on 20April, obscuring the surface over north-western IGB, while as mov-ing eastwards they are spread causing hazy conditions over theregion with limiting visibility. The influence of the local and regio-nal meteorology on the intense dust event of 20 April 2010 is alsodiscussed by Kumar et al. (2014), who used a combination of satel-lite retrievals and modelling (WRF-Chem) approach for examiningthe generation, uplift, transport and deposition of dust. Analysis ofthe dust event on 2 June 2010 in view of meteorological conditionsand dust-plume transport was also performed by Gharai et al.(2013) indicating significant scientific interest on examining thedust storms over IGB during the pre-monsoon of 2010. NCEP/NCARreanalysis revealed (Fig. 1) that the strong winds (10–15 m s�1)over the Thar desert are the main reason for the uplift and trans-port of the dust plumes towards central IGB, also supported bythe enhanced advection and boundary-layer height due to hightemperature. The air temperature, humidity, soil moisture, vegeta-tion cover, topography, erodibility, surface winds, turbulence andbuoyancy are crucial parameters for the dust erosion and life cycle(Shao et al., 2011; Calastrini et al., 2012; Cavazos-Guerra and Todd,2012), as was also shown for the dust event on 20 April (Kumaret al., 2014).

Fig. 3. Total attenuated backscatter at 532 nm and aerosol subtypes from CALIPSO (version 3.01 data) nighttime/daytime observations on 20 April 2010 (a), 28 May 2010 (b)and 2 June 2010 (c) emphasizing over IGB. The information for the coordinates of the satellite overpass (longitude and latitude) is given below the figures (x-axis).

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 21

The slight difference in the atmospheric circulation during thethree dust events (Fig. 1) is also depicted at the air-mass back tra-jectories ending at Kanpur (at 500, 1000 and 4000 m) on 20 April,28 May and 3 June 2010 (Fig. 2b). On 20 April, the air masses at allthe three altitudes show the source in far western region, trans-ported over the northern coast of the Arabian Sea and further overThar desert carrying large amount of dust from the arid regionsalong the Makran mountains and the Thar desert, as was also ver-ified via WRF-Chem simulations (Kumar et al., 2014). The airmasses seem to originate (5 days back) from south-eastern ArabianPeninsula (Rub Al Khali and Oman deserts), which have been

recognized as significant sources for long-range transport of dustover India (Prasad et al., 2007). On 28 May, the situation has beena little modified, with the 500-m air mass to originate from north-ern Iran-Afghanistan, while that at 1000 m from the Bay of Bengaland eastern Indian coast thus carrying aerosols of different charac-teristics (mostly anthropogenic mixtures). This may be the reasonfor the more homogeneously-mixed aerosol properties during thisdust event, on which the dust influence seems not to be so intense(see Figs. 6–8). On 3 June, the monsoon circulation and the highwinds over Arabian Sea (Fig. 1) drives the lower air masses reach-ing Kanpur from south-western directions after passing over Thar

Fig. 3 (continued)

Fig. 4. (a) Vertical profiles of the aerosol backscatter coefficient at 532 nm (b532) for seasonal mean (April–June, 2010) and the three dust-storm cases (20 April, 28 May and 2June), (b) scatter plot between average color ratio (CR) and particle depolarization ratio (PDR) in the 1.0–4.0 km altitude range for the seasonal mean and the three dustevents. The error bars represent the standard deviations from the mean. The profiles are averaged over IGB with centre at Kanpur.

22 S. Kumar et al. / Aeolian Research 17 (2015) 15–31

desert, while the 4000 m air mass originates from south-west Asia.The long travelling of the low air masses over the Arabian Seaenhances their moisture content, having as end result this dustevent to be appeared more humid compared to the earlier two(Fig. 8).

Fig. 2c shows the longitudinal variations of the AOD550 via TerraMODIS observations over the IGB region during the three majordust storms of pre-monsoon 2010. The lines express the longitudi-nal variation on selected days averaged over the latitudes 22.5–32.0� N, excluding the pixels over Himalayan range and Tibetanplateau. The results show significant spatial and temporal variation(range of �0.10–1.90) in the aerosol load over IGB during the dust-storm events, which amplifies on 2–4 June. However, a significantgradient in AOD from west to east, i.e. along the Ganges valleywhere the dust is mostly transported, is not so apparent, as inthe case of transported smoke plumes from agricultural burning

in the post-monsoon season of 2012 (Kaskaoutis et al., 2014).Therefore, the transport of the dust plumes over the region seemsto be somewhat more expanded covering larger areas withoutrevealing a strong sign of decreasing trend towards the east. Stron-ger winds during pre-monsoon and thermal lows developed overthe hot Indian landmass play an important role in the transportof the dust and its dilution over expanded areas. Moreover, thelarge accumulation of anthropogenic aerosols even during thepre-monsoon season (Srivastava et al., 2011b), degrade the dustcharacteristics and influence, resulting in a rather mixture of dustwith anthropogenic emissions (Giles et al., 2011). However, in gen-eral, on certain dust-storm days (e.g. 20 April 2010, 2 June 2010)the AOD is higher over the western IGB region (i.e. 70–72� E), whilethe high AODs are shifted to eastern parts (75–80� E) on the nextdays, indicating the transport of dust plumes along the IGB. Thisis more pronounced on the second dust event, where the high

Fig. 5. Daily (all point) variation of AOD500 and Ångström exponent (440–870) at Kanpur AERONET site during the three dust-storm events.

Fig. 6. Spectral variation of the single scattering albedo (SSA) for the days around the three dust events based on Kanpur AERONET retrievals.

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 23

AODs are detected over central-eastern IGB (78–85� E) on 29 May,i.e. 1–2 days after the dust outbreak from the Thar desert (see thehigh AODs on 27–28 May over Thar [72–74� E]). MODIS retrievals(Fig. 2c) show that the highest AODs are mostly detected over Kan-pur (80� E) 1–2 days later than the dust-storm exposure from theThar desert depending on the prevailing meteorology.

CALIOP nighttime and daytime observations are used to mon-itor the vertical distribution of the dust plumes over IGB duringthe dust events (20 April, 28 May and 2 June 2010) (Fig. 3a–c,respectively). During all days, the cloud presence is limited overIGB, so the dust plumes are clearly defined. The CALIOP back-scatter profiles show that the dust aerosols over IGB can reach

up to 4–5 km, and are intense enough (values of backscattercoefficient in the range of 0.04–0.06 over specific regions) tocause significant warming of the mid troposphere and modulatethe regional climate (Gautam et al., 2009a, 2010). CALIOP pro-files show a tendency of the dust layers to uplift over Himalayanslopes contributing to the elevated heat pump (Lau et al., 2006;Nigam and Bollasina, 2010; Wonsick et al., 2014) and loweringof ice albedo that enhances melting of glaciers (Gautam et al.,2013). The aerosol subtype images reveal a clear dominance ofdust (category 2, in yellow), while there is also mixing withanthropogenic aerosols modifying the aerosol subtype as ‘‘pol-luted dust’’.

Fig. 7. Aerosol volume size distribution for 22 size bins between 0.05 and 15 lm at Kanpur AERONET site during days around the three dust-storm events.

24 S. Kumar et al. / Aeolian Research 17 (2015) 15–31

Fig. 4a shows the vertical profiles of the backscatter coefficientduring three dust-storm days along with the mean (April–June2010) profile over the IGB region. A significant increase in the back-scatter signal is shown during the dust-storm days, which ismostly defined in the 1–3.5 km altitude range. The scatter plotbetween the spatially (average over IGB region with centre as Kan-pur) and vertically (1–4 km) averaged values of color ratio (CR) andparticle depolarization ratio (PDR) (Fig. 4b) reveals an increase inboth parameters during the dust events with respect to seasonalmean. Liu et al. (2008) suggested higher values of PDR and CR fordust-laden vertical aerosol profiles, while the range of these valuesremains high due to differences in the intensity, thickness of thedust plume, size, shape and mineralogy of the particles, sourceregion, ageing and mixing processes in the atmosphere (Baliset al., 2004). Liu et al. (2011) found similar results in dust verticalprofiles and mixing with anthropogenic aerosols, over the Yangtzedelta region, China during intense dust storms observed in themonths of March and April 2009.

4.3. Aerosol properties during dust storms over Kanpur

The aerosol optical and physical properties during the threedust-storm events are analyzed in this section based on KanpurAERONET retrievals. Fig. 5 shows the variation of AOD500 andÅngström exponent (AE, a) at 440–870 nm during the days aroundeach dust event. The average AOD during the pre-dusty days liesbetween 0.5 and 0.8, which rapidly increases (>1.5) on the arrivalof the dust storms. During the most intense dusty days over Kan-pur (20 April, 28 May and 3 June), the daily-averaged values ofAOD500 and AE were 1.38, 1.19, 1.90 and �0.05, 0.07, �0.02,respectively. A sharp decrease in AE to very low values (even neg-ative) is observed during the arrival of the dust storms, suggestingthat it is the best indicator for the dust presence over IGB, since the‘‘background’’ AOD is high due to mixture of various aerosols (Giles

et al., 2011). AE values near zero or slightly negative are character-istic of desert-dust presence when the UV to mid-visible AE (380–500 nm) is larger than the visible to near-infrared (Eck et al., 1999;O’Neill et al., 2001). Table 1 summarizes the aerosol characteristicsfor the days around each dust event. Statistical paired t-test(Prasad and Singh, 2007) shows that both AOD500 and AE valuesduring the dust-storm days are statistically-significant differentfrom those during the non-dusty days (Table 2) and the respectiveseasonal-mean values during pre-monsoon 2010, justifying theresults by Kaskaoutis et al. (2013) of a significant modification inthe aerosol characteristics during dust events over IGB. Similar val-ues of AOD500 and AE were reported during dust events over Delhi(Singh et al., 2005; Pandithurai et al., 2008) and Kanpur (Dey et al.,2004; Prasad and Singh, 2007).

The SSA exhibits an increase with wavelength in all days(Fig. 6), having, in general, larger values and steeper increase dur-ing the peak of the dust events (19–20 April, 27–28 May, 2–3 June,2010). SSA values larger than 0.90, along with increasing trendwith wavelength, are characteristics of dominance of dust aerosolsin the atmosphere (Dubovik et al., 2002). At shorter wavelengths(440 nm) SSA exhibits much lower values due to larger absorptionby dust in the UV and near-UV spectrum. The spectral variation ofSSA exhibits larger differences during the dust events in April andJune, compared to that occurred in the month of May, indicatingmore heterogeneous atmospheres and larger impact of the duststorms in the aerosol background over Kanpur as shown in Fig. 5.The spectral variation of SSA (SSA1020nm � SSA440nm) was found tobe greater than 0.05 for mineral dust over the desert regions(Dubovik et al., 2002). Moreover, Prasad and Singh (2007) foundthe spectral variations of SSA in the range of 0.051–0.073 for dustydays over IGB in 2005. In the present study, the DSSA was found tobe 0.104, 0.073 and 0.09 on 20 April, 28 May and 3 June, 2010,respectively suggesting increasing dominance of mineral dustand mask the effect of anthropogenic aerosols.

Fig. 8. Same as in Figs. 6 and 7, but for the real (left columns) and imaginary (right columns) parts of the refractive index.

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 25

The aerosol size distribution (ASD) reveals significant differ-ences between the dusty and pre dust-storm days with this differ-ence to be detected at the mode of the dV/dlnR and a tendency forshifting towards larger coarse-mode fraction. The ASDs show aclear dominance of coarse-mode aerosols in all the examined days,with the fine-anthropogenic contribution to be negligible (Fig. 7);the latter was also verified from the low to even negative AE values(Fig. 5) that is the usual scenario over the region during the pre-monsoon season (Singh et al., 2004; Prasad and Singh, 2007;Srivastava et al., 2011b).

In the visible spectrum, mineral dust typically shows real valuesof the refractive index [n(k)] in the order of 1.53 ± 0.05 (larger thanthe other aerosol types) and imaginary ones [k(k)] of �0.006–0.008, strongly depending on dust mineralogy and iron fraction(Dubovik et al., 2002). The n(k) shows high values in the range of1.45–1.60 during the dust events (Table 1 and Fig. 8), while k(k)is found to decrease strongly with wavelength, suggesting signifi-cant absorption at 440 nm and low at longer wavelengths, whichis characteristic for the presence of dust (Dubovik et al., 2002).The k(k) values on 2 June are much lower than those in the otherdays of the same event indicating much lower absorption, as wasshown from the higher SSA values (Table 1 and Fig. 6). The differ-ences in the values and spectral dependence of the refractive index

could be due to complex dust mineralogy (various fraction ofHematite) and mixing with anthropogenic pollutants over IGB.Dust particles from Thar are considered as more absorbing in nat-ure than those from Saharan and Middle East deserts (Chinnamet al., 2006), while the mixing of dust with anthropogenic aerosolsand black carbon over IGB further enhances its absorbing nature(Das and Jayaraman, 2011).

Prasad and Singh (2007) found larger WV content over Kanpurduring several dust storms in pre-monsoon 2005, especially whenthe trajectories were traversing the northern Arabian Sea. This is inline with AERONET observations over Bahrain (Smirnov et al.,2002), suggesting that the dust storms favored by air masses pass-ing over oceanic regions enhance the WV over landmass. In thepresent cases, the WV increases during the first dust event, whileit varies from �25 to �35 mm during the 2nd and 3rd dust events,exhibiting slightly higher values on the intense dusty days (Fig. 9a).In general, the WV exhibits a covariance with AOD500, suggestingenhancement in air-mass humidity during the dust storms, but thiseffect is not as intense as in the cases examined by Prasad andSingh (2007). However, this covariance may be partly attributedto a possible increase in AOD (scattering efficiency of the dust par-ticles) through water uptake under a more humid environment,although this is not so favored during the pre-monsoon season

Fig. 9. (a) Daily-mean variation of AOD500 and water vapor content obtained from Kanpur AERONET during days around the three dust storms in pre-monsoon 2010; (b)longitudinal variation of the water vapor via Terra MODIS observations over IGB (22.5–32.0� N, 68.0–88.0� E) during the three dust storms of pre-monsoon 2010. The lineexpresses the averaged longitudinal variation during each day.

26 S. Kumar et al. / Aeolian Research 17 (2015) 15–31

due to low RH (30–40%) and to the mostly hydrophobic nature ofdust (Titos et al., 2014). The higher WV in June is attributed toshifting of air masses to south-westerly directions (southern Ara-bian Sea and Indian Ocean, Fig. 2b), carrying humid marine airmasses over Indian landmass. The enhanced levels of WV contentduring dust storms and their effects on radiative forcing have beendiscussed by Kim et al. (2004a,b) and Yoon et al. (2006), suggestingthat the difference in humidity during the dust storms over IGB islikely to affect the radiative forcing rates. The longitudinal varia-tions of the WV via Terra MODIS observations averaged over IGB(22.5–32.0� N, 68.0–88.0� E) during the three dust events(Fig. 9b) show, in general, a progressively decreasing trend towardseast suggesting more humid air masses in the western parts com-ing from the Arabian Sea during the dust-storm days. This longitu-dinal variation is more pronounced during the 1st and the 3rd dustevents when the air masses have a longer passage over the ArabianSea (Fig. 2b).

4.4. Aerosol radiative forcing during major dust storms

The computed ARF values at surface, TOA and within the atmo-sphere during the three dust events are shown in Fig. 10a. The ARFvalues were found to be in the range of �31.85 to �101.60 W m�2

(average �65.77 ± 21.72 W m�2) at surface, �3.94 to�40.95 W m�2 (average �17.99 ± 10.46 W m�2) at TOA and+16.87 to +75.11 W m�2 (average +47.78 ± 16.24 W m�2) withinthe atmosphere, respectively. However, during 20 April, 28 Mayand 2 June, the ARF values were more extreme (�18.16, �40.95and �29.58 W m�2 at TOA, �93.27, �101.60 and �66.71 W m�2

at surface and +75.11, +60.65 and +37.13 W m�2 within the atmo-sphere), while in the non-dusty days, the corresponding values arelower, especially those referring to the surface forcing (Fig. 10a).This indicates that due to enhancement in dust aerosol, the surfaceforcing becomes larger, while the atmospheric heating is lessaffected, suggesting that the AOD plays the major role in ARF

Fig. 10. (a) Aerosol radiative forcing values (surface, top of the atmosphere and within the atmosphere, in W m�2) during the three major dust-storm events of pre-monsoon2010; (b) correlation between AOD500 and ARF values at surface, TOA and atmosphere during the dusty days in pre-monsoon of 2010.

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 27

estimates during the dust events. In this respect, a strong correla-tion between AOD (at 500 nm) and ARF is observed (Fig. 10b) atsurface (R2 = 0.85), whereas a slight lesser correlation (R2 = 0.74)is found at TOA, which becomes relatively low (R2 = 0.46) in theatmosphere. The latter suggests that the atmospheric forcing ismore sensitive with changes in SSA and refractive index controllingthe absorbing capability of the dust particles. The slope of theregression line (Fig. 10b) gives the aerosol radiative forcing effi-ciency (ARF per unit AOD) of about �44.15, �19.86 and+24.29 W m�2 at surface, TOA and atmosphere, respectively, whichare similar to those reported by Sharma et al. (2012) during intensedust storms in north-western India. The error in estimating theaerosol radiative forcing efficiency at surface was found to beabout 6%, while it is in the order of 14% for TOA due to higher scat-tering of the data points.

The large differences between TOA and surface forcing (Fig. 10a)demonstrate that the solar radiation is being strongly absorbedwithin the atmosphere, contributing to its heating (Prasad et al.,2007; Srivastava et al., 2011b; Khan et al., 2012, 2014) and tochanges in atmospheric stability and dynamics. The atmosphericheating rates were also calculated via Eq. (1) for the days shownin Fig. 10a, giving an average of 1.33 K day�1. The heating ratesare found to be much more intense on 20 April (2.11 K day�1), 28May (1.70 K day�1) and 2 June 2010 (1.04 K day�1) indicating seri-ous climate implications.

Sharma et al. (2012) computed ARF over Patiala, north-westernparts of India during the dust storms of 20–21 April 2010 and 26–28 May 2010 reporting ARF values of �100 to �50 W m�2 at sur-face and �25 to �10 W m�2 at TOA. The aerosol radiative forcingefficiency was found to be �66 W m�2 at surface and �14 W m�2

at TOA. The computed ARF values were larger than those at Kanpurindicating higher intensity of the dust storms in the western IGBdue to proximity to dust source regions. Chinnam et al. (2006) havereported ARF values in the order of �12 W m�2 (surface) and+7 W m�2 (TOA) during a dust-storm event (May 12–22, 2004)over Kanpur, which are lower at surface and of opposite sign atTOA compared to the present findings. Furthermore, Prasad et al.(2007) reported ARF values ranging between �19 and�87 W m�2 at surface and between +2 and�26 W m�2 at TOA dur-ing the period April–May 2005, which are comparable in magni-tude to the ARF values reported here. Pandithurai et al. (2008)found surface ARF between �39 W m�2 (in March) and�99 W m�2 (in June), and an atmospheric forcing between+27 W m�2 (in March) and +123 W m�2 (in June) over New Delhiduring 2006 pre-monsoon season.

Due to significant importance of dust aerosol over the Indo-Gangetic–Himalayan region, ARF calculations have been reportedat several Himalayan sites during major dust storms. In thisrespect, Srivastava et al. (2011a) reported ARF values of �45,�30, and +15W m�2 at surface, TOA and atmosphere, respectively

Table 1The aerosol characteristics for the days around each dust-storm event at Kanpur AERONET station in the pre-monsoon season of 2010.

Dust event (Kanpur) Dates (2010) Aerosol parameters WV(mm)

AOD500 AE440-870 SSA675 RI675 (Real) RI675 (Im.)

First dust event 17/4 0.785 0.279 0.860 1.596 0.00834 17.6418/4 0.437 0.275 0.878 1.600 0.00505 16.6719/4 0.969 0.042 0.913 1.600 0.00292 18.8420/4 1.380 -0.049 0.919 1.598 0.00468 19.3821/4 1.186 -0.029 0.942 1.592 0.00186 18.7922/4 1.408 -0.022 0.941 1.600 0.00198 18.7223/4 0.754 0.089 0.903 1.600 0.00384 15.3524/4 0.469 0.329 0.810 1.600 0.01331 15.40

Second dust event 25/5 0.624 0.509 0.892 1.569 0.00745 26.7226/5 1.117 0.231 0.910 1.594 0.00473 30.0127/5 1.240 0.234 0.911 1.575 0.00515 35.5128/5 1.192 0.071 0.904 1.569 0.00644 30.3629/5 1.790 0.099 0.945 1.580 0.00335 34.1430/5 1.024 0.307 0.899 1.564 0.00742 35.1931/5 0.517 0.317 0.885 1.552 0.00694 20.06

Third dust event 1/6 0.781 0.129 0.901 1.551 0.00652 18.642/6 1.310 0.023 0.969 1.547 0.00112 23.203/6 1.894 -0.017 0.935 1.536 0.00562 34.194/6 0.545 0.064 0.959 1.548 0.00206 36.67

Table 2Paired t-test for Kanpur AERONET AOD500 during non-dusty and dusty days in pre-monsoon of 2010.a,b

Pair Name N Mean Std. Dev. SEM

AOD (Non-Dusty Days) 21 0.520 0.117 0.026AOD (Dusty Days) 21 1.250 0.098 0.021Difference �0.730

a Output: t = �18.723 with degree of freedom 20 and (p 6 0.001). The confidenceinterval for difference of means at 95% confidence is: –0.812 to –0.649.

b Result: Statistically significant change (p 6 0.001) at alpha = 0.05, suggestingthat the change in AOD is due to the dust storm not by chance.

28 S. Kumar et al. / Aeolian Research 17 (2015) 15–31

and heating rate of �0.4 K/day during dusty days over Nainital.During the pre-monsoon seasons of 2008 and 2009, Dumka et al.(2014) calculated ARF values at Nainital in the order of �45.7,�7.6 and 38.1 W m�2 at surface, TOA and atmosphere, respec-tively, with heating rate of 1.07 K/day. The radiative perturbationdue to dust aerosols averaged over the region (25–30� N and 70–80� E) were �8.0 ± 3.3 W m�2 at surface, �2.9 ± 3.1 W m�2 atTOA and 5.1 ± 3.3 W m�2 in the atmosphere (Kumar et al., 2014),with severe instantaneous surface cooling of �227 W m�2 andlarge �70 W m�2 instantaneous ARF at TOA during the dust eventof 20 April 2010. Table 3 summarizes the ARF values and

Table 3Comparison of aerosol radiative forcing and atmospheric heating rates between the presseason.

Locations Pre-monsoon (dust period) TOA (W m�2) Surface (W m�2)

Kanpur April–June, 2010 �17.99 �65.77Kanpur April, 2010 �10.40 �19Kanpur April–June, 2009 �10.60 �29Kanpur April–June, 2005 �11 �23Kanpur May, 2004 +7 �12G College April–June, 2009 �13.27 �31.03Nainital Pre-monsoon, 2008–09 �7.61 �45.75Nainital June, 2006 �30 �45Patiala April, 2010 �14.80 �45.10Patiala April–May, 2010 �16.4 to �22.6 �75.9 to �102.6Delhi April, 2010 �18.70 �37.50Delhi March, 2012 NA �68 to �86Delhi March–June, 2006 �12 to +24 �39 to �99

atmospheric heating rates of several studies computed over IGBand Indo-Gangetic–Himalayan region during the pre-monsoonseason. The results show that the present findings are within pre-vious calculations of ARF during major dust storms over the north-ern parts of India. In general, the radiative forcing over IGB issomewhat larger than those found over Himalayan foothills, evenfor the same dust storms, due to larger contribution of anthropo-genic aerosols mixed with dust over IGB locations (Giles et al.,2011) and to relatively lower aerosol loading over the elevatedHimalayan sites (attenuation of dust plume above 1.5–2 km).

Fig. 11 shows the correlations between the ARF values (surfaceand TOA) as obtained via AERONET retrievals and SBDART esti-mates over Kanpur during the dusty days (Fig. 10a). It should benoted that the ARF estimates via AERONET correspond to simulta-neous ARF values at the time of the sun photometer observations(upper panels), while the SBDART estimates correspond to daily-mean values (24-h average). Therefore, for a direct comparisonbetween them, the AERONET ARF values have been re-consideredfor the whole day, supposing daytime duration of 12 h with avail-ability of sun photometer observations. Thus, the initial AERONETARF values have been divided by two in order to correspond tothe whole day (lower panels). At this case, the AERONET andSBDART ARF values lie upon the 1–1 line, also exhibiting high cor-relation with R2 = 0.89 (surface) and R2 = 0.78 (TOA), justifying therobustness of the SBDART estimates and the methodology used,

ent and previous studies over Gangetic–Himalayan region during the pre-monsoon

Atmosphere (W m�2) Heating Rate (K day�1) Reference

+47.78 1.33 Present study+8.60 NA Kumar et al. (2014)+18.40 0.55 Srivastava et al. (2011a)+12 NA Prasad et al. (2007)+19 1.02 Chinnam et al. (2006)+17.76 0.52 Srivastava et al. (2011a)+38.14 1.07 Dumka et al. (2014)+15 0.40 Srivastava et al. (2011b)+30.30 NA Kumar et al. (2014)+59.5 to +80.0 NA Sharma et al. (2012)+18.80 NA Kumar et al. (2014)NA NA Singh and Beegum (2013)+27 to +123 0.6–2.5 Pandithurai et al. (2008)

Fig. 11. Correlation between the ARF values at surface and TOA from Kanpur AERONET retrievals and SBDART estimates. The lower panels correspond to daily-meanAERONET ARF values obtained by dividing the initial values by two under the assumption of 12-h daytime duration.

S. Kumar et al. / Aeolian Research 17 (2015) 15–31 29

despite of the uncertainties in the input parameters and theassumptions in the correlations.

5. Conclusions

This paper focused on examining three major dust-storm eventsthat occurred over northern India during the pre-monsoon seasonof 2010 via the synergy of ground-based measurements, satelliteobservations and model estimates. More specifically, the NCEP/NCAR reanalysis was used to examine the specific atmospheric cir-culations that are associated or even favored the dust exposures,AERONET data over Kanpur were used to examine the aerosoloptical and physical properties during the dust events, MODISobservations were used for monitoring of the spatial distributionof the dust plumes, CALIPSO profiles have been used for verticaldistribution of dust and SBDART model for shortwave ARF estima-tions. The main findings are summarized as:

(1) During all dust events, the MSLP was low over north-wes-tern India and Pakistan, associated with strong (10–15 m s�1) winds at surface and in the lower tropospherefavoring dust exposure over Thar desert and surroundingarid regions in southwest Asia. The prevailing north-wes-terly winds disperse the dust plumes towards east, alongthe main axis of the Ganges basin, as well as up to Himala-yan foothills. The composite means and the anomalies fromthe mean climatology of MSLP and geopotential heights at850 and 700 hPa indicated some differences in the atmo-spheric circulation during the three dust events, which weredepicted at the air-mass trajectory pathways.

(2) The MODIS true color images identified the Thar desert asthe main source for the three dust events. Strong surfacewinds and enhanced convection due to high temperatureover the Thar desert, favored the erosion and uplift of signif-icant amount of dust, thus creating intense dust plumes,which initially affected the southern slopes of westernHimalayas before transporting over central and easternIGB. The three dust storms presented a vertical extent upto 5 km over IGB, as CALIPSO observations shown, thus cre-ating a dense aerosol plume able to warm the lower and midtroposphere before the monsoon onset. The aerosol vertical

profiles during the dust events showed a considerableincrease in the backscatter coefficient between 1 and 4 kmcompared to the mean profiles during pre-monsoon of2010. Furthermore, the scatter plot of area-averaged CR vs.PDR showed an increase in both CR and PDR values duringthe dust events with respect to seasonal means for the dustaffected altitudes.

(3) The aerosol physical and optical properties over Kanpurshowed pronounced modification during the dusty dayscompared to days prior to the dust-storm events. TheAOD500 exhibited large increase (>1.1) during the dusty daysassociated with considerable decrease (sometimes negativevalues) in Ångström exponent (440–870 nm) indicatinglarge fraction of coarse-mode particles that was supportedfrom the aerosol size distribution. Significant changes havebeen also found in the spectral distribution of SSA and RIduring the dust-storm days. The SSA showed high values(>0.85) and increasing trend with the wavelength duringthe dust events supported by the high values of real part(1.5–1.6) and decrease in the imaginary part (<0.006) ofthe refractive index, which are typical of desert-dust aero-sols. In general, slight higher water vapor content was foundto be associated with the dust storms over IGB mostlyfavored by the western air masses dominating during thedust storms which bring moisture from the Arabian Sea overthe mainland.

(4) The ARF at surface, TOA and within the atmosphere wasfound to be, on average, �65.77 ± 21.72 W m�2,�17.99 ± 10.46 W m�2 and +47.78 ± 16.24 W m�2, respec-tively, during the three dusty periods, with peaks reachingto �40.95 W m�2, �101.60 W m�2, +75.11 W m�2, respec-tively on certain days with accumulation of dust. A strongcorrelation has been observed between AOD and ARF values,especially at surface (R2 = 0.85) suggesting that the AOD isthe main parameter controlling the ARF during the duststorms over IGB. The correlation between AERONET andSBDART ARF, also exhibiting high correlation with R2 = 0.89(surface) and R2 = 0.78 (TOA), justifying the robustness ofthe SBDART estimates and the methodology used, despiteof the uncertainties in the input parameters and theassumptions in the correlations. The atmospheric heating

30 S. Kumar et al. / Aeolian Research 17 (2015) 15–31

corresponded to an average heating rate of 1.33 K day�1,which was found to be much more intense (2.11, 1.70 and1.04 K day�1) on certain days with more dust influence. Thissuggests significant climate implications over Gangetic–Himalayan region, which may affect the atmospheric ther-mal gradients, monsoon circulation and hydrological cycle.

Acknowledgements

We are thankful to MODIS (http://modis.gsfc.nasa.gov) andCALIPSO scientific teams as well as NASA Langley Research Center(http://eosweb.larc.nasa.gov) for providing the satellite data andthe Giovanni Online Visualization System. Kanpur AERONET sta-tion was started by one of the authors (RPS) after a joint MOUbetween NASA and IIT Kanpur. Efforts of RPS (PI) and Brent Holben(PI from NASA) and present team members are thankfullyacknowledged. The authors gratefully acknowledge the NOAA AirResources Laboratory (ARL) for the provision of the HYSPLIT trans-port and dispersion model and/or READY website (http://www.ready.noaa.gov) used in this publication. SK and AKS are thankfulto Prof. Ram Pal Singh for his suggestions and discussion duringthe preparation of the manuscript. SK is thankful to A.S. Bhardwajfor his help during SBDART calculation and to UGC, New Delhi,India for providing Rajiv Gandhi National Fellowship as a SRF.The work is partially supported by ISRO, Bangalore, India underISRO-SSPS program to BHU, main coordinator – AKS. We are grate-ful to the Associate Editor and two Reviewers for their valuablecomments and suggestions which helped us to improve the earlierversion of the manuscript.

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