The South Eastern Arabian Sea (SEAS) along the west coast of India often shows intense aerosol build-up during the pre-monsoon season, marked by brief Aerosol Optical Depth (AOD) surges. As such events are found crucial to regional weather, this study examines the causes and effects of these events, integrating data from MODIS, CALIPSO, and INSAT-3D satellites, MERRA-2 reanalysis, and back-trajectory analysis for the period 2015–2017. High AOD events over the SEAS are linked to elevated aerosol layers between 1 and 4 km, mainly polluted dust and smoke, with pure dust in most of the cases. These events are found to be fuelled by pronounced shifts in wind circulation, increased convergence, strong updrafts, and higher humidity above 1.5 km, aiding aerosol transport from upwind biomass-burning and industrial regions. Absorbing aerosols like black carbon (BC) and organic carbon (OC) strongly correlate with sulphates (SU), suggesting common origin and mixed aerosol type, pointing towards higher absorption effects. This together with the amplifying influence of underlying clouds increases the positive Atmospheric Radiative Forcing (ARFATM) from 4 to 6 W m⁻2 on normal days to 8–12 W m⁻2 during high-pollution events. This increased radiative forcing heats the lower atmosphere (0.3–2.5 km) by about 1–2 K, as seen in temperature profiles, and its changes closely follow the observed temperature variations. These exciting observational results highlight the strong role of absorbing aerosols in high pollution events modifying the atmospheric thermal structure, with probable implications on aerosol–cloud–precipitation interactions and weather patterns along the west coast of India.

Previous studies based on satellite data and model analysis have shown that the dust over Arabian Sea correlates positively with the Indian summer monsoon. During August 2018, over a period of a few days from August 13, the state of Kerala experienced anomalous rainfall, which was followed by heavy floods. In the present study, the meteorological factors and the aerosol conditions during this event are investigated. MERRA 2 reanalysis data suggests that the dust loading over the Arabian Sea during the month of August, during which the anomalous event of rainfall took place, is the highest in the last decade. Using CALIPSO aerosol vertical profiles, OMI Absorbing Aerosol Index and the HYSPLIT back trajectory analysis, higher altitude dust transport from the arid gulf region towards the Arabian sea and near to Kerala coast is confirmed during the heavy precipitation days and even prior to that. Changes in cloud properties like Cloud Fraction (CF), Cloud Top Temperature (CTT), Cloud Water Liquid Water Path (CWLWP) and Cloud Condensation Nuclei (CCN) with AOD variations during the period clearly indicate that there is a close relation between the aerosols and the cloud properties over the region. Observed CWLWP in the study on August 14 and 15, 2018 was 80% higher than the monthly average value. CCN concentration was increased by 23 percent during the severe rainy days of August when the CTT was about 16 K lower than the monthly mean value. Dust aerosols at altitudes of 2-4 km and further higher at 10-15 km, along with the deep convective clouds, point to the possibility of aerosol-cloud-precipitation interaction under conducive meteorological conditions. INSAT- 3D temperature profiles show an interesting enhancement of temperature (~1 K) at altitudes ~ 3 km, where elevated dust layers are noticed. This elevated heating could pave the way for moisture convergence and it is further confirmed from the INSAT derived Relative Humidity profiles. Investigation of vertical winds shows strong updrafts over the region during the period of heavy precipitation, indicative of the coexistence of moisture and CCN at higher altitudes. The study infers that the moisture built up over the region was excessive, enough to overwhelm the commonly observed semi direct effect caused by aerosols, and in contradiction, the aerosols amplify the cloud cover and precipitation over the Kerala region during the period of study. Thus the heavy dust loading over the South Eastern Arabian Sea region, near the west coast of Kerala, together with the conducive meteorological conditions and orography of the region led to intensified precipitation over Kerala during the mid of August 2018.

The study examines trends of scattering, absorption and total aerosol optical depths (SAOD, AAOD and AOD) over India and surrounding oceanic regions and explores role of local production, long-range transport and atmospheric dynamics on observed trends. Long-term satellite observations are used to estimate trends and assess their statistical significance. Significant spatial and seasonal changes are observed in trends of SAOD, AAOD and AOD. AOD is observed to be increasing during post monsoon and winter over most of the land mass and surrounding oceanic regions, whereas decreasing trends over land and increasing trends over oceanic regions are observed in pre-monsoon and summer months. In general, SAOD and AAOD show similar trends (if there is any) as that of AOD over most of the regions in most of the months. Strongest positive trends over land regions are observed in November with trend of AOD greater than 0.01 year−1, especially over Indo-Gangetic Plain (IGP). Increase of AOD over IGP in post monsoon is contributed significantly by absorbing aerosols with rate of increase ~ 0.005 AAOD year−1. AAODs are observed to be increasing over Arabian Sea and Bay of Bengal (BoB) in December also, with rate ~ 0.003 AAOD year−1. Strongest positive trends over Arabian Sea and BoB are observed in June with rate of increase greater than 0.02 AOD year−1, whereas strong negative trends are observed over north-west India in the same period with rate of decrease greater than 0.02 AOD year−1. Over IGP, AOD, AAOD and SAOD show contrasting trends in winter and summer seasons. AAOD exhibits strongest decreasing trend over IGP during April–June. Positive trends of AOD over Arabian Sea and BoB are favoured significantly by changes in circulation dynamics. Atmospheric convergence is observed to be strengthening over these regions in April and June, leading to more accumulation and hence positive trends of AOD. Aerosol transport over to the Arabian Sea is observed to be enhancing and contributing significantly to AOD increase over the Arabian Sea in pre-monsoon and summer months. Enhancement in aerosol transport over to the Arabian Sea is observed in pre-monsoon at higher altitudes above 3 km, whereas it is observed in summer at lower levels. However, decreasing trends of AOD over north-west India and IGP during pre-monsoon and summer are observed to be due to decrease in aerosol transport from the continental regions at the west.

Elevated aerosols assume importance as the diabatic heating due to aerosol absorption is more intense at higher altitudes where the atmosphere becomes thinner. Indian region, especially its central and northern latitudes, experiences significant loading of elevated aerosols during pre-monsoon and summer months. Genesis of elevated aerosol loading over Indian region is investigated in the present study, using multi-year satellite observations from Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS) along with reanalysis winds from MERRA. Central India is observed to have prominent aerosols loading at higher altitudes during pre-monsoon season, whereas it is during summer months over north-west India. Further analysis reveals that the elevated aerosols over Indian region in pre-monsoon and summer months are significantly contributed by transported mineral dust from the arid continental regions at west. In addition to the mineral dust advection, aerosols at higher altitudes over Indian region are enriched by strong convection and associated vertical transport of surface level aerosols. Vertical transport of aerosols observed over Indian region during pre-monsoon and summer months is aided by intense convergence at the surface level and divergence at the upper level. Moreover, aerosol source/sink strength estimated using aerosol flux continuity equation show significant aerosol production over central India during pre-monsoon. Strong vertical transport prevails during pre-monsoon uplifts the locally produced aerosols, with considerable anthropogenic fraction, to higher altitudes where their impacts would be more intense.

The study examines aerosol loading in different vertical layers of the atmosphere and explores the role of atmospheric circulation parameters in vertical distribution of aerosols and in its seasonal variability. Aerosol vertical distribution over the globe is examined, using long term satellite observations, by considering aerosol loading in different layers of atmosphere upto ∼6km altitudes from surface and fractional contribution of each of these layers to total columnar aerosol loading. Aerosols are observed residing close to the surface in most of the oceanic environments, except over certain regions which are in the close proximity of continents where upper level winds are conducive for long range aerosol transport. In contrast, considerable vertical spread in aerosol distribution with strong seasonal variability, minimum occurring in winter months and maximum in summer, is observed over the continental regions. Vertical spread in aerosol distribution is observed highest over north eastern and north western parts of Africa during northern hemispheric summer, when the convection activity peaks over these regions due to large solar insolation and associated surface heating. Seasonal variation of aerosol vertical spread over both of these regions is observed in phase with variation in atmospheric convergence and vorticity. During summer months, when the aerosol vertical spread is highest, strong surface level convergence and associated cyclonic vorticity is observed along with an upper level (700–600hPa) divergence. The surface level convergence and upper level divergence together induce an upward flow of air which carries aerosols from ground to higher altitudes. This mechanism of aerosol vertical transport is further corroborated through the correlation and regression relations of surface convergence/vorticity with aerosol loading above different elevations and hence the study reveals role of circulation parameters in aerosol vertical distribution.

A relationship between ocean surface wind speed and sea salt aerosol production is established through a study carried out using the aerosol flux continuity equation by introducing satellite data on aerosols and ocean surface wind speed into it. This mathematical approach eliminates interferences from background aerosols and aerosol variations due to advection and convergences/divergences in wind field and correctly estimates the rate of sea salt aerosol production by winds. To avoid oceanic biogenic sources and transport from other oceanic and land regions, the study is done over ocean sites substantially remote from the continents with low chlorophyll concentrations (< 0.06 mg/m(3)) and restricting to aerosols in the bottom layer (< 0.5 km altitude) of the atmosphere. The surface wind speed is found to correlate better with the estimated aerosol production rate (R-WS = 0.99, p < 0.0001) than with the bottom layer aerosol optical depth (R-WB=0.97, p < 0.0001). Aerosol production is observed at wind speeds even below 4 m/s and the production rate is found to follow a linear relationship with ocean surface wind speed with a slope 0.0053 and an intercept 0.0163 for low as well as high winds. (C) 2013 Elsevier Ltd. All rights reserved.

Intense aerosol plumes engulf a vast region of the Arabian Sea during the Asian summer monsoon season (ASM: June–September). The largest value of aerosol optical depth (AOD) at 550nm in this region generally occurs in July when the mean AOD attains its annual peak value of >0.8. However, the AOD over this region is abnormally large during the ASM in some years, especially during the June–July period. Long term satellite observations using MODIS reveal that the largest AOD during the 11-year period of March 2000–February 2011 occurred in June 2008 with a regional mean AOD of 1.1, which was ~97% larger than that of the corresponding long term mean value in June and ~49% larger than that in July. The availability of CALIPSO data since June 2006 provides a unique opportunity to quantify the role of continental aerosols transported from the West Asian desert regions in the genesis of the above abnormality over the Arabian Sea. We examine the spatial and vertical distributions of aerosols over the Arabian Sea and adjoining continents using multi-year data from MODIS and CALIPSO and explore the genesis of the above abnormal enhancement in AOD. The observed anomalies in AOD are substantially larger than that can be attributed to changes in wind-generated sea salt aerosols. The CALIPSO observations show that the anomalous enhancement in aerosol loading over the Arabian Sea during June 2008 was primarily caused by an enhancement in aerosol abundance in the altitude range of ~1–4km with a distinctly large volume depolarization ratio of >0.25, clearly indicating the dominance of highly non-spherical mineral dust. Although the aerosol loading over the Arabian Sea during the ASM is observed to be mainly caused by the mineral dust transported from the West Asian Deserts at northwest of the Arabian Sea, the abnormal enhancement in the observed AOD during June 2008 was primarily caused by a distinct increase in dust storms over the northern continents and subsequent transport into the Arabian Sea.

This research article aims at characterization of the sea-breeze circulation over Thumba (8.5°N, 76.9°E, India) in the winter season from December 2008 to February 2009, when this mesoscale circulation over the study domain was prominent. The characteristics of sea-breeze circulation cell comprising sea-breeze and compensatory return flow are investigated for clear-sky and cloudy days. The study indicated delayed onset of sea-breeze on the cloudy days as compared to the clear-sky days and the vertical thickness of sea-breeze circulation cell was found to be larger for the clear-sky days. Vertical thickness of the return flow for both clear-sky and cloudy days was larger than that of the sea-breeze flow. Simultaneous observations of upper-air meteorological parameters obtained through balloon-borne GPS sonde ascents carried out from Thumba and its adjoining coastal ocean on 29 January 2009 as part of the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) are utilized for a case study towards investigation of diurnal evolution of the sea-breeze circulation cell. Results obtained from this study indicated systematic evolution of sea-breeze circulation over Thumba, however, it was not very clear over ocean which is attributed to cloudy conditions that prevailed on the day of measurement.

Spatial and temporal variabilities in the vertical structure of Marine Atmospheric Boundary Layer (MABL) over the Bay of Bengal (BoB) are investigated through a ship-borne field experiment measurements pertaining to three different classes, namely: night, morning and afternoon conditions. High-resolution vertical profiles of meteorological parameters obtained through balloon-borne GPS Sondes during the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) formed the primary database for the present investigation. The study advocates usage of wind shear profiles in association with virtual potential temperature (θv) and specific humidity (q) profiles for determination of the mixed layer heights (MLH). The mean values of turbulent flow thickness (TFT) obtained from the vertical profiles of Bulk Richardson Number (RiB) and MLH magnitudes for the entire cruise did not show any appreciable variations for three classes. During the entire cruise period, the MLH varied in a range from 450m to 1500m with a mean of about 900m, whereas the TFT variations were confined between 125m and 1475m with a mean of about 581m. The statistical means of TFT and MLH were similar for nighttime profiles, whereas they showed significant differences in the morning and afternoon conditions. Spatio-temporal variability in the MLH showed good correlation with the surface-layer sensible heat flux which is one of the driving mechanisms in mixing processes.

As part of the winter phase of the Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB), different meteorological parameters were collected through an Automatic Weather Station mounted onboard Oceanic Research Vessel Sagar Kanya for a period of 35days spanning from 26th December 2008 to 29th January 2009 over the Bay of Bengal (BoB). The objectives of this research article are two folded: first – we make use of W-ICARB meteorological database for investigation of the diurnal variability in the air–sea interaction parameters by studying the frequency distribution of the time of occurrence of minima and maxima and second – we report the significance of North–South latitudinal gradient in meteorological parameters observed during the cruise period. A careful statistical analysis revealed a significant semi-diurnal variability in the mean sea level pressure with two minima occurring consecutively at 3LT and 15LT, whereas the two maxima were observed at 9LT and 21LT respectively. With the available database, we report a new variability index representing a quantitative measure of the diurnal variability in the air–sea interaction parameters. Analysis of the W-ICARB database indicates the diurnal variability in mean sea level pressure, wind speed, sensible and latent heat flux larger than 50%, while it was ranging from 28% to 40% for the sea surface temperature, air temperature, humidity and momentum flux. On an average, the estimates of sensible and latent heat flux were marginally higher than the earlier field experiment conducted over the BoB. During the cruise period, most of the surface layer meteorological observations barring wind speed showed a significant North–South latitudinal gradient which was not reflected in the estimates of air–sea interface fluxes.

On 15 January 2010, Thiruvananthapuram in India (8.5°N, 76.9°E) witnessed one of the longest possible noontime annular solar eclipses spanning a period of about 7 min centred at 1314 local time. Here, we present a case study on the behaviour of the atmospheric surface layer by comparing the eclipse-induced observations with similar measurements recorded on cloud-free/clear-sky days. During the peak period of the eclipse, the incoming solar irradiance was reduced by 87% of its normal values, resulting in an air-temperature decrease near the surface of 1.2°C in association with a significant reduction in turbulent kinetic energy, momentum flux and sensible heat flux. The rate of instantaneous decay in solar radiation and sensible heat flux from the first contact of the eclipse to its annularity was greater than that seen during normal evening hours.

Altitude profiles of aerosol black carbon (BC) in the atmospheric boundary layer (ABL) over a tropical coastal station, Trivandrum have been examined on two days using an aethalometer attached to a tethered balloon. One of these days (15th January, 2010) coincided with a (annular) solar eclipse, the longest of this century at this location, commenced at 11:05 local time and ended by 15:05, lasting for 7 min and 15 s (from 13:10:42), with its maximum contact occurring at ~ 13:14 IST with ~ 92% annularity, thereby providing an opportunity to understand the eclipse induced perturbations. Concurrent measurements of the ABL parameters such as air temperature, relative humidity and pressure were also made on these days to describe the response of the ABL to the eclipse. BC profiles, in general, depicted similar features up to an altitude of ~ 200 m on the eclipse day and control day, above which it differed conspicuously with profiles on eclipse day showing increasingly lower concentration as we moved to higher altitudes. Examination of the meteorological profiles showed that the altitude of maximum convection rapidly fell down during the eclipse period compared to that on control day indicating a rather shallow convection on eclipse day. Comparison of diurnal variations of BC at the surface level showed that the rate of decrease in BC during daytime on the eclipse day was smaller than that on the control day due to the reduced convection, shallow ABL and consequent reduction in the ventilation coefficient. Moreover the time of the nocturnal increase has advanced by ~ 1:30 h on the eclipse day, occurred at around 19:30 IST in contrast to all the other days of January 2010, where this increase usually occur well after 20:30 IST, with a mean value of 21:00 IST. This is attributed to the weak sea-breeze penetration during the eclipse day, which led to an early onset of the land breeze.

The circulation dynamics of an event marked by the formation of an aerosol cluster off the coast of Maharashtra on April 22, 2006, its southward migration along the Indian west coast with a mean speed of ~200 km/day and its final dissipation after reaching the end of the peninsula by April 28, 2006 as revealed by MODIS (Moderate Resolution Imaging Spectroradiometer) against the pre-monsoon conditions of April 2006 are examined in this study. The maximum aerosol concentration in the cluster was found getting confined to lower and lower altitudes during its southward movement. The NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis wind field indicates that the atmospheric circulation, especially the horizontal wind convergence is the major factor that guides the formation and the dynamics of the cluster. Fine mode fraction from MODIS suggests that the cluster mainly consists of coarse dust particles. The regional climate model, RegCM3 with an efficient dust generation module simulates the formation and movement of the cluster appreciably well. The simulations which also exhibit the altitudinally descending nature of the cluster during its southward movement confirm the mechanism which governs the cluster dynamics suggested based on MODIS and NCEP/NCAR reanalysis data.

This study explores the inevitable role of wind parameters such as wind speed, wind convergence and wind vorticity in the long range transport and distribution of aerosols in the winter time atmosphere over the Bay of Bengal (BoB) during the campaign Winter ICARB. MODIS observed aerosol optical depth (AOD), with an excellent agreement with ship borne Microtops AOD, was found to increase over the BoB particularly, in the eastern parts during the course of the campaign. The influence of atmospheric circulation on this increase is examined using the wind field from NCEP reanalysis and computed wind convergence and vorticity for first and second halves (FH and SH) of the campaign along with a back trajectory analysis using HYSPLIT transport and dispersion model. While surface winds over the BoB remained nearly the same throughout the campaign denying the possibility of enhancement in marine aerosol generation, the higher altitude winds altered significantly in SH providing a channel for aerosol transport from the Indian landmass to the BoB in addition to the increased forest fire contribution from south Asia. This suggested mechanism is supported by CALIPSO aerosol extinction profiles over the eastern BoB and the surrounding land masses. Fine particle dominance in MODIS AOD and diminished correlation between ship borne AOD and surface aerosol mass measurements during SH corroborate this inference by indicating the presence of elevated aerosol layers, which can contribute substantially to the radiative effects of the earth–atmosphere system. This study throws light on the importance of wind convergence and vorticity in the investigations on the long range transport and spatial distribution of aerosols.

On 15 January 2010, Thumba (8.5°N, 76.9°E) witnessed one of the longest known noontime annular solar eclipses (ASEs) spanning a period of about 7 min, centered at 1314 hours local time. In this research article, we present a case study on the behaviour of the atmospheric boundary layer characteristics and its vertical structure in response to this rare celestial event by making use of a suite of different in-situ instruments. During the peak period of the ASE, the incoming solar irradiance was dimmed by about 87% of its normal values, resulting in a significant reduction in the magnitudes of turbulent kinetic energy and surface-layer turbulent fluxes of heat and momentum. The intensity and vertical thickness of the sea/land breeze circulation cell over the study domain also weakened. However, the mixed layer heights determined from balloon-borne GPS Radiosonde did not show any appreciable changes. Analysis of vertical profiles of thermodynamic parameters in association with the wind direction during ASE indicated the formation of a double mixed layer between 700 and 1500 m and is attributed to horizontal advection of a different airmass at those altitudes.

Vertical profiles of meteorological parameters obtained from balloon-borne GPS Radiosonde for a period of more than two years are analyzed for characterization of the coastal atmospheric boundary layer (CABL) over Thumba (, , India). The study reports seasonal variability in the thickness of three different sublayers of the CABL, namely, mixed layer, turbulent flow, and sea breeze flow. Among the three, the vertical thickness of sea breeze flow showed considerable dominance on the other two throughout the year. Mixed layer heights derived through gradients in virtual potential temperature () showed large seasonal variability with a peak in the Summer and Post-Monsoon. On the other hand, the vertical thickness of turbulent flow remained steady all through the year. Results from the present study indicate that the magnitudes of mixed layer heights are often larger than the turbulent flow thickness.

In-situ measurements of number density, size distribution, and mass loading of near-surface aerosols were carried out at Kharagpur, a site on the eastern part of Indo-Gangetic Plains during the winter month of December 2004. The data have been used to investigate wintertime characteristics of aerosols and their effects on the occurrence of haze. The aerosol number density is found to be of the order of 109m−3 and mass loading is ~265±70μgm−3 (5–8 times that reported from south Indian sites). The diurnal patterns and day-to-day variations in aerosol number density and mass loading are closely associated with atmospheric boundary layer height. During haze events, the number density of submicron particles is found to be 2–5 times higher than that during non-hazy period. This could be attributed to the enhanced concentration of anthropogenic aerosols, low atmospheric boundary layer height/ventilation coefficient and airflow convergence.

MODIS (Moderate Resolution Imaging Spectroradiometer) level-3 aerosol data, NCEP (National Centers for Environmental Prediction) reanalysis winds and QuikSCAT ocean surface winds were made use of to examine the role of atmospheric circulation in governing aerosol variations over the Bay of Bengal (BoB) during the first phase of the ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) campaign (March 18–April 12, 2006). An inter-comparison between MODIS level-3 aerosol optical depth (AOD) data and ship-borne MICROTOPS measurements showed good agreement with correlation 0.92 (p < 0.0001) and a mean MODIS underestimation by 0.01. During the study period, the AOD over BoB showed high values in the northern/north western regions, which reduced towards the central and southern BoB. The wind patterns in lower atmospheric layers (> 850 hPa) indicated that direct transport of aerosols from central India was inhibited by the presence of a high pressure and a divergence over BoB in the lower altitudes. On the other hand, in the upper atmospheric levels, winds from central and northern India stretched south eastwards and converged over BoB with a negative vorticity indicative of a downdraft. These wind patterns pointed to the possibility of aerosol transport from central India to BoB by upper level winds. This mechanism was further confirmed by the significant correlations that AOD variations over BoB showed with aerosol flux convergence and flux vorticity at upper atmospheric levels (600–500 hPa). AOD in central and southern BoB away from continental influences displayed an exponential dependence on the QuikSCAT measured ocean surface wind speed. This study shows that particles transported from central and northern India by upper atmospheric circulations as well as the marine aerosols generated by ocean surface winds contributed to the AOD over the BoB during the first phase of ICARB.