Spatial Gradients in Aerosol-Induced Atmospheric Heating and Surface Dimming over the Oceanic Regions around India: Anthropogenic or Natural?

Abstract

Making use of the extensive shipboard and aircraft measurements of aerosol properties over the oceanic regions surrounding the Indian peninsula, under the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) field experiment during the premonsoon season (March-May), supplemented with longterm satellite data and chemical transport model simulations, investigations are made of the east-west and north-south gradients in aerosol properties and estimated radiative forcing, over the oceans around India. An eastward gradient has been noticed in most of the aerosol parameters that persisted both within the marine atmospheric boundary layer and above up to an altitude of ;6 km; the gradients being steeper at higher altitudes. It was also noticed that the north-south gradient has contrasting patterns over the Bay of Bengal and the Arabian Sea on the either side of the Indian peninsula. The aerosol-induced atmospheric heating rate increased from a low value of #0.1 K day 21 in the southwestern Arabian Sea to as high as ;0.5 K day 21 over the northeastern Bay of Bengal. The simulations of species-resolved spatial gradients have revealed that the observed gradients are the result of the strong modulations by anthropogenic species over the natural gradients, thereby emphasizing the role of human activities in imparting regional forcing. These large spatial gradients in aerosol forcing induced by mostly anthropogenic aerosols over the oceanic regions around the Indian peninsula can potentially affect the regional circulation patterns.

Vertical Structure of Aerosols and Mineral Dust Over the Bay of Bengal From Multisatellite Observations

“…Due to lack of long‐term spectral measurements of single‐scattering albedo (SSA) and phase function over Bay of Bengal, we have used a semiempirical approach in which the SSA and phase function are estimated by iterating the number concentration of the aerosol components adopted from Koepke et al () (OPAC) in such a way that the estimated aerosol parameters matches well with the measured aerosol parameters as discussed in section 2.3. When the optical properties estimated using the model matches well with the measured aerosol properties, we adopt that aerosol composition to estimate spectral variation of AOD, SSA, and asymmetry parameter required for the radiative transfer calculations (Moorthy et al, ; Nair, Babu, et al, ). For the estimation of dust radiative forcing, dust optical depth obtained at 532 nm and vertical profile of dust extinction coefficient derived using CALIOP depolarization measurements are used.…”

“…Details on the retrieval process are given by Ackerman et al () and Remer et al (). Extensive comparison of MODIS level 3 aerosol optical depth (AOD) data (at 0.55 μm) with 171 days of ship‐borne Sun photometer measurements over oceanic regions surrounding the Indian peninsula during 2003–2010 showed good agreement with a correlation coefficient of 0.91 and slope of 0.95 (Nair, Babu, et al, ). The present study made use of level 3 MODIS collection 6 (MYD08_M3) QA weighted data product over the ocean to obtain aerosol optical depth over the study region (Levy et al, ).…”

“…The parameters used to constrain the aerosol model are (i) climatological mean spectral AOD from MODIS, (ii) single scattering albedo measured at 550 nm during ICARB experiments in premonsoon 2006 and winter 2009, (iii) sea salt optical depth ( τ seasalt ) derived from the surface wind speed following Kaufman et al (), and (iv) dust optical depth ( τ dust ) derived from depolarization and backscatter coefficient of CALIPSO (section 2.4). More details on constraining the OPAC aerosol model with measured aerosol properties are available elsewhere (Moorthy et al, ; Nair, Babu et al, ). This constrained column aerosol model is used to estimate the lidar ratio ( S MODEL ) and aerosol parameters relevant for radiative forcing (refer to section 3.7).…”

Aerosol and monsoon climate interactions over Asia

“…The role of the observed increasing trend in Arabian dust activity on the interannual to decadal timescale variations in the Indian summer monsoon rainfall has revealed dynamical and positive feedback due to dust‐induced diabatic heating over the region [ Solmon et al ., ]. This is in line with the meridional and zonal gradients in the aerosol‐induced atmospheric heating rate in the lower free troposphere estimated based on extensive ship‐borne and satellite measurements made over the region [ Nair et al ., ]. The study confirmed that aerosol‐induced gradients (meridional and zonal) in surface cooling and atmospheric warming are mostly anthropogenic in origin [ Nair et al ., ].…”

Evolution of Aerosol Research in India and the RAWEX–GVAX:An Overview

“…Synthesizing the cruise measurements with those from the network and data from lidars, Nair et al 71 have shown large zonal and meridional gradients in the aerosol induced lower atmospheric heating rate across Indian Peninsula; from 0.5 K d -1 at 55E (over western Arabian Sea) increasing to 1.5 K d -1 over eastern BoB (100E) with contrasting meridional gradient on either side of the peninsula.…”