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Vijayakumar S. Nair, S. Suresh Babu, K. Krishna Moorthy, and S. S. Prijith

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 long-term 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−1 in the southwestern Arabian Sea to as high as ~0.5 K day−1 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.

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Vijayakumar S. Nair, K. Krishna Moorthy, S. Suresh Babu, and S. K. Satheesh

Abstract

Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.

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Priyanka Banerjee, S. K. Satheesh, K. Krishna Moorthy, Ravi S. Nanjundiah, and Vijayakumar S. Nair

Abstract

Synergizing satellite remote sensing data with vertical profiles of atmospheric thermodynamics and regional climate model simulations, we investigate the relative importance, transport pathways, and seasonality of contribution of dust from regional (Thar Desert and adjoining arid regions) and remote (southwest Asia and northeast Africa) sources over the northeast Indian Ocean [i.e., the Bay of Bengal (BOB)]. We show that while over the northern BOB dust from the regional sources contribute more than 50% to the total dust load during the southwest monsoon period (June–September), interestingly; the remote dust sources dominate rest of the year. On the other hand, over the southern BOB, dust transported from the remote-source regions dominate throughout the year. During June, the dry elevated layer (at altitudes between 850 and 700 hPa) of dust, transported across the Indo-Gangetic Plain to the northern BOB, arises primarily from the Thar Desert. Dust from remote sources in the far west reaches the southern BOB after traversing over and around the southern Indian Peninsula. Since dust from these distinct source regions have different mineral composition (hence optical properties) and undergo distinct changes during atmospheric transport, it is important to understand source-specific dust contribution and transport pathways to address dust–climate feedback.

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