Abstract
Abstract. Vertical structures of aerosol single scattering albedo (SSA), from near the surface through the free troposphere, have been estimated for the first time at distinct geographical locations over the Indian mainland and adjoining oceans, using in situ measurements of aerosol scattering and absorption coefficients aboard the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the South West Asian Aerosol Monsoon Interactions (SWAAMI) campaign from June to July 2016. These are used to examine the spatial variation of SSA profiles and also to characterize its transformation from just prior to the onset of Indian Summer Monsoon (June 2016) to its active phase (July 2016). Very strong aerosol absorption, with SSA values as low as 0.7, persisted in the lower altitudes (<3 km) over the Indo-Gangetic Plains (IGP), prior to the monsoon onset, with a west-to-east gradient; lower values occurred in the north-western arid regions, peaking in the central IGP and somewhat decreasing towards the eastern end. During the active phase of the monsoon, the SSA is found to increase remarkably, indicating far less absorption. Nevertheless, significant aerosol absorption persisted in the lower and middle troposphere over the IGP. Inputting these SSA and extinction profiles into a radiative transfer model, we examined the effects of using height-resolved information in estimating atmospheric heating rates due to aerosols, over similar estimates made using a single columnar value. It was noted that use of a single SSA value leads to an underestimation (overestimation) of the heating rates over regions with low (high) SSA, emphasizing the importance of height-resolved information. Further, the use of realistic profiles showed significant heating of the atmosphere by submicron aerosol absorption at the middle troposphere, which may have strong implications for clouds and climate.
Highlights
The Indian Summer Monsoon (ISM) is a large-scale climate system, having direct implications for South Asia
Panels a–b and g–h represent the conditions just before the monsoon onset over central Indo-Gangetic Plains (IGP) (Lucknow) and central India (Nagpur) respectively, while the other panels show the features during the monsoon over central IGP (LCK, panels c–d), north-western India
Vertical distributions of submicron aerosol properties at five locations Lucknow (LCK), Nagpur (NGP), Arabian Sea (AS), Bay of Bengal (BoB) and Jaipur (JPR) were studied in the period of June–July 2016 using in situ measurements made from an aircraft
Summary
The Indian Summer Monsoon (ISM) is a large-scale climate system, having direct implications for South Asia. It is important to measure the key aerosol physical and chemical properties required to assess the regional and vertical distribution of aerosolinduced atmospheric heating This can be accomplished by conducting campaign-mode measurements involving in situ measurements of aerosol properties using aircraft, supplemented with ground- and space-based measurements across the Indian domain, with special focus on the Indo-Gangetic Plains and northern Bay of Bengal, immediately before monsoon onset and during its development. The joint Indo–UK field experiment, SWAAMI was formulated to meet these goals During this campaign, extensive airborne measurements of aerosols were made in the altitude range from very close to the surface to as high as 8 km, using the BAe-146-301 atmospheric research aircraft (Highwood et al, 2012; Johnson et al, 2012) over the Indian domain, in June–July 2016, covering the pre-onset and evolving phases of the ISM (Brooks et al, 2018), perhaps for the first time at such detail. We report the vertical distribution of extinction coefficients and single scattering albedo during SWAAMI campaign and discuss the climate implications
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