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Discussion papers
https://doi.org/10.5194/acp-2019-657
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-657
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 Aug 2019

Submitted as: research article | 06 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Vertical profiles of sub-micron aerosol single scattering albedo over Indian region immediately before monsoon onset and during its development: Research from the SWAAMI field campaign

Mohanan R. Manoj1, Sreedharan K. Satheesh1,2, Krishnaswamy K. Moorthy1, and Hugh Coe3 Mohanan R. Manoj et al.
  • 1Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India
  • 2Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India
  • 3Centre for Atmospheric Science, School of Earth and Environmental Sciences, University of Manchester, Manchester, UK

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 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 onset of monsoon, 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 in 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 noticed that use of a single SSA value leads to an underestimation (over-estimation) of the heating rates over regions with low (high) SSA emphasizing the importance of height resolved information. Further, the use of realistic profiles showed the significant heating of the atmosphere by sub-micron aerosol absorption at the middle troposphere, which would have strong implications for clouds and climate.

Mohanan R. Manoj et al.
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Short summary
The study reports the observation of highly absorbing aerosol layers at high altitudes (1–2.5 km) and quantifies their climate impacts. These layers perturb the onset of monsoon through their impact on the atmospheric stability. Height resolved measurements of these properties when used in a radiative transfer model revealed a maximum heating in these layers of 1 K/day and an underestimation of heating rates as high as 0.5 K/day when height resolved data is not used.
The study reports the observation of highly absorbing aerosol layers at high altitudes...
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