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

Abstract

<p><strong>Abstract.</strong> 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.</p>

Global maps of aerosol single scattering albedo using combined CERES-MODIS retrieval

“…However, there are few exceptions. SSA values over the Bay of Bengal during SWAAMI campaign were reported as 0.84 + 0.07 during June-July by Manoj et al (2019), whereas CERES-MODIS retrieves a higher SSA of ~0.89 for the same time period. This large variation could be due to frequent cloud cover during the monsoon season, leading to fewer SSA points retrieved over the ocean and land.…”

Comments

“…Why elevated aerosols are not seen over mainland? In fact several earlier studies have reported elevated aerosol layers over mainland (Mishra et al, 2010;Ganguly et al,2006;Niranjan et al, 2007;Sinha et al, 2013;Venkat Ratnam et al, 2018). Further, it is mentioned that highest values are observed within 3 km from the surface.…”

Responses to the comments of Reviewer #2

“…The uncertainty due to the combined effect of AOD, SSA and asymmetry parameter on the top of the atmosphere forcing was found to be ∼ 1.8 W/m2. Estimation of uncertainty has been discussed in detail in many papers from the group (Moorthy et al, 2009, Satheesh et al, 2010 and hence not repeated here. We also found that the uncertainty in the instantaneous atmospheric forcing for the cases shown in figure 9 was ∼4.2 W/m2.…”