Harshavardhana Sunil Pathak
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Sreedharan Krishnakumari Satheesh
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Krishnaswamy Krishna Moorthy
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Ravi Shankar Nanjundiah
Abstract. Clear-sky, direct shortwave
aerosol radiative forcing (ARF) has been estimated over the Indian region,
for the first time employing multi-year (2009–2013) gridded, assimilated aerosol products, as an important
part of the South West Asian Aerosol Monsoon Interactions (SWAAMI) which is a joint Indo-UK research field campaign
focused at understanding the variabilities in atmospheric aerosols and their interactions with the Indian summer monsoon.
The aerosol datasets have been constructed following statistical assimilation of concurrent data from a dense network of
ground-based observatories and multi-satellite products, as described in Part 1 of this two-part paper. The ARF,
thus estimated, is assessed for its superiority or otherwise over other ARF estimates based on satellite-retrieved
aerosol products, over the Indian region, by comparing the radiative fluxes (upward) at the top of the atmosphere (TOA) estimated
using assimilated and satellite products with spatiotemporally matched radiative flux values provided by CERES
(Clouds and Earth's Radiant Energy System) single-scan footprint (SSF) product. This clearly demonstrated improved
accuracy of the forcing estimates using the assimilated vis-à-vis satellite-based aerosol datasets at regional,
subregional and seasonal scales. The regional distribution of diurnally averaged ARF estimates has revealed
(a) significant differences from similar estimates made using currently available satellite data, not only in terms
of magnitude but also the sign of TOA forcing; (b) the largest magnitudes of surface cooling and atmospheric warming over the Indo-Gangetic Plain (IGP)
and arid regions from north-western India; and (c) negative TOA forcing over most parts of the Indian region, except
for three subregions – the IGP, north-western India and eastern parts of peninsular
India where the TOA forcing changes to positive during pre-monsoon season. Aerosol-induced atmospheric warming rates,
estimated using the assimilated data, demonstrate substantial spatial heterogeneities (∼0.2 to 2.0 K d−1)
over the study domain with the IGP demonstrating relatively stronger atmospheric heating rates (∼0.6 to 2.0 K d−1).
There exists a strong seasonality as well, with atmospheric warming being highest during pre-monsoon and lowest during winter
seasons. It is to be noted that the present ARF estimates demonstrate substantially smaller uncertainties than their
satellite counterparts, which is a natural consequence of reduced uncertainties in assimilated vis-à-vis satellite
aerosol properties. The results demonstrate the potential application of the assimilated datasets and ARF estimates
for improving accuracies of climate impact assessments at regional and subregional scales.