radiative forcing is −0.26 W m
−2 (−0.06 to −0.49 W m
−2
);
however, screening based on model skill in capturing
observed AOD yields the best estimate of −0.42 W m
−2
(−0.33 to −0.50 W m
−2 ) (Shindell et al. 2013). The comparison of aerosol RF at the TOA (2005–1850) in CMIP5
models with the Modern Era Retrospective-analysis for
Research and Analysis (MERRA) indicates that most of the
CMIP5 models have underestimated aerosol RF over South
Asia (Sanap et al. 2014). The ACCMIP model shows global
mean pre-industrial to present-day aerosols direct RF (1850
and 2000) is −0.26 ± 0.14 W m
−2
(−0.06 to
−0.49 W m
−2 ), and top-of-atmosphere (TOA) is
−1.2 ± 0.5 W m
−2 although variability across models is
large in many locations (Shindell et al. 2013).
Over the Indian landmass, there is a large spatiotemporal
variation in both magnitude and sign ranging from −26 to
+14 W m
−2 at TOA and −63 to −2.8 W m
−2 at the surface.
A recent comprehensive analysis of aerosol direct radiative
effect (DRE) by Nair et al. (2016) based on 27 ARFINET
observatories, four AERONET stations, and four IMD stations estimated the seasonal variation of DRE over India.
The regional mean DRE for TOA and surface is reported to
be −8.6 ± 3 and −28.2 ± 12, −26 to +14 W m
−2 , respectively, during winter and −6.8 ± 4, −33.7 ± 12–26 to
+14 W m
−2 during spring. The effective radiative forcing
calculated by models indicates that over India, as high as
580 mW m
−2 is because of residential biofuels. However,
this is offset due to sulfate from power plants, which contribute about −30 mW m
−2 (Streets et al. 2013).
In western India, observations of aerosol surface forcing
over Ahmedabad, Gujarat, showed that surface forcing was
found to be highest during post-monsoon (−63 ± 10 W m
−2
),
followed by dry (−54 ± 6 W m
−2
) and lower values during
pre-monsoon
(−41.4 ± 5 W m
−2
)
and
monsoon
(−41 ± 11 W m
−2
) seasons (Ganguly et al. 2005).
In the northeast of India, atmospheric aerosol radiative
forcing estimated from the ARFINET observations over
India (2010–2014) shows the highest radiative forcing in the
pre-monsoon season ranging from 48.6 W m
−2 in Agartala
to 25.1 W m
−2 in Imphal. Wintertime radiative forcing follows the pre-monsoon season at these locations. The heating
rate resultant from this forcing is high at 1.2 K day
−1 and
1.0 K day
−1 over Shillong and Dibrugarh, respectively, in
this season. However, Agartala experiences higher surface
forcing (−56.5 W m
−2 ) and, consequently, larger heating of
the atmosphere (1.6 K day
−1 ) in winter (Pathak et al. 2016).
In the case of the top-of-atmosphere (TOA), radiative forcing is found to be negative during dry (−26 ± 3 W m
−2 )
and post-monsoon (−22 W m
−2 ), while positive values are
obtained during monsoon (14 W m
−2 ) and pre-monsoon
(8 W m
−2 ). Large differences between TOA and surface
forcing during monsoon and pre-monsoon indicate a large
absorption of radiant energy (*50 W m
−2 ) within the
atmosphere during these seasons. It can lead to heating rates
as high as 5.6 K day
−1 (Ganguly and Jayaraman 2006).
In north India, observations at New Delhi show a consistent increase in surface forcing, ranging from −39 W m
−2
(March) to −99 W m
−2 (June) and an increase in heating of
the atmosphere from 27 W m
−2 (March) to 123 W m
−2
(June). Heating rates in the lower atmosphere (up to 5 km)
are 0.6, 1.3, 2.1, and 2.5 K day
−1 from March, April, May,
and June 2006, respectively (Pandithurai et al. 2008).
Observations at a semi-urban location, Hisar, showed an
increase in the shortwave atmospheric forcing 16 W m
−2
during clear periods to 49 W m
−2 for foggy days. Longwave
cooling of the atmosphere increased from about −2 W m
−2
for clear conditions to about −3 W m
−2 during foggy periods (Ramachandran et al. 2006).
Observations at Hyderabad displayed diurnally averaged
values of direct shortwave radiative forcing in the range of
−15 to −40 W m
−2 at the surface, about 15% lower compared to that over the Bay of Bengal region and 22% higher
than over the Arabian Sea. TOA forcing observed was in the
range of +0.7 to −11 W m
−2
, about 50% lower compared to
both these regions. This results in a heating rate of nearly
0.8 K day
−1 for the first 2 km in the atmosphere (Ganguly
et al. 2005). Additional observations have also been made on
cruises, with the INDOEX campaign in the 1990s, and
showed that concentrations were relatively high throughout
much of the cruise, even when the ship was at considerable
distances from land. The northeast monsoonal low-level
flow can transport sulfates, mineral dust, and other aerosols
from the Indian subcontinent to the ITCZ within 6–7 days.
These transports result in an increase in AOD at the equator
by as much as 0.2 and a decrease in the solar radiative
forcing at the sea surface by about 10–20 W m
−2 (Krishnamurti et al. 1998).
The long-term historical surface temperature variations
over the Indian subcontinent reveal an absorbing
aerosol-induced statistically significant cooling of about
0.3 °C since the 1970s (Krishnan and Ramanathan 2002). In
concurrence, the temporal and spatial variability in annually
averaged global irradiance, diffuse irradiance, and bright
sunshine duration over twelve stations of solar radiation
network of India Meteorological Department (IMD) evaluated for the period 1971–2005. It showed a consistent
decrease in the decadal mean all-sky global solar radiation at
the surface for India, which was attributed to aerosols (Soni
et al. 2012). The decadal mean of the global solar radiation
for the decade was 221.5 W m
−2 for 1976–1985. From 1986
to 1995, the observed global radiation decreased by
3.6 W m
−2 and further by 9.5 W m
−2 during the decade of
1996–2005 (Soni et al. 2012). The declining trend of all-sky
global irradiance over India as a whole was 0.6 W m
−2
100
S. Fadnavis et al.
−2 (−0.06 to −0.49 W m
−2
);
however, screening based on model skill in capturing
observed AOD yields the best estimate of −0.42 W m
−2
(−0.33 to −0.50 W m
−2 ) (Shindell et al. 2013). The comparison of aerosol RF at the TOA (2005–1850) in CMIP5
models with the Modern Era Retrospective-analysis for
Research and Analysis (MERRA) indicates that most of the
CMIP5 models have underestimated aerosol RF over South
Asia (Sanap et al. 2014). The ACCMIP model shows global
mean pre-industrial to present-day aerosols direct RF (1850
and 2000) is −0.26 ± 0.14 W m
−2
(−0.06 to
−0.49 W m
−2 ), and top-of-atmosphere (TOA) is
−1.2 ± 0.5 W m
−2 although variability across models is
large in many locations (Shindell et al. 2013).
Over the Indian landmass, there is a large spatiotemporal
variation in both magnitude and sign ranging from −26 to
+14 W m
−2 at TOA and −63 to −2.8 W m
−2 at the surface.
A recent comprehensive analysis of aerosol direct radiative
effect (DRE) by Nair et al. (2016) based on 27 ARFINET
observatories, four AERONET stations, and four IMD stations estimated the seasonal variation of DRE over India.
The regional mean DRE for TOA and surface is reported to
be −8.6 ± 3 and −28.2 ± 12, −26 to +14 W m
−2 , respectively, during winter and −6.8 ± 4, −33.7 ± 12–26 to
+14 W m
−2 during spring. The effective radiative forcing
calculated by models indicates that over India, as high as
580 mW m
−2 is because of residential biofuels. However,
this is offset due to sulfate from power plants, which contribute about −30 mW m
−2 (Streets et al. 2013).
In western India, observations of aerosol surface forcing
over Ahmedabad, Gujarat, showed that surface forcing was
found to be highest during post-monsoon (−63 ± 10 W m
−2
),
followed by dry (−54 ± 6 W m
−2
) and lower values during
pre-monsoon
(−41.4 ± 5 W m
−2
)
and
monsoon
(−41 ± 11 W m
−2
) seasons (Ganguly et al. 2005).
In the northeast of India, atmospheric aerosol radiative
forcing estimated from the ARFINET observations over
India (2010–2014) shows the highest radiative forcing in the
pre-monsoon season ranging from 48.6 W m
−2 in Agartala
to 25.1 W m
−2 in Imphal. Wintertime radiative forcing follows the pre-monsoon season at these locations. The heating
rate resultant from this forcing is high at 1.2 K day
−1 and
1.0 K day
−1 over Shillong and Dibrugarh, respectively, in
this season. However, Agartala experiences higher surface
forcing (−56.5 W m
−2 ) and, consequently, larger heating of
the atmosphere (1.6 K day
−1 ) in winter (Pathak et al. 2016).
In the case of the top-of-atmosphere (TOA), radiative forcing is found to be negative during dry (−26 ± 3 W m
−2 )
and post-monsoon (−22 W m
−2 ), while positive values are
obtained during monsoon (14 W m
−2 ) and pre-monsoon
(8 W m
−2 ). Large differences between TOA and surface
forcing during monsoon and pre-monsoon indicate a large
absorption of radiant energy (*50 W m
−2 ) within the
atmosphere during these seasons. It can lead to heating rates
as high as 5.6 K day
−1 (Ganguly and Jayaraman 2006).
In north India, observations at New Delhi show a consistent increase in surface forcing, ranging from −39 W m
−2
(March) to −99 W m
−2 (June) and an increase in heating of
the atmosphere from 27 W m
−2 (March) to 123 W m
−2
(June). Heating rates in the lower atmosphere (up to 5 km)
are 0.6, 1.3, 2.1, and 2.5 K day
−1 from March, April, May,
and June 2006, respectively (Pandithurai et al. 2008).
Observations at a semi-urban location, Hisar, showed an
increase in the shortwave atmospheric forcing 16 W m
−2
during clear periods to 49 W m
−2 for foggy days. Longwave
cooling of the atmosphere increased from about −2 W m
−2
for clear conditions to about −3 W m
−2 during foggy periods (Ramachandran et al. 2006).
Observations at Hyderabad displayed diurnally averaged
values of direct shortwave radiative forcing in the range of
−15 to −40 W m
−2 at the surface, about 15% lower compared to that over the Bay of Bengal region and 22% higher
than over the Arabian Sea. TOA forcing observed was in the
range of +0.7 to −11 W m
−2
, about 50% lower compared to
both these regions. This results in a heating rate of nearly
0.8 K day
−1 for the first 2 km in the atmosphere (Ganguly
et al. 2005). Additional observations have also been made on
cruises, with the INDOEX campaign in the 1990s, and
showed that concentrations were relatively high throughout
much of the cruise, even when the ship was at considerable
distances from land. The northeast monsoonal low-level
flow can transport sulfates, mineral dust, and other aerosols
from the Indian subcontinent to the ITCZ within 6–7 days.
These transports result in an increase in AOD at the equator
by as much as 0.2 and a decrease in the solar radiative
forcing at the sea surface by about 10–20 W m
−2 (Krishnamurti et al. 1998).
The long-term historical surface temperature variations
over the Indian subcontinent reveal an absorbing
aerosol-induced statistically significant cooling of about
0.3 °C since the 1970s (Krishnan and Ramanathan 2002). In
concurrence, the temporal and spatial variability in annually
averaged global irradiance, diffuse irradiance, and bright
sunshine duration over twelve stations of solar radiation
network of India Meteorological Department (IMD) evaluated for the period 1971–2005. It showed a consistent
decrease in the decadal mean all-sky global solar radiation at
the surface for India, which was attributed to aerosols (Soni
et al. 2012). The decadal mean of the global solar radiation
for the decade was 221.5 W m
−2 for 1976–1985. From 1986
to 1995, the observed global radiation decreased by
3.6 W m
−2 and further by 9.5 W m
−2 during the decade of
1996–2005 (Soni et al. 2012). The declining trend of all-sky
global irradiance over India as a whole was 0.6 W m
−2
100
S. Fadnavis et al.