hemispheric temperature gradient. There is a southward shift
in the ITCZ location and South Asian Monsoon (Sinha et al.
2011) after the volcanic eruptions occurred during the last
millennium (Schneider et al. 2014). A host of modeling
studies shows a consistent decrease in Asian summer monsoon rainfall following volcanic (Zambri et al. 2017), barring
a few studies which report an increase in the precipitation
response in the post-eruption period, due to change in the
land–sea thermal gradient (Joseph and Zeng 2011). Volcanic
eruptions also influence climate; that is, it triggers El Niños
(Ohba et al. 2013).
One of the important impacts of volcanoes is the loss of
stratospheric ozone. The ozone loss and associated changes
in photolysis rates affect the tropospheric/stratospheric
temperature (cooling/warming) (Santer et al. 2003). The
stratospheric ozone loss is linked with chemical reactions
occurring over aerosol surfaces. There is a reduction of
nitrogen oxides and chlorine activation, which leads to an
increase in Equivalent Effective Stratospheric Chlorine
(EESC) (Tie and Brasseur 1995; Tabazadeh et al. 2002).
A drastic decrease in the stratospheric ozone over Antarctica
due to a series of volcanic eruptions has been proposed to
lead to large-scale changes in atmospheric dynamics resulting in massive de-glaciations in the past (McConnell et al.
2017). Previous work also indicates that volcanic eruptions
can serve as a source of potential predictability (Gaddis
2013) by having links with the tropical precipitation via
modulations of stratospheric ozone.
5.6 Knowledge Gaps
Aerosol-cloud-precipitation-meteorology interaction is one
of the most challenging scientific issues requiring intensive
observational and modeling with focused research from the
climate science community. The complexity in the aerosolcloud interaction arises from variations in dominant phase
changes and microphysical and dynamical processes associated with different types of clouds. Concurrent measurements of aerosol size distribution, composition, cloud
properties, microphysical parameters as well as the development of physical process scale studies based on observations over a varying space and times-scales, and translating
them to climate models are essential to gain a good understanding on the role of aerosols in modifying weather and
climate over India.
In addition, accurate representation of the absorbing
aerosol hotspots, particularly BC and dust, is crucial to
comprehend its impact on regional climate. Uncertainty in
the measurement of single scattering albedo, the parameter
determining the absorptive nature of aerosols, limits the
correct quantification of the sign of TOA radiative forcing at
regional scales. More in situ measurements of vertical
profiles of absorbing aerosols are also needed for better
evaluation of model-simulated BC profiles and understanding its effect on monsoon precipitation through interaction
with clouds and radiation. Also, the aerosol observational
data from field campaigns and long-term monitoring sites
from various sources and reanalysis products need to be
gathered to make a comprehensive quality-controlled gridded product. Future field campaigns may be planned to
address missing links in this regard and to reduce the
uncertainty in the regional estimates of the direct and indirect effect of aerosols in state of the art GCMs.
In the case of trace gases, there is a considerable variation
among the emission inventories of ozone precursors and
related trace gases. Dedicated modeling and observational
efforts are needed to improve ozone emission inventories
over the Indian region. Model simulations show seasonal
transport of chemical species over the Indian Ocean which
affects the air–sea interaction and convective processes.
However, the models show significant biases over the
Oceans. There is a need to improve chemical processes and
parameterization in the model to reduce the biases. Finally,
there are limited studies quantifying the radiative impact of
trace gases and associated climate change over the Indian
region, and further modeling and observational studies in
this direction are required.
5.7 Summary
The regional assessment of long-term in situ and remotely
sensed observations over India shows a significant increase
in aerosol loading over the subcontinent accompanied by
robust seasonal variations. The trend in AOD is
*2% year
−1 (high confidence) during the last 30. The
temporal build-up of aerosols is significantly high in the dry
winter months, while changes are smaller in the
pre-monsoon and monsoon season. This change has been
attributed to rise in fine mode particles due to rapid growth
in anthropogenic activities over the region in recent decades.
CMIP5 multi-model simulations also capture the large
increase in AOD over the Indian region between 1980 and
2000 with considerable bias in the three-dimensional
heterogeneous distribution of different aerosol species.
There is a large seasonal as well as spatiotemporal variability in the aerosol radiative forcing. In general, the estimates of aerosol radiative forcing from measurements range
from −49 to −31 W m
−2 at the surface (high confidence),
and −15 to +8 W m
−2 at top-of-atmosphere (low confidence). The positive forcing at TOA is linked with the
absorptive nature of the aerosols over the Indian region.
Aerosols produce a declining trend of all-sky global irradiance over India. During 1986–1995, the observed global
radiation decreased by 3.6 W m
−2 and further by 9.5 W m
−2
110
S. Fadnavis et al.
in the ITCZ location and South Asian Monsoon (Sinha et al.
2011) after the volcanic eruptions occurred during the last
millennium (Schneider et al. 2014). A host of modeling
studies shows a consistent decrease in Asian summer monsoon rainfall following volcanic (Zambri et al. 2017), barring
a few studies which report an increase in the precipitation
response in the post-eruption period, due to change in the
land–sea thermal gradient (Joseph and Zeng 2011). Volcanic
eruptions also influence climate; that is, it triggers El Niños
(Ohba et al. 2013).
One of the important impacts of volcanoes is the loss of
stratospheric ozone. The ozone loss and associated changes
in photolysis rates affect the tropospheric/stratospheric
temperature (cooling/warming) (Santer et al. 2003). The
stratospheric ozone loss is linked with chemical reactions
occurring over aerosol surfaces. There is a reduction of
nitrogen oxides and chlorine activation, which leads to an
increase in Equivalent Effective Stratospheric Chlorine
(EESC) (Tie and Brasseur 1995; Tabazadeh et al. 2002).
A drastic decrease in the stratospheric ozone over Antarctica
due to a series of volcanic eruptions has been proposed to
lead to large-scale changes in atmospheric dynamics resulting in massive de-glaciations in the past (McConnell et al.
2017). Previous work also indicates that volcanic eruptions
can serve as a source of potential predictability (Gaddis
2013) by having links with the tropical precipitation via
modulations of stratospheric ozone.
5.6 Knowledge Gaps
Aerosol-cloud-precipitation-meteorology interaction is one
of the most challenging scientific issues requiring intensive
observational and modeling with focused research from the
climate science community. The complexity in the aerosolcloud interaction arises from variations in dominant phase
changes and microphysical and dynamical processes associated with different types of clouds. Concurrent measurements of aerosol size distribution, composition, cloud
properties, microphysical parameters as well as the development of physical process scale studies based on observations over a varying space and times-scales, and translating
them to climate models are essential to gain a good understanding on the role of aerosols in modifying weather and
climate over India.
In addition, accurate representation of the absorbing
aerosol hotspots, particularly BC and dust, is crucial to
comprehend its impact on regional climate. Uncertainty in
the measurement of single scattering albedo, the parameter
determining the absorptive nature of aerosols, limits the
correct quantification of the sign of TOA radiative forcing at
regional scales. More in situ measurements of vertical
profiles of absorbing aerosols are also needed for better
evaluation of model-simulated BC profiles and understanding its effect on monsoon precipitation through interaction
with clouds and radiation. Also, the aerosol observational
data from field campaigns and long-term monitoring sites
from various sources and reanalysis products need to be
gathered to make a comprehensive quality-controlled gridded product. Future field campaigns may be planned to
address missing links in this regard and to reduce the
uncertainty in the regional estimates of the direct and indirect effect of aerosols in state of the art GCMs.
In the case of trace gases, there is a considerable variation
among the emission inventories of ozone precursors and
related trace gases. Dedicated modeling and observational
efforts are needed to improve ozone emission inventories
over the Indian region. Model simulations show seasonal
transport of chemical species over the Indian Ocean which
affects the air–sea interaction and convective processes.
However, the models show significant biases over the
Oceans. There is a need to improve chemical processes and
parameterization in the model to reduce the biases. Finally,
there are limited studies quantifying the radiative impact of
trace gases and associated climate change over the Indian
region, and further modeling and observational studies in
this direction are required.
5.7 Summary
The regional assessment of long-term in situ and remotely
sensed observations over India shows a significant increase
in aerosol loading over the subcontinent accompanied by
robust seasonal variations. The trend in AOD is
*2% year
−1 (high confidence) during the last 30. The
temporal build-up of aerosols is significantly high in the dry
winter months, while changes are smaller in the
pre-monsoon and monsoon season. This change has been
attributed to rise in fine mode particles due to rapid growth
in anthropogenic activities over the region in recent decades.
CMIP5 multi-model simulations also capture the large
increase in AOD over the Indian region between 1980 and
2000 with considerable bias in the three-dimensional
heterogeneous distribution of different aerosol species.
There is a large seasonal as well as spatiotemporal variability in the aerosol radiative forcing. In general, the estimates of aerosol radiative forcing from measurements range
from −49 to −31 W m
−2 at the surface (high confidence),
and −15 to +8 W m
−2 at top-of-atmosphere (low confidence). The positive forcing at TOA is linked with the
absorptive nature of the aerosols over the Indian region.
Aerosols produce a declining trend of all-sky global irradiance over India. During 1986–1995, the observed global
radiation decreased by 3.6 W m
−2 and further by 9.5 W m
−2
110
S. Fadnavis et al.