and 0.1 (−0.06 to +0.14) W m
−2 . For NO x , due to its role in
nitrate aerosol formation, the best estimate is a resultant
negative forcing of −0.15 (−0.34 to +0.02) W m
−2 . For
sulfur dioxide, the estimate is −0.41(−0.62 to −0.21) W m
−2
(IPCC 2013).
5.4 Influence of Transport Processes
Monsoon sustains a remarkably efficient cleansing mechanism in which contaminants are rapidly oxidized and
deposited to Earth’s surface. However, some pollutants are
lifted above the monsoon clouds due to deep convection and
are chemically processed in a reactive reservoir before being
redistributed globally, including to the stratosphere. Numbers of studies based on satellite remote sensing indicate the
transport of CO, H 2 O, PAN, Hydrogen cyanide (HCN),
CH 4 , NO x , etc., from the Asian boundary layer to the UTLS
(Fadnavis et al. 2013, 2015, 2017). Enhancement of trace
gases in the UTLS during the monsoon season alters local
heating rates and radiative balance. Transported NO x and
associated ozone variations in the UTLS enhances the ozone
heating rates by *1–1.4 K day
−1 in the upper troposphere
(400–200 hPa) and radiative forcing *16.3 m W m
−2 over
the Indian region. There is a positive impact of ozone
heating rates and radiative forcing on the Indian monsoon
circulation (Roy et al. 2017). There is a convective injection
of polluted water vapor from the Asian region into the
UTLS, which is then dispersed into the global stratosphere
by the large-scale upward motion (Fu et al. 2006). The H 2 O
feedback amplifies the radiative forcing of anthropogenic
greenhouse gases by a factor of *2.
In the lower troposphere, the transport of trace gases
occurs to and from India with seasonal variations in the
wind. The seasonal variation in most trace gases shows a dip
during the monsoon season due to efficient wet scavenging
by precipitation and the transport of clean marine air. Integrated Campaign for Aerosols, gases, and Radiation Budget
(ICARB) measurements during the pre-monsoon season
show elevated levels of CO (*100 ppb) over the Bay of
Bengal and the Arabian Sea. These studies reveal that high
amounts of marine CO are attributed to transport from the
Indian subcontinent (Aneesh et al. 2008). Satellite observations also show high values of CO (130–160 ppb) and ozone
(120–130 ppb) at 825 hPa near the location of cyclones
occurring in the Bay of Bengal and Arabian Sea (Fadnavis
et al. 2011).
The in situ observations are unable to explain the different
atmospheric processes accountable for high pollution events.
Therefore, chemistry transport models are valuable for providing a large-scale view of the regional impact of these gases
and are useful for the interpretation of observations on local
to global scale (Yarragunta et al. 2017). The simulated ozone
concentrations from the MOZART4 model when evaluated
against ground-based observations revealed that the model
captures the seasonal cycle of ozone amounts but overestimates the values of ozone concentration. The magnitude of
observed ozone is in the range of 7–60 ppbv, whereas the
quantity of simulated ozone is *27–53 ppbv (Yarragunta
et al. 2018). Lower tropospheric ozone over India during
2006–2010 as observed from OMI showed the highest concentrations (54.1 ppbv) in the pre-summer monsoon season
(May) and the lowest concentrations (40.5 ppbv) in the
summer monsoon season (August). Analyses from the
GEOS-Chem model showed that the onset of the summer
monsoon brings ozone-unfavorable meteorological conditions which all lead to substantial decreases in the lower
tropospheric ozone burden (Lu et al. 2018). The influence of
springtime (MAM) biomass burning in central India, the
Indo-Gangetic region and the Bay of Bengal, on regional
ozone distribution has been evaluated using a regional
chemical transport model (WRF-Chem), and the Fire
Inventory from NCAR (FINNv1). These simulations
demonstrated that the springtime fire emissions have a significant impact on the ozone in this region (Jena et al. 2015).
5.4.1 Influence of Stratosphere to Troposphere
Transport
Transport associated with tropopause folding produces a
significant variation in ozone, humidity, and temperature.
MLS and AIRS satellites show intrusion events of
ozone-rich dry stratospheric air over northern India and the
Tibetan Plateau region occurring every winter and
pre-monsoon season. It enhances ozone amounts by *100–
200 ppmv in the UTLS (300–100 hPa) (Fadnavis et al.
2010). Tropopause folding in the subtropical westerly jet
during the monsoon seasons sheds eddies into the deep
troposphere (*700 hPa) which are a carrier of ozone-rich
cold and dry air. These eddies spread stratospheric air in the
upper troposphere, increasing the static stability of the troposphere (Fig. 5.8). These stratospheric dry air intrusions are
associated with monsoon breaks and are evident in observations during 1979–2007 (Fadnavis and Chattopadhyay
2017).
The stratospheric folding tends to occur on the northwestern side of the upper-level anticyclone resulting in
intensified subsidence and reduces extreme rainfall upstream
of the fold, while it enhances the precipitation at downstream
of the fold. A typical pattern of suppression of extreme
rainfall upstream and promotion downstream of the fold
persists for about 1–2 days. Rossby wave breaking over
West Asia inhibits deep monsoonal convection and thereby
leading to a dry spell over India (1998–2010) (Samanta et al.
2016).
108
S. Fadnavis et al.
−2 . For NO x , due to its role in
nitrate aerosol formation, the best estimate is a resultant
negative forcing of −0.15 (−0.34 to +0.02) W m
−2 . For
sulfur dioxide, the estimate is −0.41(−0.62 to −0.21) W m
−2
(IPCC 2013).
5.4 Influence of Transport Processes
Monsoon sustains a remarkably efficient cleansing mechanism in which contaminants are rapidly oxidized and
deposited to Earth’s surface. However, some pollutants are
lifted above the monsoon clouds due to deep convection and
are chemically processed in a reactive reservoir before being
redistributed globally, including to the stratosphere. Numbers of studies based on satellite remote sensing indicate the
transport of CO, H 2 O, PAN, Hydrogen cyanide (HCN),
CH 4 , NO x , etc., from the Asian boundary layer to the UTLS
(Fadnavis et al. 2013, 2015, 2017). Enhancement of trace
gases in the UTLS during the monsoon season alters local
heating rates and radiative balance. Transported NO x and
associated ozone variations in the UTLS enhances the ozone
heating rates by *1–1.4 K day
−1 in the upper troposphere
(400–200 hPa) and radiative forcing *16.3 m W m
−2 over
the Indian region. There is a positive impact of ozone
heating rates and radiative forcing on the Indian monsoon
circulation (Roy et al. 2017). There is a convective injection
of polluted water vapor from the Asian region into the
UTLS, which is then dispersed into the global stratosphere
by the large-scale upward motion (Fu et al. 2006). The H 2 O
feedback amplifies the radiative forcing of anthropogenic
greenhouse gases by a factor of *2.
In the lower troposphere, the transport of trace gases
occurs to and from India with seasonal variations in the
wind. The seasonal variation in most trace gases shows a dip
during the monsoon season due to efficient wet scavenging
by precipitation and the transport of clean marine air. Integrated Campaign for Aerosols, gases, and Radiation Budget
(ICARB) measurements during the pre-monsoon season
show elevated levels of CO (*100 ppb) over the Bay of
Bengal and the Arabian Sea. These studies reveal that high
amounts of marine CO are attributed to transport from the
Indian subcontinent (Aneesh et al. 2008). Satellite observations also show high values of CO (130–160 ppb) and ozone
(120–130 ppb) at 825 hPa near the location of cyclones
occurring in the Bay of Bengal and Arabian Sea (Fadnavis
et al. 2011).
The in situ observations are unable to explain the different
atmospheric processes accountable for high pollution events.
Therefore, chemistry transport models are valuable for providing a large-scale view of the regional impact of these gases
and are useful for the interpretation of observations on local
to global scale (Yarragunta et al. 2017). The simulated ozone
concentrations from the MOZART4 model when evaluated
against ground-based observations revealed that the model
captures the seasonal cycle of ozone amounts but overestimates the values of ozone concentration. The magnitude of
observed ozone is in the range of 7–60 ppbv, whereas the
quantity of simulated ozone is *27–53 ppbv (Yarragunta
et al. 2018). Lower tropospheric ozone over India during
2006–2010 as observed from OMI showed the highest concentrations (54.1 ppbv) in the pre-summer monsoon season
(May) and the lowest concentrations (40.5 ppbv) in the
summer monsoon season (August). Analyses from the
GEOS-Chem model showed that the onset of the summer
monsoon brings ozone-unfavorable meteorological conditions which all lead to substantial decreases in the lower
tropospheric ozone burden (Lu et al. 2018). The influence of
springtime (MAM) biomass burning in central India, the
Indo-Gangetic region and the Bay of Bengal, on regional
ozone distribution has been evaluated using a regional
chemical transport model (WRF-Chem), and the Fire
Inventory from NCAR (FINNv1). These simulations
demonstrated that the springtime fire emissions have a significant impact on the ozone in this region (Jena et al. 2015).
5.4.1 Influence of Stratosphere to Troposphere
Transport
Transport associated with tropopause folding produces a
significant variation in ozone, humidity, and temperature.
MLS and AIRS satellites show intrusion events of
ozone-rich dry stratospheric air over northern India and the
Tibetan Plateau region occurring every winter and
pre-monsoon season. It enhances ozone amounts by *100–
200 ppmv in the UTLS (300–100 hPa) (Fadnavis et al.
2010). Tropopause folding in the subtropical westerly jet
during the monsoon seasons sheds eddies into the deep
troposphere (*700 hPa) which are a carrier of ozone-rich
cold and dry air. These eddies spread stratospheric air in the
upper troposphere, increasing the static stability of the troposphere (Fig. 5.8). These stratospheric dry air intrusions are
associated with monsoon breaks and are evident in observations during 1979–2007 (Fadnavis and Chattopadhyay
2017).
The stratospheric folding tends to occur on the northwestern side of the upper-level anticyclone resulting in
intensified subsidence and reduces extreme rainfall upstream
of the fold, while it enhances the precipitation at downstream
of the fold. A typical pattern of suppression of extreme
rainfall upstream and promotion downstream of the fold
persists for about 1–2 days. Rossby wave breaking over
West Asia inhibits deep monsoonal convection and thereby
leading to a dry spell over India (1998–2010) (Samanta et al.
2016).
108
S. Fadnavis et al.