extreme rainfall events. But they can bring substantial rains
to the Indian landmass, and monsoon lows contribute to
about 40% of monsoon seasonal rains over the central Indian
landmass (Hurley and Boos 2015).
Generally, the development and intensification of LPS
have associations with warm sea surface temperatures (SSTs),
and environmental factors, such as the presence of low level
(850 hPa) cyclonic vorticity, high mid-tropospheric (500 hPa)
humidity and strong vertical wind shear (difference in the
zonal winds between 850 and 200 hPa) (Sikka 1977). Further,
other large-scale synoptic environments which favor the LPS
genesis also includes the following: (i) upper-tropospheric
easterly waves, (ii) westward-moving residual low of tropical
cyclones from the Western Tropical Pacific–South China Sea
(WTP-SCS) region and (iii) slow descent of mid-tropospheric
cyclonic circulations (Sikka 2006). While in all other northern
hemispheric basins the cyclone activity peaks in July–August,
the strong vertical wind shear during summer monsoon season generally restricts the LPS activity over the Arabian Sea
and the BoB to further intensify into tropical cyclones (Gray
1968; Sikka 1977; Ding and Sikka 2006; see Chap. 8).
Accordingly, intense systems such as Cyclonic Storms and/or
Severe Cyclonic Storms (commonly referred hereafter as
simply Cyclonic storms throughout the text) very rarely form
in the summer monsoon season (e.g. Sikka 2006).
The spatio-temporal variations in monsoon rainfall are
often associated with the genesis and movement of the LPS,
and the associated rainfall distribution over its domain of
influence. According to the pioneering study by Eliot (1884),
the heaviest rainfall occurs in the southern quadrant of
monsoon depressions over the head BoB in the formative
stage, and in the southwest quadrant during its
west/west-northwest translation. Monsoon depressions typically produce heavy rainfall amounts of 30–60 cm day
−1
within the 200–300 km radius located in the southwestern
sector of depressions (Sikka 2006).
In addition to LPS, there is another distinct class of
summer monsoon (JJAS) synoptic systems known as
mid-tropospheric
cyclones
(MTCs)
which
are
quasi-stationary cold-core systems associated with the
strongest cyclonic vorticity between 700 and 500 hPa levels
(Miller and Keshavamurthy 1968; Krishnamurti and Hawkins 1970; Carr 1977; Mak 1983; Choudhury et al. 2018).
Further, MTCs show strong midlevel convergence, with
anomalous temperature field exhibiting cold (warm) signatures below (above) 500 hPa. MTCs seen over the Arabian
Sea have received special attention in recent times, as they
often produce flood-producing rainfall situations over the
western states of India (Maharashtra and Gujarat) during
JJAS. Choudhury et al. (2018) showed that some of the
heaviest 3-day rain accumulations over the western Indian
regions (e.g. south Gujarat and adjoining areas) during
1998–2007 co-occurred with MTC signatures. For example,
the MTC occurrence during the 24 June–3 July 2005 period
was associated with record 3-day rainfall accumulations of
700 mm at 72.7° E, 20.87° N located just north of Mumbai
on 28 June 2005. A few other cases include: the MTC event
during 9–20th July 2018 produced heavy rainfall over
Saurashtra, Kutch, Gujarat, and interior Maharashtra. The
extreme rainfall events over Mumbai that occurred on 29th
June, 1st July, and 5th September 2019 (24-h rainfall
accumulations exceeding 200 mm, as recorded at the Santa
Cruz observatory in Mumbai; Indian Daily Weather Report,
IMD) have co-occurred with MTCs seen over north Konkan
and adjoining south Gujarat region. Choudhury et al. (2018)
also showed that the formation of heavily precipitating
MTCs over western India has linkage to stratiform heating
structure within the northward propagating organized monsoon convection on sub-seasonal timescales. There are,
however, very limited studies on MTCs and ascertaining its
association with extreme rainfall events over western India
(Miller and Keshavamurthy 1968; Krishnamurti and Hawkins 1970; Carr 1977; Choudhury et al. 2018), and so far no
studies have documented the future projections in the MTCs.
Hence for JJAS period, we mainly focus on the present and
future changes in LPS characteristics.
7.1.2 Western Disturbances
During boreal winter and early spring season (December to
April; DJFMA), high-pressure conditions are prevalent over
north India and the associated weather is usually clear skies
and dry. The conditions of cloudy, dense fog, snow, and
light to heavy precipitation also occur intermittently during
this season by the eastward passage of synoptic-scale
weather disturbances, known as ‘western disturbances
(WDs)’, originating from the Mediterranean (Pisharoty and
Desai 1956; Mooley 1957; Singh and Kumar 1977; Kalsi
1980; Kalsi and Halder 1992; De et al. 2005; Schiemann
et al. 2009; Madhura et al. 2015; Cannon et al. 2015, 2016;
Dimri 2007, 2008, Dimri et al. 2015, Dimri and Chevuturi
2016; Krishnan et al. 2019; Hunt et al. 2018a, b). IMD
defined WDs as follows: a cyclonic circulation/trough in the
mid and lower tropospheric levels or as a low-pressure area
on the surface, which occurs in middle latitude westerlies
and originates over the Mediterranean Sea, Caspian Sea, and
Black Sea and moves eastwards across north India (http://
imd.gov.in/section/nhac/wxfaq.pdf). The WDs are basically
synoptic-scale perturbations embedded in subtropical westerly jet stream (STJs) at upper levels, and also latitudinal
positioning of these STJs has a greater influence on the
frequency of WDs (Hunt et al. 2018a).
The WDs are modulated by the tropical air mass and the
Himalayas. Accordingly, the WDs are preceded by warm
and moist air mass of tropical origin and succeeded by the
7 Synoptic Scale Systems
145
to the Indian landmass, and monsoon lows contribute to
about 40% of monsoon seasonal rains over the central Indian
landmass (Hurley and Boos 2015).
Generally, the development and intensification of LPS
have associations with warm sea surface temperatures (SSTs),
and environmental factors, such as the presence of low level
(850 hPa) cyclonic vorticity, high mid-tropospheric (500 hPa)
humidity and strong vertical wind shear (difference in the
zonal winds between 850 and 200 hPa) (Sikka 1977). Further,
other large-scale synoptic environments which favor the LPS
genesis also includes the following: (i) upper-tropospheric
easterly waves, (ii) westward-moving residual low of tropical
cyclones from the Western Tropical Pacific–South China Sea
(WTP-SCS) region and (iii) slow descent of mid-tropospheric
cyclonic circulations (Sikka 2006). While in all other northern
hemispheric basins the cyclone activity peaks in July–August,
the strong vertical wind shear during summer monsoon season generally restricts the LPS activity over the Arabian Sea
and the BoB to further intensify into tropical cyclones (Gray
1968; Sikka 1977; Ding and Sikka 2006; see Chap. 8).
Accordingly, intense systems such as Cyclonic Storms and/or
Severe Cyclonic Storms (commonly referred hereafter as
simply Cyclonic storms throughout the text) very rarely form
in the summer monsoon season (e.g. Sikka 2006).
The spatio-temporal variations in monsoon rainfall are
often associated with the genesis and movement of the LPS,
and the associated rainfall distribution over its domain of
influence. According to the pioneering study by Eliot (1884),
the heaviest rainfall occurs in the southern quadrant of
monsoon depressions over the head BoB in the formative
stage, and in the southwest quadrant during its
west/west-northwest translation. Monsoon depressions typically produce heavy rainfall amounts of 30–60 cm day
−1
within the 200–300 km radius located in the southwestern
sector of depressions (Sikka 2006).
In addition to LPS, there is another distinct class of
summer monsoon (JJAS) synoptic systems known as
mid-tropospheric
cyclones
(MTCs)
which
are
quasi-stationary cold-core systems associated with the
strongest cyclonic vorticity between 700 and 500 hPa levels
(Miller and Keshavamurthy 1968; Krishnamurti and Hawkins 1970; Carr 1977; Mak 1983; Choudhury et al. 2018).
Further, MTCs show strong midlevel convergence, with
anomalous temperature field exhibiting cold (warm) signatures below (above) 500 hPa. MTCs seen over the Arabian
Sea have received special attention in recent times, as they
often produce flood-producing rainfall situations over the
western states of India (Maharashtra and Gujarat) during
JJAS. Choudhury et al. (2018) showed that some of the
heaviest 3-day rain accumulations over the western Indian
regions (e.g. south Gujarat and adjoining areas) during
1998–2007 co-occurred with MTC signatures. For example,
the MTC occurrence during the 24 June–3 July 2005 period
was associated with record 3-day rainfall accumulations of
700 mm at 72.7° E, 20.87° N located just north of Mumbai
on 28 June 2005. A few other cases include: the MTC event
during 9–20th July 2018 produced heavy rainfall over
Saurashtra, Kutch, Gujarat, and interior Maharashtra. The
extreme rainfall events over Mumbai that occurred on 29th
June, 1st July, and 5th September 2019 (24-h rainfall
accumulations exceeding 200 mm, as recorded at the Santa
Cruz observatory in Mumbai; Indian Daily Weather Report,
IMD) have co-occurred with MTCs seen over north Konkan
and adjoining south Gujarat region. Choudhury et al. (2018)
also showed that the formation of heavily precipitating
MTCs over western India has linkage to stratiform heating
structure within the northward propagating organized monsoon convection on sub-seasonal timescales. There are,
however, very limited studies on MTCs and ascertaining its
association with extreme rainfall events over western India
(Miller and Keshavamurthy 1968; Krishnamurti and Hawkins 1970; Carr 1977; Choudhury et al. 2018), and so far no
studies have documented the future projections in the MTCs.
Hence for JJAS period, we mainly focus on the present and
future changes in LPS characteristics.
7.1.2 Western Disturbances
During boreal winter and early spring season (December to
April; DJFMA), high-pressure conditions are prevalent over
north India and the associated weather is usually clear skies
and dry. The conditions of cloudy, dense fog, snow, and
light to heavy precipitation also occur intermittently during
this season by the eastward passage of synoptic-scale
weather disturbances, known as ‘western disturbances
(WDs)’, originating from the Mediterranean (Pisharoty and
Desai 1956; Mooley 1957; Singh and Kumar 1977; Kalsi
1980; Kalsi and Halder 1992; De et al. 2005; Schiemann
et al. 2009; Madhura et al. 2015; Cannon et al. 2015, 2016;
Dimri 2007, 2008, Dimri et al. 2015, Dimri and Chevuturi
2016; Krishnan et al. 2019; Hunt et al. 2018a, b). IMD
defined WDs as follows: a cyclonic circulation/trough in the
mid and lower tropospheric levels or as a low-pressure area
on the surface, which occurs in middle latitude westerlies
and originates over the Mediterranean Sea, Caspian Sea, and
Black Sea and moves eastwards across north India (http://
imd.gov.in/section/nhac/wxfaq.pdf). The WDs are basically
synoptic-scale perturbations embedded in subtropical westerly jet stream (STJs) at upper levels, and also latitudinal
positioning of these STJs has a greater influence on the
frequency of WDs (Hunt et al. 2018a).
The WDs are modulated by the tropical air mass and the
Himalayas. Accordingly, the WDs are preceded by warm
and moist air mass of tropical origin and succeeded by the
7 Synoptic Scale Systems
145