pronounced during the onset and withdrawal phase months
(June and September). The eastern and northeastern states of
India (West Bengal, Bihar, Assam, Jharkhand, and Orissa)
and Bangladesh experience a special type of violent thunderstorms known as “Nor’westers (also known as
Kal-Baisakhi by the locals; Desai 1950)” during the
pre-monsoon months (March–May). These storms are triggered by the intense convection activity over the
Chhotanagpur Plateau (Jharkhand region) and are generally
associated with sudden violent gusty winds from northwest,
known as squalls, with wind speeds exceeding 28 m s
−1 .
These are usually accompanied with heavy rainfall (about
100 mm h
−1 ) and oftentimes hail of sizes 30 mm in diameter, causing flash floods, severe property and agricultural
damages (e.g., Chakrabarty et al. 2007; Pradhan et al. 2012).
Thunderstorm is always characterized by cumulonimbus
cloud type, in which individual cells produce copious rains,
and sometimes hail, strong winds, and even occasional tornadoes in northeast India and Bangladesh from Nor’westers,
e.g., a F3 tornado (on the Fujita–Pearson scale) over Rajkanika block of Kendrapara district of Orissa, India, on 31
March 2009 (Litta et al. 2012). Over the southern and
northeastern parts of India, the pre-monsoon thundershowers
are called by various names in conjunction with rain
favorable to local plantations, viz Tea showers in Assam,
Mango showers in Kerala, Konkan, and Goa, and Coffee
showers in Kerala. Thunderstorms in India generally fall
under two categories: strong rains and maximum wind
speeds ranging from 8 to 20 m s
−1 with loud blasts of
thunder and frequent lightning flashes in the category of
moderate thunderstorms, while continuous thunder and
lightning with strong rains and wind speeds >20 m s
−1 in the
category of severe thunderstorms. The lightning activity
from thunderstorms is also significantly noticeable despite
lower values of convective available potential energy, over
the northern, central India and Bangladesh as compared to
other parts of Indian subcontinent (Murugavel et al. 2014).
Based on the genesis and life cycle of thunderstorms, they
are grouped into different types, viz single-cell thunderstorms (a single cloud cell moving independently without
combining with any other cell with a life cycle of 30 min to
1 h, moves slowly with mean environmental wind, and
generally the vertical wind shear is small), multiple-cell
thunderstorms (consists of a series of evolving cells with a
life cycle of 3–6 h, possible hail storms and damaging winds
and isolated tornadoes and travel over a few hundreds of
kilometers), and squall lines (a chain of thunderstorms
connected together and moving like a single entity usually
accompanied by high-speed winds). Regardless of the type
of thunderstorms, they go through three stages: the cumulus
stage, the mature stage, and the dissipation stage.
In addition, two types of high-impact dust storms known
as “Andhi” co-occur with thunderstorms of northwest India
during the later part of the pre-monsoon season following
the development of a heat low over this region. These dust
storms are essentially thunderstorms in the arid regions of
northwest India, where the local instability mechanisms are
similar to thunderstorms and occur in association with large
cumulus or cumulonimbus clouds (Raipal and Deka 1980;
Middleton 1986). The first type is pressure-gradient dust
storm which is characterized by higher temperatures and
relatively low moisture content in the lowest atmospheric
layers which makes the thunderstorms to have much higher
cloud bases above the ground (3–4 km). On occasions, a
strong pressure gradient develops to the south of the heat
low causing strong winds lifting the dust and sand that are
amply available in this region. Another type of dust storm is
convective dust storm which develops in association with
strong negatively buoyant downdraft of cumulonimbus
cloud. The pressure-gradient dust storms generally last for
longer periods with suspended dust in air, whereas convective dust storms last for a few minutes to few hours. The
annual mean frequencies of Andhi are higher over the Indian
states of Rajasthan, Punjab, Haryana, and Delhi. India is also
among the countries in the world having large frequency of
hail storms, associated with severe thunderstorms in association with multiple updrafts and downdrafts. These are
more commonly seen along the Himalayan foothills causing
severe large-scale agricultural damages (De et al. 2005). The
frequency of hailstorms is small in winter; however, it
increases from February and peaks during the pre-monsoon
months (March and April). Hailstorms are rare during the
ISM season. Unprecedented hailstorms lasting for a week or
more are observed on occasions over north peninsular India
(Kulkarni et al. 2015).
Generally, cloudburst is a severe weather outbreak feature
commonly seen along the southern Himalayan slopes of the
Indian subcontinent during the ISM months (Dimri et al.
2017; Deshpande et al. 2018). These are associated with
thunderstorms where strong updrafts from intense vortices
on smaller scale tend to hold up a large amount of water and
upon sudden cessation results in catastrophic rainfall in a
short period of time (greater than 10 cm h
−1 ) concentrated
over a limited geographical area (Bhan et al. 2004; Dimri
et al. 2017; see references therein). Romatschke and Houze
(2011) indicated that precipitation is predominantly associated with smaller spatial scale but highly convective systems
along the western Himalayan region and bulk of the cloudburst episodes are observed between 1000 m and 2500 m
elevations of the Greater Himalayas. Das et al. (2006) quote
“Cloudbursts in India occur when monsoon clouds associated with low-pressure area travel northward from the Bay of
Bengal across the Ganges Plains onto the Himalayas and
‘burst’ in heavy downpours.” For example, there were seven
cloudburst instances witnessed over the western Himalayan
state of Uttarakhand during the ISM season of 2018
8 Extreme Storms
165
(June and September). The eastern and northeastern states of
India (West Bengal, Bihar, Assam, Jharkhand, and Orissa)
and Bangladesh experience a special type of violent thunderstorms known as “Nor’westers (also known as
Kal-Baisakhi by the locals; Desai 1950)” during the
pre-monsoon months (March–May). These storms are triggered by the intense convection activity over the
Chhotanagpur Plateau (Jharkhand region) and are generally
associated with sudden violent gusty winds from northwest,
known as squalls, with wind speeds exceeding 28 m s
−1 .
These are usually accompanied with heavy rainfall (about
100 mm h
−1 ) and oftentimes hail of sizes 30 mm in diameter, causing flash floods, severe property and agricultural
damages (e.g., Chakrabarty et al. 2007; Pradhan et al. 2012).
Thunderstorm is always characterized by cumulonimbus
cloud type, in which individual cells produce copious rains,
and sometimes hail, strong winds, and even occasional tornadoes in northeast India and Bangladesh from Nor’westers,
e.g., a F3 tornado (on the Fujita–Pearson scale) over Rajkanika block of Kendrapara district of Orissa, India, on 31
March 2009 (Litta et al. 2012). Over the southern and
northeastern parts of India, the pre-monsoon thundershowers
are called by various names in conjunction with rain
favorable to local plantations, viz Tea showers in Assam,
Mango showers in Kerala, Konkan, and Goa, and Coffee
showers in Kerala. Thunderstorms in India generally fall
under two categories: strong rains and maximum wind
speeds ranging from 8 to 20 m s
−1 with loud blasts of
thunder and frequent lightning flashes in the category of
moderate thunderstorms, while continuous thunder and
lightning with strong rains and wind speeds >20 m s
−1 in the
category of severe thunderstorms. The lightning activity
from thunderstorms is also significantly noticeable despite
lower values of convective available potential energy, over
the northern, central India and Bangladesh as compared to
other parts of Indian subcontinent (Murugavel et al. 2014).
Based on the genesis and life cycle of thunderstorms, they
are grouped into different types, viz single-cell thunderstorms (a single cloud cell moving independently without
combining with any other cell with a life cycle of 30 min to
1 h, moves slowly with mean environmental wind, and
generally the vertical wind shear is small), multiple-cell
thunderstorms (consists of a series of evolving cells with a
life cycle of 3–6 h, possible hail storms and damaging winds
and isolated tornadoes and travel over a few hundreds of
kilometers), and squall lines (a chain of thunderstorms
connected together and moving like a single entity usually
accompanied by high-speed winds). Regardless of the type
of thunderstorms, they go through three stages: the cumulus
stage, the mature stage, and the dissipation stage.
In addition, two types of high-impact dust storms known
as “Andhi” co-occur with thunderstorms of northwest India
during the later part of the pre-monsoon season following
the development of a heat low over this region. These dust
storms are essentially thunderstorms in the arid regions of
northwest India, where the local instability mechanisms are
similar to thunderstorms and occur in association with large
cumulus or cumulonimbus clouds (Raipal and Deka 1980;
Middleton 1986). The first type is pressure-gradient dust
storm which is characterized by higher temperatures and
relatively low moisture content in the lowest atmospheric
layers which makes the thunderstorms to have much higher
cloud bases above the ground (3–4 km). On occasions, a
strong pressure gradient develops to the south of the heat
low causing strong winds lifting the dust and sand that are
amply available in this region. Another type of dust storm is
convective dust storm which develops in association with
strong negatively buoyant downdraft of cumulonimbus
cloud. The pressure-gradient dust storms generally last for
longer periods with suspended dust in air, whereas convective dust storms last for a few minutes to few hours. The
annual mean frequencies of Andhi are higher over the Indian
states of Rajasthan, Punjab, Haryana, and Delhi. India is also
among the countries in the world having large frequency of
hail storms, associated with severe thunderstorms in association with multiple updrafts and downdrafts. These are
more commonly seen along the Himalayan foothills causing
severe large-scale agricultural damages (De et al. 2005). The
frequency of hailstorms is small in winter; however, it
increases from February and peaks during the pre-monsoon
months (March and April). Hailstorms are rare during the
ISM season. Unprecedented hailstorms lasting for a week or
more are observed on occasions over north peninsular India
(Kulkarni et al. 2015).
Generally, cloudburst is a severe weather outbreak feature
commonly seen along the southern Himalayan slopes of the
Indian subcontinent during the ISM months (Dimri et al.
2017; Deshpande et al. 2018). These are associated with
thunderstorms where strong updrafts from intense vortices
on smaller scale tend to hold up a large amount of water and
upon sudden cessation results in catastrophic rainfall in a
short period of time (greater than 10 cm h
−1 ) concentrated
over a limited geographical area (Bhan et al. 2004; Dimri
et al. 2017; see references therein). Romatschke and Houze
(2011) indicated that precipitation is predominantly associated with smaller spatial scale but highly convective systems
along the western Himalayan region and bulk of the cloudburst episodes are observed between 1000 m and 2500 m
elevations of the Greater Himalayas. Das et al. (2006) quote
“Cloudbursts in India occur when monsoon clouds associated with low-pressure area travel northward from the Bay of
Bengal across the Ganges Plains onto the Himalayas and
‘burst’ in heavy downpours.” For example, there were seven
cloudburst instances witnessed over the western Himalayan
state of Uttarakhand during the ISM season of 2018
8 Extreme Storms
165