(https://sandrp.in/2018/07/21/uttrakhand-cloudburst-incidents2018/).
A comprehensive documentation of 30 instances of
cloudbursts and associated damages in the Himalayas from
1970 is shown in Dimri et al. (2017). Several researchers
also highlight the role of large-scale forcing and anomalous
atmospheric circulation conditions leading to flood situations
in and along the western Himalayas (e.g., Houze et al. 2011;
Vellore et al. 2016) with implications to cloudburst instances
though not directly. The modeling community observes
large limitations to capture the cloudburst events due to
outstanding issues such as understanding of microscale
interactions with rugged and variable orography, paucity of
observations, and representation of physical processes at a
much-localized scale setting. The essentiality of more
modeling studies with multiple events and observations
for better understanding these severe convective events over
the Himalayan region is also clearly suggested. Indian scientists have additionally coined another new term
“mini-cloudbursts (more than 5 cm in two consecutive
hours)” (Deshpande et al. 2018) to define incidences of
heavy rains over short period of time. Instances include
heavy rainfall in Mumbai during the ISM season, e.g.,
944 mm rainfall on July 27, 2005, and 304 mm rainfall on
September 20, 2017, that caused massive urban flood. The
new definition distinguishes the cloudbursts associated with
high topography (e.g., Leh event in August 2010; Uttarakhand event in June 2013) and rainfall incidences over the
plain region exceeding 10 cm h
−1 that can result in urban
flooding. The aforesaid Mumbai events fall under
mini-cloudburst classification.
8.3.1 Historical Changes
The thunderstorm data available for the Indian region has
been comprehensively documented in the recent studies of
Tyagi (2007), Bharadwaj et al. (2017), Bharadwaj and Singh
(2018). Based on the compiled dataset, Fig. 8.5 shows the
state-wise distribution of thunderstorm events over the
Indian region during the 1978–2012 period. Five states
(West Bengal, Orissa, Bihar, Assam, and Jharkhand) were
the worst hit of thunderstorms in terms of fatalities, injuries,
and casualties. West Bengal experienced most intense
thunderstorm events and highest casualties during this period. Maharashtra and Kerala experienced 147 and 79 events,
respectively, during this period, the fatalities and casualties
are comparatively less in these states. Delhi experienced
only seven events with significant injuries and casualties
(Bhardwaj et al. 2017). Figure 8.6 shows mean number of
thunderstorm days for different seasons over the Indian
region for two periods 1951–1980 and 1981–2010. On the
annual scale, high thunderstorm activity days (100 days or
more) are seen over northern Assam, Meghalaya, and West
Bengal. The annual frequency of thunderstorm days latitudinally increases from lower to higher latitudes; however,
there is a sharp decline in their frequency noticed from
1980s, i.e., during the 1981–2010 period, relative to 1951–
1980 time period, there was 34% decline of frequency of
thunderstorm days (Bhardwaj and Singh 2018; Singh and
Bhardwaj 2019). The observed changes in thunderstorm
activity are found to be mostly dependent upon latitude and
season, and they are consistent with the seasonal migration
of the inter-tropical convergence zone (ITCZ) and the solar
heating of the Indian landmass (Manohar et al. 1999;
Manohar and Kesarkar 2005) along with strong regional
influences from the topography. The thunderstorm activity
also exhibits an increasing number from the western side of
the subcontinent and moves northeastwards toward the
Himalayan foothills, and more notably the highest and
lowest number of thunderstorm days are observed over the
mountainous terrain of Jammu and Kashmir and Ladakh
region, respectively. Over the Gangetic plains, West Bengal
and surrounding regions record between 80 and 100 days of
thunderstorm activity annually while Kerala records the
highest (80–100 days) thunderstorm frequency over the
peninsular regions of India (Tyagi 2007).
The pre-monsoon increase in thunderstorm activity is
primarily attributed to the topography, insolation, and
advection of moisture under favorable wind conditions. The
spatial distribution of thunderstorm occurrences during
pre-monsoon season indicates a maximum occurrence over
the north, northeast, and southern parts of India (Fig. 8.6).
The highest frequency of thunderstorms is noticed over the
country during monsoon season as a whole, and mostly over
the northern and northeastern parts of India. During the
post-monsoon season, highest number of thunderstorms
occurs over Kerala and the neighboring state of Tamil Nadu.
Lowest number of thunderstorms over India is generally
observed during the winter season due to stable and dry
atmospheric conditions prevailing over most parts of the
Indian subcontinent. The declining thunderstorm activity
over the Indian subcontinent is suggestively attributed to
reductions in rainfall activity and in the moisture amount,
due to a fall in the frequency of monsoon depressions, and
enhanced intensities of natural variability sources such as
ENSO, and PDO. Also, a recent study notes that there is a
decline in the dust loading of the atmosphere, or a decrease
in intensity of dust storms, during the period 2000–2017 due
to increasing pre-monsoon rains over the northwestern states
and the Indo-Gangetic Plains (Pandey et al. 2017). Das
(2015a) suggests that frequency of cloudburst events in the
western Himalayan region has been on the continuous rise
due to faster evaporation rates from glacial lakes at high
altitudes, as a consequence of global warming. It is also
noted that most of the cloudburst events are reported from
166
R. K. Vellore et al.
A comprehensive documentation of 30 instances of
cloudbursts and associated damages in the Himalayas from
1970 is shown in Dimri et al. (2017). Several researchers
also highlight the role of large-scale forcing and anomalous
atmospheric circulation conditions leading to flood situations
in and along the western Himalayas (e.g., Houze et al. 2011;
Vellore et al. 2016) with implications to cloudburst instances
though not directly. The modeling community observes
large limitations to capture the cloudburst events due to
outstanding issues such as understanding of microscale
interactions with rugged and variable orography, paucity of
observations, and representation of physical processes at a
much-localized scale setting. The essentiality of more
modeling studies with multiple events and observations
for better understanding these severe convective events over
the Himalayan region is also clearly suggested. Indian scientists have additionally coined another new term
“mini-cloudbursts (more than 5 cm in two consecutive
hours)” (Deshpande et al. 2018) to define incidences of
heavy rains over short period of time. Instances include
heavy rainfall in Mumbai during the ISM season, e.g.,
944 mm rainfall on July 27, 2005, and 304 mm rainfall on
September 20, 2017, that caused massive urban flood. The
new definition distinguishes the cloudbursts associated with
high topography (e.g., Leh event in August 2010; Uttarakhand event in June 2013) and rainfall incidences over the
plain region exceeding 10 cm h
−1 that can result in urban
flooding. The aforesaid Mumbai events fall under
mini-cloudburst classification.
8.3.1 Historical Changes
The thunderstorm data available for the Indian region has
been comprehensively documented in the recent studies of
Tyagi (2007), Bharadwaj et al. (2017), Bharadwaj and Singh
(2018). Based on the compiled dataset, Fig. 8.5 shows the
state-wise distribution of thunderstorm events over the
Indian region during the 1978–2012 period. Five states
(West Bengal, Orissa, Bihar, Assam, and Jharkhand) were
the worst hit of thunderstorms in terms of fatalities, injuries,
and casualties. West Bengal experienced most intense
thunderstorm events and highest casualties during this period. Maharashtra and Kerala experienced 147 and 79 events,
respectively, during this period, the fatalities and casualties
are comparatively less in these states. Delhi experienced
only seven events with significant injuries and casualties
(Bhardwaj et al. 2017). Figure 8.6 shows mean number of
thunderstorm days for different seasons over the Indian
region for two periods 1951–1980 and 1981–2010. On the
annual scale, high thunderstorm activity days (100 days or
more) are seen over northern Assam, Meghalaya, and West
Bengal. The annual frequency of thunderstorm days latitudinally increases from lower to higher latitudes; however,
there is a sharp decline in their frequency noticed from
1980s, i.e., during the 1981–2010 period, relative to 1951–
1980 time period, there was 34% decline of frequency of
thunderstorm days (Bhardwaj and Singh 2018; Singh and
Bhardwaj 2019). The observed changes in thunderstorm
activity are found to be mostly dependent upon latitude and
season, and they are consistent with the seasonal migration
of the inter-tropical convergence zone (ITCZ) and the solar
heating of the Indian landmass (Manohar et al. 1999;
Manohar and Kesarkar 2005) along with strong regional
influences from the topography. The thunderstorm activity
also exhibits an increasing number from the western side of
the subcontinent and moves northeastwards toward the
Himalayan foothills, and more notably the highest and
lowest number of thunderstorm days are observed over the
mountainous terrain of Jammu and Kashmir and Ladakh
region, respectively. Over the Gangetic plains, West Bengal
and surrounding regions record between 80 and 100 days of
thunderstorm activity annually while Kerala records the
highest (80–100 days) thunderstorm frequency over the
peninsular regions of India (Tyagi 2007).
The pre-monsoon increase in thunderstorm activity is
primarily attributed to the topography, insolation, and
advection of moisture under favorable wind conditions. The
spatial distribution of thunderstorm occurrences during
pre-monsoon season indicates a maximum occurrence over
the north, northeast, and southern parts of India (Fig. 8.6).
The highest frequency of thunderstorms is noticed over the
country during monsoon season as a whole, and mostly over
the northern and northeastern parts of India. During the
post-monsoon season, highest number of thunderstorms
occurs over Kerala and the neighboring state of Tamil Nadu.
Lowest number of thunderstorms over India is generally
observed during the winter season due to stable and dry
atmospheric conditions prevailing over most parts of the
Indian subcontinent. The declining thunderstorm activity
over the Indian subcontinent is suggestively attributed to
reductions in rainfall activity and in the moisture amount,
due to a fall in the frequency of monsoon depressions, and
enhanced intensities of natural variability sources such as
ENSO, and PDO. Also, a recent study notes that there is a
decline in the dust loading of the atmosphere, or a decrease
in intensity of dust storms, during the period 2000–2017 due
to increasing pre-monsoon rains over the northwestern states
and the Indo-Gangetic Plains (Pandey et al. 2017). Das
(2015a) suggests that frequency of cloudburst events in the
western Himalayan region has been on the continuous rise
due to faster evaporation rates from glacial lakes at high
altitudes, as a consequence of global warming. It is also
noted that most of the cloudburst events are reported from
166
R. K. Vellore et al.