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Variability and Trends of Extreme Rainfall and Rainstorms
Abstract
In this chapter, variability and long-term trends of heavy rainfall events and rainstorms over India during the monsoon season (June to September) are documented. For analyzing extreme rainfall events, rain gauge station and gridded data for the period 1901–2010 have been used. For studying rainstorms, another gridded rainfall data set of 1951–2010 also has been used. The trend analyses revealed increasing trends in the frequency of dry days in most parts of the country during the winter, pre-monsoon, and southwest monsoon seasons. Frequency of very light rain and light-to-moderate rain events has decreased significantly over most of the country. Both the station and gridded data have shown significant increasing trends of very heavy to extremely heavy rainfall events over most parts of the country. Over Central India, extreme rainfall events show significant decadal variations which could be related to variations in sea surface temperatures over the tropical oceans. Over the period 1901–2010, heavy rainfall events show an increasing trend of six percent per decade. Further, an analysis is made on rainstorms over North India where majority of rainstorms cause floods over North India. Frequency of rainstorms has shown an increasing trend of 4 rainstorms (50 % increase) in 65 years (1951–2015). Similarly, the duration of rainstorms has shown an increase of about 15 days (80 % increase) during the period 1951–2015, which is also significant. Both the increases are statistically significant at the 99 % confidence level.
... As per a study that analyzed rainstorms over North India which lead to largescale floods, it was found that the frequency of these rainstorms increased at 6% per decade between 1901-2009 and the duration of rainstorms increased by 15 days over the period 1951-2015 (Guhathakurta & Pai, 2017 An IPCC report titled "The Ocean and Cryosphere in a Changing Climate" investigates how climate change impacts oceans and frozen bodies. India will face the brunt from melting of the ice in the Hindu Kush region, which holds largest reserves of water in the region. ...
Traditionally, water sector research was focused on minimizing human health
and ecosystem risks, but recent focus on climate variability added another
dimension of water-health complexities. Consequently, significant progress
has been made in understanding the intricacies of water-health system in a
changing climate. The consequences of climate variability and induced water
insecurity will be more severe for water sustainability and health risks. In this
background, water utilities of the future need to develop the capabilities and
foster innovation to deal with the magnitudes of climate variability. Academic
research highlighted the significance of sustainability of drinking water service
provisioning and recommend water safety plan (WSP) to achieve the goal.
However, little attention was paid to the capabilities and innovation required
to achieve integrated WSP and to deal with the intricacies of water-health
systems under climate variability, specifically in developing countries such as
India. The Indian water utilities are struggling with the lack of capabilities to
deal with inefficient management, inter-institutional coordination, engaging
stakeholders, achieving financial stability, and holistic implementation of
strategies. To investigate such limitations, the case-study approach is adopted
to understand the complexity of the water-health system and capabilities
required for implementing climate integrated WSP in the Indian context.
The study showed that performance-based public-private partnership was
effective in identifying and assessing risks at earlier stages and improving
the capabilities to integrate climate variability in planning. The rationale
and illustration calls for engaging expertise, collaboration, and technology
diffusion in Indian water utilities to achieve Sustainable Development Goal 6.
... As per a study that analyzed rainstorms over North India which lead to largescale floods, it was found that the frequency of these rainstorms increased at 6% per decade between 1901-2009 and the duration of rainstorms increased by 15 days over the period 1951-2015 (Guhathakurta & Pai, 2017 An IPCC report titled "The Ocean and Cryosphere in a Changing Climate" investigates how climate change impacts oceans and frozen bodies. India will face the brunt from melting of the ice in the Hindu Kush region, which holds largest reserves of water in the region. ...
... 70 For the period 1901-2010, there is a decrease in frequencies of lower intensity rainfall and an increase in higher intensities (heavy to very heavy rainfall) during the southwest monsoon season over most parts of India. 71 The All India Summer Monsoon Rain (AISMR) for the period 1871-1993 has seen a significant decreasing trend over central and major parts of North India for annual frequency of rain, rainy and heavy rainfall days, and increasing trends over Peninsular India for annual frequency of rain, rainy and heavy rainfall days. 72 The findings also indicate that generally, the area coverage of very heavy rainfall events is more during the recent 55 years (1961-2015) compared to 6 decades before that. ...
This note intends to pave a conceptual and methodological frame for an
inaugural multidisciplinary scholarship on the inequality and autonomous
adaptation to climate change in the Indian scenario. It begins with
providing a brief background on climate change responses and uncovers
the disconnect in the existing climate change knowledge in social sciences.
It argues that the knowledge on climate change in social sciencesespecially on inequality, vulnerability, and adaptation, lacks the discourses
from the ground. This disconnect perhaps take a toll on the existing
adaptation policies. Therefore, it argues that the intuitive and spontaneous
responses of people and communities towards coping and adapting to
climate change known as autonomous adaptation has potential scope for
improving the adequate and equitable resilience policies and 'governances
of adaptation' in the regime of climate change. It also raises the need for
shifting our attention and scholarly engagements towards investigating
autonomous adaptation to climate change as potential scope for bridging
the existing disconnect in climate change knowledge. It further presents
the geographical locations in India which are more exposed towards
the effects of climate change as an appropriate site for ethnographic
inquiry on the process and other dynamics of autonomous adaptation.
Besides, it draws the theoretical and methodological approaches emerging
in different disciplines in social sciences such as economics, political
science and anthropology to move forward to study both inequality and
autonomous adaptation to climate change.
... As per a study that analyzed rainstorms over North India which lead to largescale floods, it was found that the frequency of these rainstorms increased at 6% per decade between 1901-2009 and the duration of rainstorms increased by 15 days over the period 1951-2015 (Guhathakurta & Pai, 2017 An IPCC report titled "The Ocean and Cryosphere in a Changing Climate" investigates how climate change impacts oceans and frozen bodies. India will face the brunt from melting of the ice in the Hindu Kush region, which holds largest reserves of water in the region. ...
Upward changes in mean, maximum or minimum temperature in a given place
and time, has been traditionally used to quantify heat wave. Heat wave is
an extreme weather condition, and adversely affecting human health and
its ecology. This has prompted several countries to adopt alert and warning
mechanisms. Objectively defining the heat wave is very diverse, and complex.
India and other countries issue an area specific heat wave warning notice
by measuring changes in maximum surface temperature and heat index.
However, lack of uniform definition makes it difficult to comprehend this
global phenomenon from a public health intervention point of view. In this
context we examined the definition which is more suitable for public health
preparedness purposes. This study analyzed diverse heat wave definitions
from literature and national weather forecasting systems and compared the
public health significance of each definition to identify the most suitable
scientific definition for Indian context. The realistic measures consider
intensity, duration, frequency, humidity and/or spatial extent to characterize
heat wave. All the existing heat wave definitions consider at least one form
of temperature (daily max, min, or average) with a predefined threshold for
certain duration. There is evidence of a dual-threshold of increasing mortality
risk. Rise in minimum temperature has more deleterious effect than rise in
maximum temperature. Heat index that considers humidity has been proved to
be more meaningful to describe the deleterious effect of heat wave. Machine
learning-based location-specific dynamic threshold for minimum and maximum
temperature with due consideration for humidity and linked morbidity and
mortality would be more useful to characterize heat wave in Indian settings.
... As per a study that analyzed rainstorms over North India which lead to largescale floods, it was found that the frequency of these rainstorms increased at 6% per decade between 1901-2009 and the duration of rainstorms increased by 15 days over the period 1951-2015 (Guhathakurta & Pai, 2017 An IPCC report titled "The Ocean and Cryosphere in a Changing Climate" investigates how climate change impacts oceans and frozen bodies. India will face the brunt from melting of the ice in the Hindu Kush region, which holds largest reserves of water in the region. ...
Compendium of Case Studies, under HER-CAP (Health Resilience and Capacity Building) project supported by WHO at Centre for Excellence on Climate Resilience (CECR) at NIDM New Delhi. Comprises 24 Case studies and thematic chapters. Editors: Gupta, Anil Kumar, et al. 2021. ISBN 978-93-82571-52-0
... The anticyclonic circulation anomaly prevailing over NEC resulted in more frequent precipitation, a finding similar to the results of Wang et al. (2017). The western Pacific subtropical high is one of the most important circulation systems in the western Pacific and East Asia (Gao and Shi, 2016;Guhathakurta et al., 2017;Wang et al., 2017). The stronger and longer the western Pacific subtropical high, the more frequent and the stronger the precipitation is in eastern China (Ding et al., 2010). ...
Understanding the trends of precipitation concentration play a key role in watershed development and management. In this study, spatiotemporal and abrupt changes in the concentration index (CI) and their relationships with summer large-scale atmospheric circulations over Northeast China (NEC) were discovered. We used daily precipitation data from 71 meteorological stations and five large-scale atmosphere circulation indices for the period 1961–2016 across NEC. The results show that annual daily precipitation concentration in NEC decreased slightly in most areas. The spatial distribution of the CI ranges from 0.64 to 0.7 and the regions of the highest value were in Liaoning Province. The correlation between CI and different large-scale atmospheric indices show significant differences and mainly depend on the regional differences. Pacific Decadal Oscillation (PDO) and North Atlantic Oscillation (NAO) are negatively correlated with the CI in most regions of NEC, in contrast to the East Asian summer monsoon index (EASMI) and Multivariate ENSO Index (MEI). The Southern Oscillation Index (SOI) is negatively correlated with the CI in Liaoning Province and positivity correlated with the CI in Jilin and most of Heilongjiang Province. Compared to before 1975, a Eurasian continent anticyclonic circulation anomaly caused more clear days and higher solar radiation during 1976–2016.
... Past studies have demonstrated that there is an increase in the moisture content of the atmosphere over the Indian region 23 , and this rise is attributed for the increasing trend in extreme rainfall events over central India 24,25 . In addition, studies have showed that an increase in surface warming leads to a rise in the moisture content of the atmosphere over the Indian region 26,27 . Hunt 28 confirmed from a composite of 106 depressions over the BoB that there is a high resemblance between RH and cloud cover in the south western quadrant of MDs with a precipitation maxima. ...
In this study, a comprehensive investigation is carried out to examine the sensitivity of tropospheric relative humidity (RH) on monsoon depressions (MDs) under a changing climate regime through surrogate climate change approach over the Indian region. Composite analysis of four MDs show a persistent warming (RH2+) and cooling (RH2-) throughout the troposphere in the sensitivity experiments. In-depth analysis of a MD over the Arabian Sea (AS) exhibits sustained warming for RH2+, which is accredited to 2.6% increase in stratiform clouds accounting for 13% increment in heating, whereas 5% increment in convective clouds hardly contribute to total heating. Frozen hydrometeors (graupel and snow) are speculated to be the major contributors to this heating. Stratiform clouds showed greater sensitivity to RH perturbations in the lower troposphere (1000-750 hPa), albeit very less sensitivity for convective clouds, both in the lower and mid-troposphere (700-500 hPa). Precipitation is enhanced in a moist situation (RH2+) owing to positive feedbacks induced by moisture influx and precipitation efficiency, while negative feedbacks suppressed precipitation in a dry troposphere (RH2-). In a nutshell, it is inferred that under moist (dry) situations, it is highly likely that intense (weak) MDs will occur in the near future over the Indian region.
The flood risk across the globe is intensified due to global warming and subsequent increase in extreme temperature and precipitation. The long-term trends in extreme rainfall (1944–2013) and temperature (1969–2012) indices have been investigated at annual, seasonal, and monthly time scales using nonparametric Mann-Kendall (MK), modified Mann-Kendall (MMK), and Sen’s slope estimator tests. The extreme rainfall and temperature indices, recommended by the Expert Team on Climate Change Detection Monitoring Indices (ETCCDMI), have been analyzed at finer spatial scales for trend detection. The results of trend analyses indicate decreasing trend in annual total rainfall, significant decreasing trend in rainy days, and increasing trend in rainfall intensity over the basin. The seasonal rainfall has been found to decrease for all the seasons except postmonsoon, which could affect the rain-fed agriculture in the basin. The 1- and 5-day annual maximum rainfalls exhibit mixed trends, wherein part of the basin experiences increasing trend, while other parts experience a decreasing trend. The increase in dry spells and concurrent decrease in wet spells are also observed over the basin. The extreme temperature indices revealed increasing trends in hottest and coldest days, while decreasing trends in coldest night are found over most parts of the basin. Further, the diurnal temperature range is also found to increase due to warming tendency in maximum temperature (Tmax) at a faster rate compared to the minimum temperature (Tmin). The increase in frequency and magnitude of extreme rainfall in the basin has been attributed to the increasing trend in maximum and minimum temperatures, reducing forest cover, rapid pace of urbanization, increase in human population, and thereby increase in the aerosol content in the atmosphere. The findings of the present study would significantly help in sustainable water resource planning, better decision-making for policy framework, and setting up infrastructure against flood disasters in Upper Tapi Basin, India.
Socioeconomic challenges continue to mount for half a billion residents of central India because of a decline in the total rainfall and a concurrent rise in the magnitude and frequency of extreme rainfall events. Alongside a weakening monsoon circulation, the locally available moisture and the frequency of moisture-laden depressions from the Bay of Bengal have also declined. Here we show that despite these negative trends, there is a threefold increase in widespread extreme rain events over central India during 1950–2015. The rise in these events is due to an increasing variability of the low-level monsoon westerlies over the Arabian Sea, driving surges of moisture supply, leading to extreme rainfall episodes across the entire central subcontinent. The homogeneity of these severe weather events and their association with the ocean temperatures underscores the potential predictability of these events by two-to-three weeks, which offers hope in mitigating their catastrophic impact on life, agriculture and property.
Using a high-resolution daily gridded rainfall data set for the period 1951–2015, new objective criteria were developed to determine rainstorms over the Indian region during the south-west monsoon season (June to September). The rainstorms thus identified have rainfall of 125 mm/day or more at the centre, cover minimum 50,000 km2 in area with rainfall of 25 mm or more and sustain for at least two consecutive days. These rainstorms have potential to cause large-scale floods and associated risk over the region in which they are present. The analysis identified 395 rainstorms during the period, 1951–2015, on an average 6 storms per season. About 40% of the rainstorms are associated with the low-pressure systems which form over the North Bay of Bengal and move north-westwards along the seasonal monsoon trough. The present analysis suggests that over northern parts of India, where majority of rainstorms form, frequency and duration of rainstorms have shown statistically significant increasing trends. During the period of 1951–2015, there was an increase from 4 to 8 rainstorms per year and from 12 to 27 rainstorm days per year. This increase has significant repercussions in terms of added risk of large-scale floods and associated causalities. Further analysis suggests that rainstorm activity over northern parts of India is strongly influenced by the colder sea surface temperature anomalies over the east equatorial Indian Ocean and associated moisture divergent flow and strong moisture convergence over the Indian landmass and the Bay of Bengal.
A new method to track the smooth eastward propagation of Madden Julian
Oscillation (MJO) is proposed in this report in order to use it for real-time prediction. The
new method essentially removes any requirement of pre-filtering of data and is based on
the extended empirical orthogonal function (EEOF) analysis of the zonal wind at 850hPa,
200hPa and the velocity potential (chi) at 200hPa. The lag used for creating the extended
data matrix is 6 days (day 0 …day 5). The EEOF method not only captures the MJO but also
smooths the temporal propagation in the phase space defined by first two principal
components (PCs) of the EEOFs. Along with traditional tracking in a phase space we extend
the method to get the MJO-filtered spatial distribution of precipitation and other three
dimensional variables associated with MJO. Examples of the TOGA-COARE and DYNAMO
period are presented to compare the newly developed method with earlier ones.
We investigate trends in extreme precipitation in Iran for 1951–2007 using the recently released APHRODITE daily rainfall time series. We find that seven different indices of extreme precipitation all show an upward trend through the study period. The seven different precipitation indices include annual precipitation total, number of days above a certain threshold, maximum precipitation received over a certain period of time, maximum one-day precipitation, and number of days with precipitation above the 90th percentile. A principal components analysis reveals one eigenvector explaining much of the variance in the seven indices and reveals that this component exhibits a strong upward trend for the whole of Iran. On a regional level, we find that the upward trend in extreme precipitation has a strong southwest-to-northeast gradient across the country for all the indices. We repeated all the analyses for 42 stations across the country to compare with the results from the gridded data; trends in extreme rainfall generated from the station data compare favorably with the results from the APHRODITE daily rainfall time series thereby reinforcing the robustness of our conclusions.
The study discusses development of a new daily gridded rainfall data set (IMD4) at a high spatial resolution (0.25° × 0.25°, latitude × longitude) covering a longer period of 110 years (1901-2010) over the Indian main land. A comparison of IMD4 with 4 other existing daily gridded rainfall data sets of different spatial resolutions and time periods has also been discussed. For preparing the new gridded data, daily rainfall records from 6955 rain gauge stations in India were used, highest number of stations used by any studies so far for such a purpose. The gridded data set was developed after making quality control of basic rain-gauge stations. The comparison of IMD4 with other data sets suggested that the climatological and variability features of rainfall over India derived from IMD4 were comparable with the existing gridded daily rainfall data sets. In addition, the spatial rainfall distribution like heavy rainfall areas in the orographic regions of the west coast and over northeast, low rainfall in the lee ward side of the Western Ghats etc. were more realistic and better presented in IMD4 due to its higher spatial resolution and to the higher density of rainfall stations used for its development.
The daily rainfall records of about 5000 stations in India, during the monsoon periods of several decades, have been examined for occurrence of heavy rainfall. The point values of (i) the highest annual rainfalls, (ii) the highest rainfalls of one, two, and three day durations, and (iii) the highest hourly values, are presented in the form of tables and charts. The highest depth-area-duration values for twelve storm-centres are also given.
The trends and epochal variability of southwest monsoon over the country as a whole and four homogeneous regions are examined using monthly rainfall data (1901–2011) of 640 political districts of India. The district rainfall data is computed from station rainfall data. The same station data is used to analyse the trends in the frequency of rainfall events of different intensities for examining extreme rainfall events. The existence of the multidecadal epochal variability of rainfall is clearly established in the all-India monsoon rainfall as well as monsoon rainfall over the four homogenous regions. However, over different homogenous regions, the phases of multidecadal variability are found to be different. Principal component analysis brings out Northeast India (NEI) rainfall as more dominant mode for all-India rainfall. Significant decrease in southwest monsoon rainfall over NEI is observed during the post 1950 period. Decreasing trends are also observed over the monsoon core region during the post-1950 period. Over these regions, monsoon rainfall has increased significantly during the pre-1950 period. It has been shown that the decreasing trend in monsoon rainfall during the post 1950 period is the result of multidecadal epochal variability. Geographical regions that experienced significant changes in the frequency of days of rainfall with different intensities are also identified. Significant change/turning points are also detected in the southwest monsoon rainfall. Frequency of moderate rainfall events (5 mm ≤ daily rainfall < 100 mm) decreased significantly during the period 1951–2010 over the monsoon core region of India whereas no significant changes are observed in the frequencies of heavy (daily rainfall >100 mm) or very heavy rain (daily rainfall >150 mm) during the southwest monsoon season. Climatic shift or change point in monsoon rainfall in India is also detected by an established statistical test.
The occurrence of exceptionally heavy rainfall events and associated flash floods in many areas during recent years motivate us to study long-term changes in extreme rainfall over India. The analysis of the frequency of rainy days, rain days and heavy rainfall days as well as one-day extreme rainfall and return period has been carried out in this study to observe the impact of climate change on extreme rainfall events and flood risk in India. The frequency of heavy rainfall events are decreasing in major parts of central and north India while they are increasing in peninsular, east and north east India. The study tries to bring out some of the interesting findings which are very useful for hydrological planning and disaster managements. Extreme rainfall and flood risk are increasing significantly in the country except some parts of central India.
Daily gridded (1° × 1°) rainfall data prepared by the India Meteorological Department for the period 1951–2004 have been used in this study to examine possible changes in the frequency of rain events in India in terms of their duration and intensity per day. So far as the duration is concerned, a rain event is classified as short, long, dry, or prolonged dry spell. Similarly in terms of intensity, a rainy day is considered as low, moderate, or heavy. Changes in the frequency of these events have great relevance from the point of view of climate change. Threshold and limiting values for defining the heavy and moderate rain days are calculated in accordance with the gamma probability distribution. Results show that the frequencies of moderate and low rain days considered over the entire country have significantly decreased in the last half century. On the basis of the duration of rain events it is inferred that long spells show a significant decreasing trend over India as a whole while short and dry spells indicate an increasing tendency with 5% significance. The characteristics of rain events are also examined over six homogenous rainfall zones separately since the spatial distribution of rainfall over India shows large variability. In this study, the changes in the frequencies of different categories of rain events suggest weakening of the summer monsoon circulation over India. This hypothesis of a weakening of monsoon circulation is supported by significant reduction in the 850 hPa wind fields in the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyzed data set.
In this paper, changes in the long and short spells of different rain intensities are statistically analyzed using daily gridded
rainfall data prepared by the India Meteorological Department for the period 1951–2008. In order to study regional changes,
analyses have been conducted over nine selected agro-meteorological (agro-met) divisions, five homogeneous zones, and also
over the whole of India. Rain events of different intensities with continuous rainfall of more than or equal to 4 days are
classified here as long spells. Those with less than 4 days are termed as short spells. Those results which are statistically
significant at 95% confidence level are discussed in this paper. Trend analysis shows that during the summer monsoon months
of June to September, short spell rain events with heavy intensity have increased over India as a whole. On the other hand,
long spell rain events with moderate and low intensities have decreased in numbers. Results further show that the contributions
of long spell moderate and short spell low-intensity rain events to the total rainfall have decreased whereas the contributions
of short spell heavy and moderate-intensity rain events to the total seasonal rainfall have increased. Percentage changes
in various categories of long and short spells in the decade 1991–2000 compared with the earlier decade 1951–1960, highlight
the maximum increase in heavy-intensity short spell category and decrease in moderate-intensity long spell category in India
as a whole and in most of the homogeneous zones and agro-met divisions. The changes in different types of rain events differ
in the six homogeneous zones and nine selected agro-met divisions. However, in three homogeneous zones and three agro-met
divisions, the short spell heavy-intensity rain events dominate as in the entire country. There are also changes observed
in the monthly occurrences of above categories of rain events during the 4 months of summer monsoon. Such results with details
of changes in rain categories in different parts of India have important implications in agriculture sector in the country.
The Indian economy largely depends on agriculture which is highly influenced by the spatio-temporal variability of precipitation. Kharif and rabi are the two main crop-growing seasons which require major proportion of rainfall. Increase in heavy precipitation events, however, can have adverse effects on the crops. This study, therefore, is mainly focused on understanding the variation of extremes in precipitation during summer monsoon season and its impact on kharif foodgrain yield over India. For this, several objectively defined indices of observed precipitation extremes, in terms of frequencies and intensities, have been computed for the period 1951–2003 using daily gridded data of 1° latitude × 1° longitude resolution. Efforts have also been made to prepare the climatology of extremes in precipitation (1961–1990) to determine their basic characteristics over the Indian region.