**1. Introduction**

The Western Himalayas (WH) are a strong modulator of weather and climate patterns over northern India and surrounding regions. WH are highly rich in biodiversity and covered with forests, agricultural landscapes, glaciers, wetlands and urbanized land, which underlines the significance of the region. The regional spatiotemporal distribution of precipitation over WH depicts high variability [1, 2] which can be partly attributed to influences from the atmosphere-land surface exchange

processes over the region, keeping in mind the diverse land surface characteristics and large geographical variability [3, 4]. Additionally, complex interplay of regional topography with moist airflow and temperature gradient magnifies this variability further [5–7].

WH receives precipitation during both summer and winter monsoons [8]. The Indian summer monsoon (ISM), spanning through June-September (JJAS), contributes about 67–75% to the annual precipitation received over WH [2]. These mountainous environments have a significant impact on the spatiotemporal distribution of precipitation [1, 9]. Majorly, precipitation over the region during the ISM is contributed by convection followed by an orographically locked system, with Himalayas as barriers forcing moisture-laden southwest monsoon winds to dissipate moisture. Additionally, strong Tibetan high combined with the monsoon trough in northern India, creates a strong moist flow from Bay of Bengal and Arabian Sea into the Himalayas [10, 11]. Occasionally, interaction of tropical monsoon depressions and extratropical disturbances leads to the formation of heavy precipitation over the region during summer [12].

The region also receives a significant amount of precipitation during the winter season (December to February), primarily through extra-tropical cyclonic systems called Western Disturbances (WD; e.g. [1, 13]). WDs, embedded in upper tropospheric sub-tropical westerly jet stream, propagate eastward towards WH carrying moisture mainly from Mediterranean sea, Caspian sea, and Black sea (e.g. [13–15]). The interaction of these disturbances with elevated topography of WH results in their intensification and subsequent precipitation [16]. This precipitation holds key significance for sustenance of regional glaciers through snow accumulation and agricultural activities. The glacial mass balance is particularly crucial for regional river runoff and flows [17]. Any major precipitation variations in these glaciers can lead to adverse consequences on freshwater availability in downstream areas (e.g. [18]). Moreover, the vast river basin of WH acts as a watershed for a large population, assists in sustaining the regional biodiversity and provides various key ecosystem services to the surrounding north Indian plains. However, changing climate and expected hydroclimatic variability raises serious concerns related to impacts on this richly biodiverse and fragile mountainous landscape.

WH is highly prone to extreme precipitation events (EPEs) due to its intricate topography and altitude-dependent climate [10, 19, 20], which can give rise to surface runoff during such events, causing additional natural hazards such as landslides and floods (e.g. [21, 22]). Sharp regional weather fluctuations over the Himalayas makes this region unpredictable leading to sudden occurrences of heavy precipitation events. Various states in WH including Uttarakhand (UK), Jammu and Kashmir (J&K), Himachal Pradesh (HP), often face the problem of river flooding and landslides due to torrential downpours and localized occurrences of intense precipitation along the southern slopes of the Himalayas. Several case studies of EPEs over WH have highlighted massive losses through cloudbursts triggered by terrain-locked deep convective systems in valleys, as well as flash floods triggered by extratropical disturbances [23, 24]. Many scientific reports suggest an enhancement of precipitation in the order of 5–20% in the Himalayas in the 21st century (e.g. [25]). Moreover, the changes in the intensity and frequency of EPEs may vary seasonally. The increased susceptibility of WH to heavy precipitation during ISM has been discussed in various studies (e.g., [22, 26–32]). The variability aspects of WH winter precipitation under climate change scenarios, including a possibility of enhancement in precipitation extremes has also been frequently highlighted [14, 33, 34]. An increase in avalanche

#### *Hydrological Extremes in Western Himalayas-Trends and Their Physical Factors DOI: http://dx.doi.org/10.5772/intechopen.109445*

activity over WH slopes related to enhanced frequency of wet-snow conditions during recent decades has also been reported by [35]. Being thickly populated, WH and surrounding regions are highly vulnerable to climate extremities. Such extreme precipitation events (EPEs) can affect both natural and anthropogenic ecosystems through damage to life, infrastructure, agriculture, energy sectors, etc.

Many massive disasters related to unexpected heavy precipitation such as Leh flood (August 2010), Kedarnath disaster (June 2013), Chamoli river floods in Uttarakhand (July 2016), Nadum disaster (August 2018), Bilaspur and Shimla floods (August 2019), Jammu & Kashmir floods in Kishtwar district (July 2021) and Dharmsala floods (August 2022) have been reported in the recent times during the ISM, leading to immense life and economic losses. Increased frequency of summer EPEs (see [10, 36]) has been attributed to a number of possible causes including enhanced water vapor transport in the northern Hindu Kush Himalayan region [37], formation and movement of local deep convective systems along the orography [10, 23, 24], convergence of low-level monsoon westerly winds and northeasterly winds along the foothills combined with enhanced vertical wind shear and interaction of tropical systems with extratropical disturbances (see [9, 10, 12, 23, 26, 34]). Furthermore, elevated Tibetan plateau plays a key role during ISM by producing mesoscale precipitation through small-scale circulations and enhancing the synoptic weather conditions, leading to extreme precipitation in the WH [19, 38, 39]). Additionally, [12] found that EPEs over the Himalayas are associated with a southward extension of the western upper trough and a simultaneous northward migration of the lower monsoon trough towards the foothills of the Himalayas. These two systems amplify the low-level moisture flux from the Arabian Sea and Bay of Bengal towards the foothills of the Himalayas and contribute to cloud development and heavy precipitation.

Studies pertaining to the wintertime trends of extreme precipitation intensity and frequency are comparatively fewer. Shekhar et al. [40] reported a significantly increasing trend for heavy precipitation events (>70 mm) in the Pir-Panjal range of WH, however, other altitudes and ranges did not portray any clear trends. A significantly increasing trend for winter to early spring precipitation extremes intensity (exceeding 90th percentile threshold) has also been observed by [14] between 1900 and 2011, attributable to higher baroclinicity and in turn enhanced variability of WD activity during the recent decades. Krishnan et al. [33] also studied the impact of climate change on WD activity over WH and reported an increasing trend of winter precipitation extremes in the recent decades appertaining largely to anthropogenic forcings in conjunction with natural factors. Increasing trends of different extreme precipitation indices (exceeding 90th and 95th percentiles) using station-based records were further observed by [41] for all western Himalayan ranges during the month of February. Further, [42] demonstrated an increased frequency of atmospheric rivers (ARs) over the Himalayas during winter and underlined the association of intense ARs with EPEs over the Ganga and Indus basins.

Weather and climate extremes are generally a result of variations in different atmospheric dynamic and thermodynamic variables, as well as of some surface properties or states. Several studies, involving observations or climate model simulations, indicate that the frequency of these events would intensify with global warming due to an increment in the atmospheric moisture holding capacity as per Clausius Clapeyron relationship [43–46]. Additionally, the influence of local thermodynamics and orographic forcing in the WH produces abrupt changes in synoptic circulation, which have the potential to produce EPEs that can last for a few days [12, 47].

However, certain limitations are associated with the study of precipitation extremes over complex topographic regimes of WH. The remoteness of the region and the sparse coverage of rain gauges and automatic weather stations in mountainous areas makes precipitation monitoring in this region quite difficult [10, 48, 49]. The lack of availability of data directly affects the research studies investigating EPEs to under-perform in the Himalayan region. As a result, studies of extreme precipitation patterns, trends and possible causes in the WH remain limited and insufficient. Furthermore, very few studies have been conducted on the spatial distribution of EPE trends over WH. The inadequate representation of regional orography in the available coarser resolution datasets adds to the uncertainties associated with assessment of EPEs over any region [50]. Most of the past studies for EPEs over WH utilize comparatively coarser resolution datasets and there is a lack of high-resolution data-based studies for these extremes. Thus, it becomes important to utilize high-resolution datasets for a finer and accurate understanding of extreme precipitation trends and their possible causes over WH. The latest advancements in meteorological satellites and reanalysis products at finer resolutions with improved precipitation estimation algorithms have further facilitated the research on extreme weather events.

Our study focuses on changes in the spatial and temporal distributions of extreme precipitation events over WH during 1979–2020 in various high-resolution multisource climate datasets, including the potential of recently released high-resolution regional reanalysis, Indian Monsoon Data Assimilation and Analysis (IMDAA). We evaluate extreme precipitation distribution and associated changes in key atmospheric parameters of EPEs over WH. Such knowledge about the climatological features of precipitation extremes and their associated dynamical and thermodynamic changes is crucial to interpret how precipitation patterns are changing in a varying climate scenario, further giving way to carry out vulnerability impact assessment studies.
