Interaction of a Low-Pressure System, an Offshore Trough, and Mid-Tropospheric Dry Air Intrusion: the Kerala Flood of August

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Article Interaction of a Low-Pressure System, an Oﬀshore Trough, and Mid-Tropospheric Dry Air Intrusion: The Kerala Flood of August 2018

Vinay Kumar 1,* , Prabodha Kumar Pradhan 2 , Tushar Sinha 1, S. Vijaya Bhaskara Rao 2 and Hao-Po Chang 1

1 Department of Environmental Engineering, Texas A&M University, Kingsville, TX 78363, USA; [email protected] (T.S.); [email protected] (H.-P.C.) 2 Department of Physics, Sri Venkateswara University, Tirupati 517502, India; [email protected] (P.K.P.); [email protected] (S.V.B.R.) * Correspondence: [email protected]

Received: 28 May 2020; Accepted: 9 July 2020; Published: 13 July 2020

Abstract: The present study examines the Kerala Flood Event (KFE, 15–16 August 2018, in India) that occurred along the west coast of India and resulted in ~400 mm of rainfall in one day. The KFE was unique in comparison to previous floods in India, not only due to the rainfall duration and amount, but also due to the fact that the dams failed to mitigate the flood, which made it the worst in history. The main goal of this study is to analyze and elucidate the KFE based on meteorological and hydrological parameters. A propagating low-pressure system (LPS) from the Bay of Bengal (BoB) caused the streak of plenty of rainfall over Kerala, the west coast, central India, and the BoB. Additionally, the upper-tropospheric anti-cyclonic system over the Middle East region inhibited a northward advancement of LPS. On the western coast of India, a non-propagating (with diurnal fluctuations) offshore trough was observed over the west coast (from Kerala to Gujarat state). Therefore, a synergic interaction between LPS, an intrusion of dry air in the middle-troposphere, and the offshore trough was the main reason for KFE. However, after around ten days, rainfall saturated the dam capacities; thus, the released water, along with the amount of precipitation on the day of the event, was one of the other possible reasons which worsened the flood over Kerala.

Keywords: Kerala Flood Event; oﬀ-shore trough; low-pressure systems

1. Introduction The quantity and availability of surface precipitation play important roles not only in hydrometeorology, but also in water resource management. Be it irrigation, agriculture, controlling floods, coping with droughts, or administering a freshwater supply, the timely assessment of events is vital to answering critical questions, such as when, where, and how much will it rain. Over the Indian subcontinent, the northward propagation of the monsoon and surface rainfall amounts are governed by remote teleconnections and internal dynamics of the monsoon itself [1]. During the summer monsoon, extreme flood events have significant social, economic, ecological, environmental, hydrological, and health impacts on regional and local communities. Therefore, the sudden kick-off of a natural disaster requires a detailed prioritization list of flood forecasts, assessments of the damages, risks, and improved management of floods. July and August are the rainiest months in India, and bring moderate to plenty of rainfall and sometimes floods over various parts of India. Recently, the state of Kerala (in India) witnessed the worst flooding of the century, which occurred from 15 to 16 August in 2018. The flood destroyed thousands of houses, leading to about 500 deaths, inundated water in cities and villages,

Atmosphere 2020, 11, 740; doi:10.3390/atmos11070740 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 740 2 of 21 collapsed bridges, and severely damaged communications [2]. The impact of the Kerala Flood Event (KFE) was much more severe in the following districts in Kerala: Idukki, Palakkad, Kollam, Kottayam, Malappuram, Ernakulam, and Pathanamthitta. The damages that occurred during the KFE were so severe that restoration facilities were obtained from various parts of India and transported to Kerala state. After the flood, in the last week of August, drought and a high temperature took over the Idduki district, where the water levels in the wells were reported to have fallen by 20 feet (from various online websites, e.g., Business Insider and Swarajyamag). Extreme weather events are associated with low-pressure systems, cloud bursts, cyclones, and landslides, which have significant socioeconomic impacts [3,4]. However, over India, low-pressure systems (LPS) embedded in mean monsoonal flow produce abundant rainfall over land and oceanic regions; almost every year, LPS remains a leading cause of the flood. The KFE region is located along the orography of the western coast (~2.5 km height) of India. The torrential rainfall that occurred during the KFE caused landslides in the mountain regions and loosened soil in the hill slopes. Flash floods collect mud and debris along their way to the river. Several research works have appeared on KFE, considering various aspects of this flood, e.g., hydrology [5–7], natural disasters [8], meteorological modeling [9], and a climate change perspective [10]. They have analyzed rainfall data of more than 100 years and used generalized extreme value distributions [5,6]. They have presented a return period of 1 to 3 days of extreme rainfall and contributions of dams in KFE. In terms of the analysis of hydrological datasets for the Periyar basin, the probability of such extreme rainfall in August is quite low, with a value of 0.6% over Kerala [7]. Kerala state has almost seven river basins (Achencoil River, Bharathappuzha, Ithikkara, Karuvannur, Karamana, Manimala, Muvattupuzha, Pallikkal and Kallada, and Periyar). A dataset obtained from the Sentinel-2A satellite and the modified normalized difference water index demonstrated that the water was lodged in the low-lying coastal area of Kerala based on [8]. Furthermore, the role of monsoonal depression over the Bay of Bengal (BoB), mid-tropospheric moisture transport, and the modeling of vorticity have been used to explain features of Kerala floods [9]. A recent study hinted at a possible increase in flooding events over the Kerala region in the future, based on climate projections and scenarios [10]. All of the studies, as mentioned above, were either limited to a smaller portion of the flood region or considered other aspects of the flood than we elucidate here. Additionally, the present study used high-resolution rainfall station datasets to explain rainfall over a larger area of the flood and considered other crucial atmospheric factors to explain KFE, in order to draw unique conclusions. We will now revisit some of the well-known flood events of India, other than KFE, and their causes and details. The Mumbai flood (~900 mm/day), which occurred on 26 July 2005, is a widely known example of extreme rainfall which drew the attention of the Indian community and researchers. The eastward movement and intensification of the offshore trough is considered to be one of the main reasons for this flood [11]. Floods, followed by continuous heavy precipitation for days (1 or 2 days), remain the costliest natural disasters in India, such as the Uttarakhand flood (15 June 2013, ~400 mm/day), Jammu and Kashmir (J&K) flood (5 September 2014, ~200 mm/day), and Chennai flood (2 December 2015, ~350 mm/day) [12–16]. In the case of Uttarakhand’s flood, the moisture supply was the result of a combined contribution from nearby water bodies, the Arabian Sea, and the Bay of Bengal. A fast-moving LPS and Himalayan orographic effect played an essential role in this flood [17]. Extreme rainfall events in the orographic region may be attributed to cloud burst, e.g., the Leh flood (6 August 2010, ~150 mm/day) [18]. Some extreme rainfall events are the result of scale interactions between several events, e.g., a synoptic scale, mesoscale, large scale, and LPS [15]. In most flood events, LPS and it’s interactions with other local systems remain a crucial reason for the occurrence of floods. The KFE of 2018 attracted attention from the media, scientific community, and policymakers, because it was one of the worst floods of the century over Kerala [5]. Due to continuous rainfall over the Western Ghats, major dams in Kerala reached full capacity before the KFE [5]. Furthermore, it has been noted Atmosphere 2020, 11, 740 3 of 21 that during the active southwest monsoon season, heavy rainfall events (HREs) along the west coast of India cause flash floods; such floods are the most common natural disasters of the region [19]. To study HREs, over the west coast of India, during the monsoon season, a field experiment, namely ARMEX-I (Arabian Monsoon Experiment, Phase I), was performed by the Indian Climate Research Programme (ICRP), Govt. of India [20]. As part of the ARMEX-I (observational data that was collected from June to August in 2002), particular observations were collected to study the HREs associated with synoptic systems, such as offshore troughs, strong low-level jets, and low-pressure-systems [21–25]. Several modeling studies on HREs along the west coast, notably the Mumbai flood case, have been reported [22,26]. These studies suggest that mesoscale model techniques have commendable skills for simulating the HREs 48 h in advance. Around 80% of intense rainfall events along the west coast of India are associated with offshore troughs [12,19,20,27,28], strong cross-equatorial flow over the Arabian Sea [29], and the low-pressure system over the Arabian Sea near the Gujarat and Maharashtra coast [30]. Other well-known low-pressure systems that form over BoB and move inward across land along the monsoonal trough-lane are parts of the monsoonal system [31]. Among the many dynamic features of a monsoon, strong cross-equatorial flow in the lower troposphere, widely known as the low-level monsoon jet, plays a vital role in the Indian summer monsoon rainfall (ISMR), which occurs during June–September [32]. All extreme rainfall events are diverse in their initiation, spread, and impacts. In recent work, it has been speculated that a rise in the surface temperature and moisture over orographic regions favors flood events, due to an increase in convective available potential energy [33]. Many stations in west coast regions have exhibited a rise in temperature in the last 100 years [34]. Some of the valid questions that can be raised regarding KFE are as follows: Did intensification of the offshore trough, LPS over BoB, and mid-tropospheric dry air intrusion air from the Middle East region trigger extreme precipitation over the Kerala region? Did the release of water from dams add fuel to KFE? Furthermore, what were the favorable conditions for the serial clustering of convection along the west coast of India? Can the strength of the monsoonal kinetic energy, net-tropospheric moisture, and mean tropospheric temperature field across BoB, the Arabian Sea, and the Indian monsoon zone play any crucial role in the enhancement of the convection over Kerala? Is there a synergic role of the interaction of LPS, the offshore trough, and mid-tropospheric dry air intrusion from Middle East regions for KFE? Such probable issues associated with KFE are analyzed and discussed in this research. Furthermore, the present research work addresses the regional and dominant features of the synoptic conditions over the Indian subcontinent. As a whole, this study tried to fill the gap in the understanding of the interaction between monsoonal LPS, the offshore trough, and other regional features.

2. Data and Methodology The daily rainfall dataset from the Global Precipitation Measurement (GPM) satellite, Indian Meteorological Society (IMD) rain gauge in-situ datasets, and the IMD + TRMM (Tropical Rainfall Measuring Mission) merge rainfall dataset were used in this study [35]. GPM is a constellation-based satellite mission speciﬁcally designed to provide a new generation of observations of rainfall and snowfall from space to improve our understanding of Earth’s water and energy cycle and has been in operation since 28 February 2014. These datasets provide near real-time precipitation at a spatial-temporal resolution of 25 25 km horizontal resolutions. Furthermore, the Indian Meteorological × Society’s satellite products (esp. the water vapor channel) were used to study the synoptic features of KFE. These datasets were used for understanding the dynamic characteristics of the convective systems that caused severe rainfall along the west coast of Kerala state. INSAT-3D images are an improved design of the Very High-Resolution Radiometer (VHRR-2) capable of generating images of the Earth in six wavelength bands. These images are available in visible, shortwave infrared, middle infrared, water vapor, and two bands in thermal infrared regions. They oﬀer an improved 1-km resolution in the visible band for the monitoring of mesoscale phenomena and severe local storms [36]. For KFE, we used water vapor and thermal infrared bands to understand the synoptic features and convection. To address the issues involving the dynamics of HREs during Atmosphere 2020, 11, 740 4 of 21

KFE, we used the European Centre for Medium-Range Weather Forecast (ECMWF) ERA-Interim reanalysis of the global atmosphere covering the data-rich period since 1979 (https://apps.ecmwf.int). The ECMWF reanalysis (ERA-Interim) dataset [37], along with ground-based and satellite observations, were used in this study. The ERA-Interim reanalysis uses a state-of-the-art model with four-variational data assimilation (4DVAR) scheme. The datasets have high spatial resolutions and are easy to access for use in research. Additionally, these ERA-Interim meteorological variables are available at a T255 resolution, but we optimally interpolated all required variables at a 25 25 km horizontal resolution. The data on × runoﬀ was obtained from ECMWF reanalysis. Data for the ﬂooded water for Kerala’s dam were acquired from http://tamcnhp.com/wris/#/waterData.

3. Overview of the Kerala Flood Event The daily accumulated rainfall, 850 mb wind anomalies, and track of the monsoon depression through 13–18 August 2018 over the Indian region are portrayed in Figure1a. The path of low pressure is ~4◦ south of the mean position of LPS tracks (the black dashed line is the mean position of monsoon depressions) and lasted for six days. As usual, most of the heavy rainfall is located in the southern part of the LPS track. A low-pressure system embedded in the mean monsoon flow brings copious rainfall to its southwest sector. Over BoB, LPS lingered for a while; thus, heavy rainfall is evident over that area. Furthermore, it will be interesting to investigate the rainfall over west-coastal regions and Kerala, as LPS advances westward. It has been observed that when LPS over BoB and the offshore trough near the Gujrat region are active, then the Indian landmass receives plenty of rainfall. Some of the prominent features of rainfall are visible over central BoB; these are extended inland along the track of monsoonal depression, central India, and over the western coast (Figure1a). The authors focus on the spatial spread of rainfall over Kerala and adjoining states through 13–17 August, 2018 (Figure1b). The maximum accumulation of rainfall (>350 mm) was recorded over northern Kerala and southern coastal regions of Karnataka. An adjacent state, Tamilnadu, being under the rain-shadow influence (receives low rainfall due to the subsidence of cold and dry air), received less rainfall. A central water commission report on the Kerala flood also recorded rainfall of 214 to 398 mm over various cities of the Kerala districts [38]. Notably, on a single day (15 August), the recorded rainfall was more than the climatological precipitation of August. The daily area-averaged rainfall over the Kerala region (8◦ N–13◦ N; 74◦ E–78◦ E) is depicted in Figure1c, which shows moderate to heavy rainfall during 8–19 August 2018, while heavy rainfall on 15 August and 16 August only. Rain-gauge rainfall (from IMD) for some of the flood-affected cities of Kerala shows an abundant amount of rainfall (>300 mm) on 15–16 August (Figure1d). A previous study documented that, during active spells, the western coast, between 14◦ N–16◦ N and 18◦ N–20◦ N, is the most favorable location of intense rainfall due to deeper cloud systems [39]. It has been noted that the heavy rainfalls over Kerala state and the adjoining west coast of India are associated with a quasi-stationary trough, in which a cyclonic vortex is sometimes embedded [27]. Atmosphere 2020, 11, x FOR PEER REVIEW 4 of 21

(https://apps.ecmwf.int). The ECMWF reanalysis (ERA-Interim) dataset [37], along with ground- based and satellite observations, were used in this study. The ERA-Interim reanalysis uses a state-of- the-art model with four-variational data assimilation (4DVAR) scheme. The datasets have high spatial resolutions and are easy to access for use in research. Additionally, these ERA-Interim meteorological variables are available at a T255 resolution, but we optimally interpolated all required variables at a 25 × 25 km horizontal resolution. The data on runoff was obtained from ECMWF reanalysis. Data for the flooded water for Kerala’s dam were acquired from http://tamcnhp.com/wris/#/waterData.

3. Overview of the Kerala Flood Event The daily accumulated rainfall, 850 mb wind anomalies, and track of the monsoon depression through 13–18 August 2018 over the Indian region are portrayed in Figure 1a. The path of low pressure is ~4° south of the mean position of LPS tracks (the black dashed line is the mean position of monsoon depressions) and lasted for six days. As usual, most of the heavy rainfall is located in the southern part of the LPS track. A low-pressure system embedded in the mean monsoon flow brings copious rainfall to its southwest sector. Over BoB, LPS lingered for a while; thus, heavy rainfall is evident over that area. Furthermore, it will be interesting to investigate the rainfall over west-coastal regions and Kerala, as LPS advances westward. It has been observed that when LPS over BoB and the offshore trough near the Gujrat region are active, then the Indian landmass receives plenty of rainfall. Some of the prominent features of rainfall are visible over central BoB; these are extended inland along the track of monsoonal depression, central India, and over the western coast (Figure 1a). The authors focus on the spatial spread of rainfall over Kerala and adjoining states through 13–17 August, 2018 (Figure 1b). The maximum accumulation of rainfall (>350 mm) was recorded over northern Kerala and southern coastal regions of Karnataka. An adjacent state, Tamilnadu, being under the rain-shadow influence (receives low rainfall due to the subsidence of cold and dry air), received less rainfall. A central water commission report on the Kerala flood also recorded rainfall of 214 to 398 mm over various cities of the Kerala districts [38]. Notably, on a single day (15 August), the recorded rainfall was more than the climatological precipitation of August. The daily area- averaged rainfall over the Kerala region (8° N–13° N; 74° E–78° E) is depicted in Figure 1c, which shows moderate to heavy rainfall during 8–19 August 2018, while heavy rainfall on 15 August and 16 August only. Rain-gauge rainfall (from IMD) for some of the flood-affected cities of Kerala shows an abundant amount of rainfall (>300 mm) on 15–16 August (Figure 1d). A previous study documented that, during active spells, the western coast, between 14° N–16° N and 18° N–20° N, is the most favorable location of intense rainfall due to deeper cloud systems [39]. It has been noted that the heavy rainfalls over Kerala state and the adjoining west coast of India are associated with a quasi- Atmosphere 2020, 11, 740 5 of 21 stationary trough, in which a cyclonic vortex is sometimes embedded [27].

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Figure 1. (a) Accumulated rainfall (mm, shaded), wind anomaly (850 mb),mb), and low-pressurelow-pressure system (LPS)(LPS) track (blue line) over India during 13–1813–18 August 2018, where the dark dashed line represents the mean positions positions of of monsoon monsoon depressions. depressions. ( (bb)) Kerala state derived from IndianIndian Meteorological Meteorological Society ( (IMD)IMD) ++ GlobalGlobal Precipitation Precipitation Measurement Measurement (GPM (GPM)) merged merged products products at a at 25 a km 25 kmresolution resolution for for13–1713–17 August August 2018. 2018 (c).( Dailyc) Daily area area-averaged-averaged rainfall rainfall (mm) (mm) over over the theKerala Kerala state state (8° N (8–◦13N–13° N ◦andN and 74° 74E–◦78E–78° E)◦ duringE) during August August 2018. 2018. (d) (dDifferent) Different station station rainfall rainfall over over Kerala Kerala derived derived from from an an IMD IMD rain gauge during 03 UTC 15–1615–16 August 2018 (stations(stations withwith <<77 cm of rainfall are ignored ignored).). In all the figures figures “Aug”“Aug” isis usedused asas thethe shortshort formform ofof “August”.“August”. 4. Discussion of Results 4. Discussion of Results 4.1. Heavy Rainfall Detection Using Satellite Images 4.1. Heavy Rainfall Detection Using Satellite Images The INSAT-3D cloud imagery of the water vapor channel from 14 to 17 August 2018 at 0300 UTC The INSAT-3D cloud imagery of the water vapor channel from 14 to 17 August 2018 at 0300 UTC is shown in Figure S1a–d (see Supplementary Materials). The INSAT 3-D satellite imagery exhibits is shown in Figure S1a–d (see Supplementary Materials). The INSAT 3-D satellite imagery exhibits the the distribution of dense clouds over BoB along the monsoon trough. However, deep clouds along distribution of dense clouds over BoB along the monsoon trough. However, deep clouds along the west the west coast of India (around 8–20◦ N) display convective activity associated with LPS and active coast of India (around 8–20° N) display convective activity associated with LPS and active monsoon monsoon conditions over the southwest monsoon region (Figure1a). On 12 August 2018, at 0300 UTC, conditions over the southwest monsoon region (Figure 1a). On 12 August, 2018, at 0300 UTC, the the offshore vortex was noticed along (in a red circular box in Figure S1) the west coast from Kerala offshore vortex was noticed along (in a red circular box in Figure S1) the west coast from Kerala to to Konkan. During the next 24 h, the offshore trough weakened, which led the convection to remain Konkan. During the next 24 h, the offshore trough weakened, which led the convection to remain isolated on 13 August 2018 at 0300 UTC (Figure S1). On the other hand, well-organized dense cloud isolated on 13 August 2018 at 0300 UTC (Figure S1). On the other hand, well-organized dense cloud puffs can be observed over the central BoB on 14 August and 15 August 2018 at 0300 UTC. Furthermore, puffs can be observed over the central BoB on 14 August and 15 August 2018 at 0300 UTC. clouds appeared over Andhra Pradesh, the Tamilnadu coast, and the adjoining region (Figure S1c,d). Furthermore, clouds appeared over Andhra Pradesh, the Tamilnadu coast, and the adjoining region On these two days, scattered but intense convection was observed over the east and central part of (Figure S1c,d). On these two days, scattered but intense convection was observed over the east and BoB, which can be attributed to the formation of well-organized monsoonal LPS, reported by IMD, central part of BoB, which can be attributed to the formation of well-organized monsoonal LPS, which is a prominent feature of a vigorous monsoon over BoB. reported by IMD, which is a prominent feature of a vigorous monsoon over BoB.

4.2. Large and Synoptic-Scale Systems Concomitant with the Kerala Flood

Low-Level Jet (LLJ), Low-Pressure-System, Offshore Trough, and Blocking High During the summer monsoon LPS over BoB (at ~996 mb), the offshore trough (green dashed line), and dry air intrusion from the middle-East (at 600 mb) are as displayed in Figure 2a–e. A well-organized feature of ISMR, the offshore trough, is visible in the MSLP from Kerala to the Gujrat region (Figure 2, see along the west coast of India). Besides, the intrusion of dry air from the Middle East is well-mixed at 600 mb into the LPS over the Indian region. A previous study showed that in such situations, dry air intrusion can introduce non-uniform saturated instability to produce heavy rainfall [40]. Along with LPS and the offshore trough, the mixing of cold-dry air from the Middle East and warm-moist air from as it is one of the critical features in the formation of non-uniformly saturated air and static instability that can be exploited to lead to heavy rainfall. This kind of instability is not a very common element in the generation of flood events. The interaction between monsoon depression, the offshore trough, and dry air intrusion is responsible for the heavy rainfall over Kerala state (Figure 2).

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The daily mean wind flow pattern at 850 to 300 hPa levels from 13 to 17 August 2018 is displayed in Figure 3a–t. At the 850 hPa level, wind flow characterized by strong south-westerlies (strong wind speed within a range of 15–20 ms−1) along the west coast of India, known as a low-level jet [41], can be seen over AS. The wind speed of the core of the low-level jet is ~20 ms−1 from 5° N to 15° N over

AtmosphereAS and the2020 west, 11, 740 coast of India. A strong dry-cold, north-westerly wind flow pattern from Middle6 of 21 East countries merged over the Indian landmass, with highly moist-warm, south-westerly wind causing unstable atmospheric conditions between the longitude of 50° E –70° E (Figures 2 and 3 and Figure4.2. Large S2). and The Synoptic-Scale combination Systems of two contrasting Concomitant air with masses the Kerala in a Floodboundary layer flow produces heavy rainfallLow-Level over the Jetwestern (LLJ), Low-Pressure-System,coast, termed the coasta Olff phaseshore Trough,[42]. The and anti-cyclonic Blocking High circulation over the MiddleDuring East theregion summer and Tibet monsoon region LPS at over300 mb BoB served (at ~996 as mb),the anomalous the offshore blocking trough (green high, limiting dashed line),LPS toand the dry south air intrusion of the mean from theposition middle-East (black (atdashed 600 mb) line are in as Figure displayed 1a). inOn Figure many2 a–e.of the A well-organizedoccasions, it is perceivedfeature of ISMR,that the the blocking offshore high trough, produces is visible a indry the spell MSLP of fromthe monsoon, Kerala to thedroughts, Gujrat regionheatwaves, (Figure and2, floodssee along in the the pre-monsoon west coast of India).and monsoon Besides, season. the intrusion A previous of dry study air from advocated the Middle that the East moist is well-mixed and cold air-massat 600 mb advected into the LPS from over the the maritime Indian region.region Acould previous be referred study showed to as LLJ, that and in such another situations, dry-cold dry air- air massintrusion from can the introduce Middle East non-uniform desert could saturated be referred instability to as to continental produce heavy tropical rainfall [43]. [Both40]. Alongair-masses with wereLPS and observed the off shorefrom trough,850 to 500 the mixinghPa levels of cold-dry and the airsystem from theof different Middle East air masses and warm-moist merged into air fromLPS overas it isthe one eastern of the coast critical of India features (Figure in the 3a–d). formation For the of non-uniformlymiddle to upper saturated troposphere, air and two static high-pressure instability systemsthat can beare exploited prominent: to leadone toover heavy the rainfall.Middle Eastern This kind area of instability(30° N –35 is °N not and a very 50° commonE –60 °E) elementand Tibet in regionthe generation (Figure of3d), flood and events. the other The acting interaction as the betweenblocking monsoon high and depression, forcing monsoon the offshore depression trough, to and be limiteddry air intrusionto the south is responsible of the mean for po thesition heavy of monsoonal rainfall over depressions. Kerala state (Figure2).

Figure 2. Accumulated total rainfall (shaded; mm d 1), mean sea level pressure (contours), and wind Figure 2. Accumulated total rainfall (shaded; mm·d· −1), mean sea level pressure (contours), and wind flowﬂow pattern pattern at at the the 600 600 hPa hPa level level valid valid at 00 at 00UTC UTC on ( ona) 13 (a )August, 13 August, (b) 14 (b August,) 14 August, (c) 15 (Augustc) 15 August (d) 16 (d) 16 August, (e) 17 August, and (f) 18 August 2018 derived from ERA-Interim reanalysis at 25 km resolutions. Dashed green lines indicate the position of the oﬀshore trough. Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 740 7 of 21

The daily mean wind flow pattern at 850 to 300 hPa levels from 13 to 17 August 2018 is displayed in Figure3a–t. At the 850 hPa level, wind flow characterized by strong south-westerlies (strong wind 1 speed within a range of 15–20 ms− ) along the west coast of India, known as a low-level jet [41], can be 1 seen over AS. The wind speed of the core of the low-level jet is ~20 ms− from 5◦ N to 15◦ N over AS and the west coast of India. A strong dry-cold, north-westerly wind flow pattern from Middle East countries merged over the Indian landmass, with highly moist-warm, south-westerly wind causing unstable atmospheric conditions between the longitude of 50◦ E–70◦ E (Figures2 and3 and Figure S2). The combination of two contrasting air masses in a boundary layer flow produces heavy rainfall over the western coast, termed the coastal phase [42]. The anti-cyclonic circulation over the Middle East region and Tibet region at 300 mb served as the anomalous blocking high, limiting LPS to the south of the mean position (black dashed line in Figure1a). On many of the occasions, it is perceived that the blocking high produces a dry spell of the monsoon, droughts, heatwaves, and floods in the pre-monsoon and monsoon season. A previous study advocated that the moist and cold air-mass advected from the maritime region could be referred to as LLJ, and another dry-cold air-mass from the Middle East desert could be referred to as continental tropical [43]. Both air-masses were observed from 850 to 500 hPa levels and the system of different air masses merged into LPS over the eastern coast of India (Figure3a–d). For the middle to upper troposphere, two high-pressure systems are prominent: one over the Middle Eastern area (30◦ N–35◦ N and 50◦ E–60◦ E) and Tibet region (Figure3d), and the other acting as the blocking high and forcing monsoon depression to be limited to the south of the mean position of monsoonal depressions. 1 As an analogy, KFE carries a silent feature of an active monsoon viz. a high speed of LLJ, 18 ms− (Figure3i), and relative vorticity of 100 10 6 s 1 (Figure S3a,b), which are way higher than the × − · − corresponding typical values recorded during the monsoon season [30]. The west coast rainfall is associated with the offshore trough, the interaction between TEJ and STWJ, and LPS embedded with mesoscale bands, which are linked to the movement of a disturbance from the BoB to Indian landmass. Atmosphere 2020, 11, x FOR PEER REVIEW 7 of 21

AtmosphereAugust,2020 , (11e), 74017 August, and (f) 18 August 2018 derived from ERA-Interim reanalysis at 25 km8 of 21 resolutions. Dashed green lines indicate the position of the offshore trough.

−11 FigureFigure 3.3. WindWind speedspeed (shaded;(shaded; msms− ) and direction at (a) 850,850, ((b)) 700,700, ((cc)) 500,500, andand ((dd)) 300300 hPahPa levelslevels validvalid atat 0000 UTCUTC onon ((aa)) 1313 August,August, 2018;2018; similarly,similarly, ((ee––hh,,i i––ll,,m m––ppand andq q––tt)) presentpresent thethe 14,14, 15,15, 16,16, andand 1717 AugustAugust 20182018 data data derived derived from from ERA-Interim ERA-Interim reanalysis reanalysis at at 25 25 km km resolutions. resolutions.

4.3. RoleAs an of Kineticanalogy, Energy KFE Duringcarries a the silent Kerala feature Flood of an active monsoon viz. a high speed of LLJ, 18 ms−1 (FigureThe 3i), diurnal and variabilityrelative vorticity of kinetic of 100 energy × 10 (KE−6·s− at1 (Figure 850 hPa), S3a–b), net tropospheric which are way moisture higher (from than 1000 the tocorresponding 300 hPa), and typical mean troposphericvalues recorded temperature during the (from monsoon 1000 to season 100hPa) [30]. were The averaged west coast for therainfall active is periodassociated of 8–21 with August the offshore 2018. trough, The average the interact wasion obtained between for TEJ over and the STWJ, Arabian and LPS Sea embedded (50◦ E–72.5 with◦ E; 5mesoscale◦ N–15◦ N), bands, Bay of which Bengal are (0–19.5 linked◦ N, 78to◦ theE–96 moveme◦ E), andnt Indian of a landmassdisturbance (72.5 from◦ E–82.5 the◦ BoBE; 5◦ toN–15 Indian◦ N) (Figurelandmass.4a–c), respectively. We followed the procedure presented in [ 44,45] to compute the kinetic energy, net tropospheric moisture, and mean tropospheric temperature. Figure4a depicts the daily variations of kinetic energy over AS, BoB, and the Indian landmass at 0000 UTC on 8–21 August 2018. It shows a significant difference in kinetic energy over the Kerala flood region and other nearby regions. It is suggested that the threshold value of kinetic energy (at 850 hPa) during the onset period was Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, x FOR PEER REVIEW 8 of 21

4.3. Role of Kinetic Energy During the Kerala Flood The diurnal variability of kinetic energy (KE at 850 hPa), net tropospheric moisture (from 1000 to 300 hPa), and mean tropospheric temperature (from 1000 to 100hPa) were averaged for the active period of 8–21 August 2018. The average was obtained for over the Arabian Sea (50° E –72.5° E; 5° N –15° N), Bay of Bengal (0–19.5° N, 78° E –96° E), and Indian landmass (72.5° E –82.5° E; 5° N –15° N) (Figure 4a–c), respectively. We followed the procedure presented in [44,45] to compute the kinetic energy, net tropospheric moisture, and mean tropospheric temperature. Figure 4a depicts the daily variations of kinetic energy over AS, BoB, and the Indian landmass at 0000 UTC on 8–21 August 2018. It shows a significant differenceAtmosphere in kinetic2020 ,energy11, 740 over the Kerala flood region and other nearby regions. It is 9 of 21 suggested that the threshold value of kinetic energy (at 850 hPa) during the onset period was about ~40 m2s−2 over the Indian monsoon2 domain2 [45]; during KFE, the kinetic energy was always >80 m2s−2. about ~40 m s− over the Indian monsoon domain [45]; during KFE, the kinetic energy was always Additionally, the kinetic energy2 2 was quite strong over the BoB, with a peak of ~150 m2s−2, as shown 2 2 >80 m s− . Additionally, the kinetic energy was quite strong over the BoB, with a peak of ~150 m s− , in Figure 4a. The magnitudeas shown of in kinetic Figure4 energya. The magnitudeand net mean of kinetic temperature energy andduring net the mean Kerala temperature flood during the Kerala were higher than theirflood corresponding were higher thanclimatologic their correspondingal/average values climatological (Figure/ averageS4) for the values same (Figure days S4) for the same days and over the same region.and over It is theevident same from region. the Itdi issplay evident that from the kinetic the display energy that over the the kinetic entire energy Indian over the entire Indian monsoon region (AS, BoB, and Indian landmass) varied from 65 to 160 m2s−2, which indicates2 highly2 monsoon region (AS, BoB, and Indian landmass) varied from 65 to 160 m s− , which indicates highly active conditions of theactive Indian conditions monsoon. of the The Indian mean monsoon.tropospheric The temperature mean tropospheric remained temperature lower than remained lower than its climatology, whichits is climatology, due to the excess which rainfall is due toover the AS, excess BoB, rainfall and the over Indian AS, landmass BoB, and the(Figure Indian 4c). landmass (Figure4c). Two apparent indicatorsTwo of apparent the extreme indicators rainfall ofevent the (here, extreme KFE), rainfall viz. the event net (here,tropospheric KFE), viz. moisture the net and tropospheric moisture kinetic energy, were significantlyand kinetic energy, superior were to their significantly corresponding superior climatological to their corresponding values (Figure climatological 4b,c). In values (Figure4b,c). the case of KFE, theIn transport the case ofof KFE,moisture the transportwas distinct of moisture from the was nearby distinct Arabian from Sea, the nearbywhile the Arabian Sea, while the offshore trough was oresponsibleffshore trough for the was convergence responsible forof the the flux convergence of moisture. of the flux of moisture.

Atmosphere 2020, 11, x FOR PEER REVIEW 9 of 21

2 2 Figure 4. (a) 850 hPa daily variation of kinetic energy (m s− ) over the Arabian(c) Sea (50◦ E–72.5◦ E; 5◦ N–15◦ N; in black), Bay of Bengal (BoB) (82.5◦ E–97.5◦ E; 5◦ N–15◦ N; in red), and Indian landmass 2 −2 (72.5◦ E–82.5◦ E; 5◦ N–15◦ N;Figure in blue) 4. ( werea) 850 derived hPa daily from variation ERA-Interim of kinetic reanalysis energy (m fors 00) over UTC the of Arabian Sea (50° E –72.5° E; 5 ° 1 8–21 August 2018, as were (b) theN net–15° tropospheric N; in black), moisture Bay of Bengal (NTM; ( gmBoB kg) (−82.5) and° E (–c97.5°) mean E; tropospheric 5° N –15° N; in red), and Indian landmass temperature (MTT; ◦K). (72.5° E –82.5° E; 5° N –15° N; in blue) were derived from ERA-Interim reanalysis for 00 UTC of 08– 21 August 2018, as were (b) the net tropospheric moisture (NTM; gm Kg−1) and (c) mean tropospheric 5. Precipitation and Tropospherictemperature Features (MTT; °K). An accurate estimate of the precipitation intensity and accumulation is essential to understanding the cycle of global water fluxes5. Precipitation and the energy and balance Tropospheric of Earth’s systemFeatures [46 ]. A rainfall of >150 mm/day Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere is defined as extreme rainfall overA centraln accurate India [ estimate47,48]. However, of the inprecipitat the monsoonion intensity period, the and amount accumulation is essential to and duration of extreme rainfall,understand whiching can the produce cycle floods, of global diff ers water from fluxes one geographical and the energy location balance to of Earth’s system [46]. A another. In the case of KFE, therainfall total dailyof >150 rainfall mm/day was moreis defined than 200as extreme mm/day, rainfall which occurredover central over India the [47,48]. However, in the west coast of India (Figure5monsoona–i). Additionally, period, the in amount (Figure 5anda–i), duration the rainfall of extreme is continuous, rainfall, stationary,which can produce floods, differs from and intense over Kerala stateone and geographical adjacent regions location of India.to another. At the In same the case time, of over KFE, other the total areas daily (e.g., rainfall was more than 200 BoB), the rainfall remains nonstationary.mm/day, which On occurred 14 August, over moderatethe west coast to heavy of India rainfall (Figure occurred 5a–i). Additionally, over in (Figure 5a–i), the coastal regions of Karnatakarainfall state and is continuous, continued overstationary Kerala, and state intense for the over next Kerala two days state (Figure and adjacent5a). regions of India. At the The next day, very heavy rainfallsame wastime, recorded over other over area boths (e.g. Karnataka, BoB), the and rainfall Kerala statesremain (Figures nonstationary.5b), but On 14 August, moderate to heavy rainfall occurred over coastal regions of Karnataka state and continued over Kerala state for the next two days (Figure 5a). The next day, very heavy rainfall was recorded over both Karnataka and Kerala states (Figure 5b), but the core of the intense rainfall centered in the middle of Kerala state (Figure 1b and Figure 5b). On 16 August, the rainfall was reduced over the Kerala state, as represented in Figure 5c. Figure 5a–c shows the clustering of total rainfall (sum of large-scale precipitation and convective precipitation) during KFE, closely agreeing with IMD + TRMM merge analysis (Figure 5a–c). Before and on the day of the flood, most of the rainfall was due to convective precipitation (Figure 5d,e) rather than large-scale stratiform rainfall (Figure 5g,h). On the next day, 16 August, rainfall from CP and LSP reduced in intensity and space (Figure 5f,i). Over the west coast, rainfall is associated with an offshore trough produced by shallow convective clouds (cloud top ~6 km). Figure 5g–i shows large-scale rainfall along the west coast and east coast that is attributed to offshore troughs. However, on 17 August (Figure is not shown), the quantity of rainfall reduced drastically, because the intensity of the LPS and offshore trough was diminished. The combination of synoptic systems, such as monsoon depression, strong LLJ, offshore troughs along the west coast, and the low-pressure system over BoB caused convective rainfall over the west coast. However, the large-scale precipitation represents the regular monsoonal rainfall of Western Ghats due to LLJ and offshore troughs.

Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 740 10 of 21 the core of the intense rainfall centered in the middle of Kerala state (Figures1b and5b). On 16 August, the rainfall was reduced over the Kerala state, as represented in Figure5c. Figure5a–c shows the clustering of total rainfall (sum of large-scale precipitation and convective precipitation) during KFE, closely agreeing with IMD + TRMM merge analysis (Figure5a–c). Before and on the day of the flood, most of the rainfall was due to convective precipitation (Figure5d,e) rather than large-scale stratiform rainfall (Figure5g,h). On the next day, 16 August, rainfall from CP and LSP reduced in intensity and space (Figure5f,i). Over the west coast, rainfall is associated with an o ffshore trough produced by shallow convective clouds (cloud top ~6 km). Figure5g–i shows large-scale rainfall along the west coast and east coast that is attributed to offshore troughs. However, on 17 August (Figure is not shown), the quantity of rainfall reduced drastically, because the intensity of the LPS and offshore trough was diminished. The combination of synoptic systems, such as monsoon depression, strong LLJ, offshore troughs along the west coast, and the low-pressure system over BoB caused convective rainfall over the west coast. However, the large-scale precipitation represents the regular monsoonal rainfall of WesternAtmosphere Ghats 2020, 11 due, x FOR to LLJPEER and REVIEW offshore troughs. 10 of 21

FigureFigure 5. (a–c).) 24-h24-h accumulatedaccumulated rainfall rainfall (mm (mm·dd −11) distributions validvalid atat 0300 UTC onon 14 August 2018, 2018, · − 03000300 UTCUTC on on 15 15 August August 2018, 2018, and and 0300 0300 UTC UTC on 16 on August 16 August 2018, 2018, respectively, respectively, derived derived from IMD from+ TRMM IMD + mergedTRMM analysis.merged analysis. Figure ( dFigure–f,g–i )( isd– thef,g– samei) is the as (asame–c), butas ( fora–c convective), but for convective and large-scale and large-scale stratiform precipitationstratiform precipitation (mm d 1 ERA-Interim (mm·d−1 ERA-Interim reanalysis), reanalysis), respectively. respective Both thely. datasests Both the used datasests 25 km resolutions.used 25 km · − resolutions. Finally, we examined some of the valued hydrological variables over the KFE region. The total runoﬀFinally,, temperature we examined at 2 m, some and evaporationof the valued are hydrol displayedogical invariables Figure 6overa–i. the One KFE of the region. recognizable The total runoff, temperature at 2 m, and evaporation are displayed in Figure 6a–i. One of the recognizable hydrological impacts of the flood is runoff, which reached the maximum over the Kerala region, especially on 15 and 16 August (Figure 6b,c). On the next day of KFE, runoff increased over a large part of the land surface. Generally, temperature and evaporation are temporarily lowered over locations of moderate to heavy rainfall (Figure 6d–f,g–i). A low value of 2 m shows the areas of Western Ghats with a higher peak and maximum rainfall spots. However, a reduction of temperature and evaporation due to the occurrence of rainfall can be seen, whose maxima do not match the peaks of rainfall. The magnitude of temperature and evaporation shows a reduction in most of the rainfall episodes.

Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 740 11 of 21 hydrological impacts of the flood is runoff, which reached the maximum over the Kerala region, especially on 15 and 16 August (Figure6b,c). On the next day of KFE, runo ff increased over a large part of the land surface. Generally, temperature and evaporation are temporarily lowered over locations of moderate to heavy rainfall (Figure6d–f,g–i). A low value of 2 m shows the areas of Western Ghats with a higher peak and maximum rainfall spots. However, a reduction of temperature and evaporation due to the occurrence of rainfall can be seen, whose maxima do not match the peaks of rainfall.

TheAtmosphere magnitude 2020, 11, of x FOR temperature PEER REVIEW and evaporation shows a reduction in most of the rainfall episodes.11 of 21

Figure 6. Total surface runoﬀ (mm d 1) distributions over the Kerala land region valid at 00 UTC on · − (a) 14 August, (b) 15 August, and (c) 16 August 2018 derived from ERA-Interim reanalysis at 25 km Figure 6. Total surface runoff (mm·d−1) distributions over the Kerala land region valid at 00 UTC on resolutions; both Figure (d–f) and (g–i) are the same as (a–c), but for temperature (◦C) and evaporation (a) 14 August,1 (b) 15 August, and (c) 16 August 2018 derived from ERA-Interim reanalysis at 25 km (mm d− ), respectively. resolutions;· both Figure (d–f) and (g–i) are the same as (a–c), but for temperature (°C) and evaporation (mm·d−1), respectively.

6. Vertical Cross-Sections Over the Flood Event and LPS Meteorological variables related to heavy rainfall have a relatively large magnitude and penetrate deep in the troposphere. The cross-sections (longitudinal and pressure level) at the location of KFE and LPS of the relative humidity, abnormal temperature, and wind flow are shown in Figure 7a–f. The center of LPS significantly demonstrates strong LLJ at the lower troposphere during the intrusion of colder and drier air in the middle troposphere. The dry air imposition in the middle troposphere is seen on all days over AS and BoB. At a 300 mb level, a divergence of wind is observed on 15 August; however, at a lower level, LLJ dominates over convergence.

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6. Vertical Cross-Sections Over the Flood Event and LPS Meteorological variables related to heavy rainfall have a relatively large magnitude and penetrate Atmosphere 2020, 11, x FOR PEER REVIEW 12 of 21 deep in the troposphere. The cross-sections (longitudinal and pressure level) at the location of KFE and LPSAdditionally, of the relative lower-level humidity, westerly abnormal wind temperature, and an accumulation and wind of moisture ﬂow are with shown vigorous in Figure vertical7a–f. The centermotion of are LPS evident signiﬁcantly from 14 to demonstrates 16 August (Figure strong 7d LLJ–f). The at the values lower of tropospherethe temperature during anomaly the intrusionat the of coldercenter and of the drier monsoon air in the depression middle troposphere. are significantl They higher dry air (~1.0 imposition °K) than in those the middleof the environment troposphere is seen(surroundings), on all days over whereas AS and significant BoB. At a 300cooling mb is level, noticed a divergence in the upper of wind troposphere. is observed These on features 15 August; however,closely at agree a lower with level, the study LLJ dominates [44] on the formation over convergence. of LPS during flood events. The location of KFE at the lower level accumulated plenty of moisture transported from the AS (Figure 7a–c).

FigureFigure 7. ( a7.– c()a–c Longitudinal). Longitudinal and and pressure pressure level level cross-section cross-section of of the the relative relative humidity humidity (%; (%; shaded), shaded), −11 temperaturetemperature anomaly anomaly (◦K; (°K; blue blue contour), contour), and and wind wind flow flow patternpattern (ms−) )along along the the coast coast of ofthe the Arabian Arabian Sea atSea 10.5 at◦ 10.5°N valid N valid at 00 at UTC 00 UTC on 14–16 on 14–16 August August 2018; 2018; (d–f )(d are–f) theare samethe same as (a as–c ),(a– butc), forbut thefor cross-sectionthe cross- oversection the east over coast the of east BoB coast at 20.5 of◦ NBoB derived at 20.5° from N de ERA-Interimrived from ERA-Interim reanalysis at reanalysis a 25 km resolution. at a 25 km resolution. Additionally, lower-level westerly wind and an accumulation of moisture with vigorous vertical motion areThe evident time-evolution from 14 toof 16the August vertical (Figure variation7d–f). of Thethe specific values ofhumidity the temperature over KFE anomalydisplays atan the enhancement of the moisture content from the surface to the 500 hPa level (Figure 8a). The center of the monsoon depression are significantly higher (~1.0 ◦K) than those of the environment accumulation of specific humidity reached up to 350 mb over the Kerala region, while over BoB, it (surroundings), whereas significant cooling is noticed in the upper troposphere. These features closely was up to 600 mb during the KFE. Notably, the middle troposphere was wetter than the boundary agree with the study [44] on the formation of LPS during flood events. The location of KFE at the lower layer (Figure 8a), with vigorous upward flow depicting heavy rainfall from 14 to 16 August compared levelto accumulated BoB. Concomitantly, plenty of on moisture 15 August transported 2018, there was from a thesubstantial AS (Figure build-up7a–c). of moisture in the lower troposphere that conforms to the accumulation of moisture associated with upward motion (Figure

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8a). The regions of vigorous convection show the strong vertical velocity on the order of 5 and 10 m/s over BoB and KFE, respectively. In contrast, the order of magnitude of specific humidity is almost constant, but a little extended in the vertical direction. These numbers are very typical and sufficient for the initiation of active rainfall events. Furthermore, such moisture build-up in the atmospheric column may support flood events on a local scale. A spatial pattern and vertical cross-section of the total column water vapor (kg·m−2) and cloud Atmosphere 2020, 11, 740 13 of 21 ice water content (mg/kg) are displayed in Figure 9. These variables reveal the kind of cloud that microphysics carry during KFE and are displayed spatially and vertically. The cloud liquid water contentThe and time-evolution cloud ice water of thecontent vertical are evident variation alon ofg thethe specificwest coast humidity of India, over but KFEnot over displays the KFE an enhancementlocation (Figure of the9a–c). moisture This excludes content from the possibilit the surfacey of to any the 500big hPasystem level being (Figure responsible8a). The accumulation for the KFE. ofThe specific KFE seems humidity to have reached been up influenced to 350 mb over regional the Keralaly and region, locally while by low-pressure over BoB, it was systems, up to 600which mb duringfavored the torrential KFE. Notably, downpour. the middle The figure troposphere indicates was th wetterat the thanmaximum the boundary values of layer moisture (Figure layers8a), with are vigorousindeed overlapping upward flow over depicting the locations heavy of rainfall the offsho fromre 14 trough to 16 Augustand monsoon compared depression to BoB. Concomitantly,along the west oncoast 15 of August India 2018,and east there coast was of asubstantial BoB. The intensity build-up of of the moisture offshore in thetrough lower does troposphere not vary thatmuch conforms (Figure toS5a), the but accumulation its small deviation of moisture when associated moving withinland upward can produce motion heavy (Figure rainfall.8a). The The regions total ofperceptible vigorous convectionwater content show did the surge strong a little vertical during velocity the KFE on the (Figure order S5b). of 5 and 10 m/s over BoB and KFE, respectively.

Figure 8. Pressure-time cross-section of the area averagedaveraged over Kerala and adjoining the Arabian Sea 1 (9(9°◦ N–12N –12°◦ N; N; 72 72°◦ E–77 E –77◦ E): °E): (a) Specific(a) Specific humidity humidity anomaly anomaly (shaded; (shaded; gm kg gm·Kg− ) and−1) vertical and vertical velocity velocity ( 100; 1 · × m(×100;− s; bluem−1s; contour)blue contour) at 00 at UTC 00 UTC on 13–18 on 13–18 August August 2018 2018 derived derived from from ERA-Interim ERA-Interim reanalysis reanalysis at 25 at km 25 resolutions.km resolutions. Figure Figure (b) is(b the) is same the same as Figure as Figure (a), but (a shows), but area-averagedshows area-averaged values ofvalues the low-pressure of the low- systempressure over system the Bayover of the Bengal Bay of (17.5 Bengal◦ N–26 (17.5°◦ N; N 72 –26°◦ E–88.5 N; 72°◦ E). E –88.5° E). In contrast, the order of magnitude of specific humidity is almost constant, but a little extended in the vertical direction. These numbers are very typical and sufficient for the initiation of active rainfall events.Atmosphere Furthermore, 2020, 11, x; doi:such FOR PEER moisture REVIEW build-up in the atmospheric columnwww.mdpi.com/journ may support floodal/atmosphere events on a local scale. A spatial pattern and vertical cross-section of the total column water vapor (kg m 2) and cloud · − ice water content (mg/kg) are displayed in Figure9. These variables reveal the kind of cloud that microphysics carry during KFE and are displayed spatially and vertically. The cloud liquid water content and cloud ice water content are evident along the west coast of India, but not over the KFE location (Figure9a–c). This excludes the possibility of any big system being responsible for the KFE. Atmosphere 2020, 11, 740 14 of 21

The KFE seems to have been influenced regionally and locally by low-pressure systems, which favored torrential downpour. The figure indicates that the maximum values of moisture layers are indeed overlapping over the locations of the offshore trough and monsoon depression along the west coast of India and east coast of BoB. The intensity of the offshore trough does not vary much (Figure S5a), but its small deviation when moving inland can produce heavy rainfall. The total perceptible water contentAtmosphere did 2020 surge, 11, x aFOR little PEER during REVIEW the KFE (Figure S5b). 14 of 21

FigureFigure 9.9. TheThe cloudcloud liquidliquid waterwater contentcontent (shaded)(shaded) andand cloudcloud iceice waterwater contentcontent (contour)(contour) horizontalhorizontal ((aa)) and vertical-cross section section (b (b) )valid valid at at 00 00 UTC UTC on on 14 14August August 2018. 2018. Both Both (c–d) ( cand–d) ( ande–f) (aree–f )the are same the sameas (a– asb), ( abut–b), are but valid are valid for 00 for UTC 00 UTC 15–16 15–16 on August on August 2018 2018 derived derived from from ERA-Interim ERA-Interim reanalysis reanalysis at 25 at 25km km resolutions. resolutions.

7. Other Details of the Extreme Flood Event There are many other issues (hydrological, social, economic, and environmental) that made the KFE the worst flood of the century and left footprints on the community in the short and long term. There were two episodes of heavy rainfall: one through 8–9 August and another that included very heavy rainfall through 15–17 August. Therefore, it appeared that most of the Kerala reservoirs were saturated and reached their capacity of the Full Reservoir Level (FRL) before KFE. A CWC report (Central Water Commission, India) indicates that the major dams of Kerala were filled with water ~95% of their storage capacity. Gates of about 35 dams were suddenly opened (without warming)

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7. Other Details of the Extreme Flood Event There are many other issues (hydrological, social, economic, and environmental) that made the KFE the worst flood of the century and left footprints on the community in the short and long term. There were two episodes of heavy rainfall: one through 8–9 August and another that included very heavy rainfall through 15–17 August. Therefore, it appeared that most of the Kerala reservoirs were saturated and reached their capacity of the Full Reservoir Level (FRL) before KFE. A CWC report (CentralAtmosphere Water 2020, 11 Commission,, x FOR PEER REVIEW India) indicates that the major dams of Kerala were filled with water15 ~95% of 21 of their storage capacity. Gates of about 35 dams were suddenly opened (without warming) during during KFE due to a massive inflow of water from the first week of August rainfall, specifically from KFE due to a massive inflow of water from the first week of August rainfall, specifically from severe severe rainfall on 15–16 August 2018. The CWC (Central Water Commission) report adds that the rainfall on 15–16 August 2018. The CWC (Central Water Commission) report adds that the dams did dams did not help in mitigating the flood; however, dams are not to be blamed for KFE dams being not help in mitigating the flood; however, dams are not to be blamed for KFE dams being at their full at their full capacity before the KFE. In such a case, dams have to release excess water or they may capacity before the KFE. In such a case, dams have to release excess water or they may collapse [7]. collapse [7]. Dams, reservoirs, and wetlands are, in general, created for the generation of electricity, flood Dams, reservoirs, and wetlands are, in general, created for the generation of electricity, flood control, and maintaining a water supply. However, heavy rainfall for an extended period and the control, and maintaining a water supply. However, heavy rainfall for an extended period and the release of excess water from overflowing dams may be answerable to some extent [5,6]. The path of release of excess water from overflowing dams may be answerable to some extent [5,6]. The path of rivers, location of four dams, and orography of Kerala are shown in Figure 10. Idukki, Periyar, and rivers, location of four dams, and orography of Kerala are shown in Figure 10. Idukki, Periyar, and Kakki represent the center of excess rainfall. Kakki is the major reservoir in the Pamba sub-basin. Kakki represent the center of excess rainfall. Kakki is the major reservoir in the Pamba sub-basin. The The collected water in the four major and well-known dams (Kakki, Periyar, Idukki, and Malampuzha) collected water in the four major and well-known dams (Kakki, Periyar, Idukki, and Malampuzha) during the Kerala event in 2018 surpassed the average value of accumulated water from 2009–2018 and during the Kerala event in 2018 surpassed the average value of accumulated water from 2009–2018 the previous year (2017) (Figure 11a–d). The maximum water discharged from Idukki and Idamalayar and the previous year (2017) (Figure 11a–d). The maximum water discharged from Idukki and dams into Pariyar River was 8800 cumes on 16 August 2019, and the maximum amount of water Idamalayar dams into Pariyar River was 8800 cumes on 16 August 2019, and the maximum amount attained in the river was 12.40 m high in comparison to the last flood on 27 July 1974, which had a of water attained in the river was 12.40 m high in comparison to the last flood on 27 July 1974, which value of 11.11 m high. had a value of 11.11 m high.

Figure 10. Thenetwork of of rivers, the elevation of the co coastalastal region, and the location of four major reservoirsreservoirs/dams/dams in Kerala.

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(a) (b)

Figure 11. ((aa––dd).). The The storage storage capacity capacity of offour four reservoirs/dams reservoirs/dams in inKerala: Kerala: (a) (Kakki,a) Kakki, (b) (Periyar,b) Periyar, (c) (Idukki,c) Idukki, and and (d) (Malampuzha.d) Malampuzha.

Before and during the the time time of of KFE, KFE, the the water water collected collected in in several several dams dams reached reached FRL. FRL. In Interms terms of ofthe the Idukki Idukki dam, dam, about about 345 345million million cubic cubic meters meters (mcm (mcm,, for spill) for spill)of water of waterwas released was released in 3 days in 3(15–17 days (August15–17 August 2018) [38]. 2018 The)[38 water]. The runoff water runoin Figureff in Figure5j–l indicates5j–l indicates a jump a jumpon 15 onAugust 15 August and 16 and August. 16 August. This Thisrunoff runo waterff water due to due heavy to heavy precipitation precipitation shows shows the de thevelopment development of a flood of a due flood to duethe filling to the of filling rivers, of rivers,reservoirs, reservoirs, and dams. and Idukki dams. Idukkidam and dam Mullaperiya and Mullaperiya dam are dam located are located across the across Periyar the Periyar River. There River. Therewas a wasrelease a release of water of into water Periyar into Periyar and Pamba and Pambarivers from rivers their from associated their associated dams; thus, dams; the thus, villages the villagesand lives and in their lives incatchment their catchment area were area affected were a fftheected most the (Figure most (Figure11) [38]. 11 Flooding)[38]. Flooding execrated execrated due to duethe release to therelease of water of from water dams from can dams be canattributed be attributed to the underestimation to the underestimation of water of watermanagement management in the inKFE the case. KFE A case. NASA A NASAreport on report the Kerala on the flood Kerala and flood this and study this demonstrate study demonstrate the role of the water role ofreleased water releasedfrom the fromdams, the as dams,well as as water well asfrom water heavy from precipitation, heavy precipitation, resulted resultedin it becoming in it becoming the worst the flood worst of floodthe century of the centuryin Kerala in [7]. Kerala [7]. Apart fromfrom the the heavy heavy precipitation precipitation over Kerala, over theKerala, flood wasthe exacerbatedflood was byexacerbated the mismanagement by the ofmismanagement water released fromof water dams, released landslides, from and dams, stormwater. landslides, Another and stormwater. issue left behind Another by theissue KFE left was behind that theby the course KFE of was rivers that was the altered,course of due rivers to the was erosion altered, of soildue and to the storm erosion surge. of Mostsoil and of thestorm dams surge. of Kerala Most areof the dedicated dams of to Kerala hydro-electric are dedicated power generation,to hydro-el andectric the power rest are generation, used to regulate and the irrigation rest are practices. used to Therefore,regulate irrigation other aspects practices. of Kerala Therefore, dams and other reservoirs, aspects e.g.,of Kerala flood controldams an andd reservoirs, water availability, e.g., flood are debatable.control and The water old availability, and poor management are debatable. practices The old of and the poor dams management in terms of floodwater practices of management the dams in mayterms also of floodwater have contributed. management Poor dammay also and have reservoir contributed. management Poor dam may haveand reservoir aggravated management the flood. However,may have evenaggravated if the authoritythe flood. of However, dams had even released if the theauthority total water of dams collected had released before KFE,the total the water dams stillcollected would before not have KFE, mitigated the dams this still flood would [7], because not have unexpected mitigated rainfall this flood distributed [7], because a lot of unexpected water over therainfall region. distributed a lot of water over the region.

8. Concluding Remarks and Discussion Continuous rainfallrainfall of of more more than than a week a week occurred occurred during during 9–18 August, 9–18 Augu 2018,st, over 2018, Kerala, over including Kerala, aincluding heavy rainfall a heavy event rainfall from event 15 to from 16 August. 15 to 16 The Augu KEFst. didThe notKEF manifest did not anymanifest single any prominent single prominent synoptic featuresynoptic which feature one which can rely one upon. can rely The upon. interactions The interactions between the between quasi-stationary the quasi-stationary oﬀshore trough offshore and deviatedtrough and LPS deviated embedded LPS in theembedded large-scale in ﬂowthe large-sc and mid-troposphericale flow and mid-tropospheric cold-dry air triggered cold-dry the KFE air fortriggered almost the two KFE days. for Based almost on two the days. diagnostics Based ofon MSLP, the diagnostics the winds of from MSLP, 850 the to 300 winds hPa, from kinetic 850 energy, to 300 hPa, kinetic energy, net tropospheric moisture, and mean tropospheric temperature can be employed to paint a realistic picture of Kerala’s extreme rainfall event. The results are summarized in the following points: Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 740 17 of 21 Atmosphere 2020, 11, x FOR PEER REVIEW 17 of 21

• net troposphericThe wind and moisture, vorticity and field mean over tropospheric the coast of temperature the BoB (18° can N; be86° employed E) was consistent to paint a with realistic the picturemaximum of Kerala’s rainfall extreme in the rainfall south-west event. sector The results of the arelong-lasting summarized monsoon in the depression. following points: The cyclonic circulation somehow extended from the BoB to AS and persisted for up to two days, which Thecaused wind severe and vorticityrainfall fieldover overKerala the and coast the of theWestern BoB (18Ghats.◦ N; 86Monsoonal◦ E) was consistent depression with has the a • maximumsouthwest rainfalltilt in the in vertical the south-west direction; sector thus, of th thee pumping long-lasting of moisture monsoon by depression. the depression Thecyclonic into the circulationmiddle troposphere somehow is extended a possible from aspect the (Figures BoB to 1 AS and and Figure persisted 2); for up to two days, which • causedThe KFE severe was rainfallan outcome over Keralaof the interaction and the Western between Ghats. the Monsoonaloffshore trough, depression LPS, dry has air a southwest intrusion tiltin the in themiddle vertical troposphere, direction; and thus, ot theher pumpingregional features of moisture (Figure by 12). the depressionAdditionally, into the the mixing middle of tropospherecold-dry air isfrom a possible the Middle aspect East (Figures region1 andin th2e); middle troposphere with south-westerly winds Thein the KFE lower was troposphere an outcome was of the sufficient interaction to create between an unstable the offshore atmosphere trough, during LPS, dry KFE air (Figure intrusion 3); • • inDue the to middle continuous troposphere, rainfall, most and other of the regional major dams features in the (Figure Kerala 12 state). Additionally, were filled with the mixingrainwater of cold-drybefore the air KFE. from Water the Middle released East from region major in the reservoirs middle troposphere into the rivers with might south-westerly be one of the winds other in thepossible lower factors troposphere which wasworsened sufficient this to flood create (Figure an unstable 11). atmosphere during KFE (Figure3);

FigureFigure 12.12. Interaction of the mid-tropospheric mid-tropospheric cold-dry ai air,r, ooff-shoreﬀ-shore trough,trough, andand low-pressure systemsystem overover thethe IndianIndian regionregion duringduring thethe KeralaKerala FloodFlood EventEvent (KFE).(KFE).

DueIn the to last continuous few decades, rainfall, the most mean of surface the major temp damserature in the over Kerala the Western state were Ghats filled has with significantly rainwater • escalated.before Thermodynamic the KFE. Water releasedsituations from favor major intense reservoirs and deep into precipitation the rivers might over bethe one Western of the Ghats, other whichpossible makes factorsthe region which prone worsened to flash this floods flood at present (Figure and11). in the future [34]. Additionally, landslide and storm surges are other possible reasons for the KFE. The floodwater remained turbidly muddy. In the last few decades, the mean surface temperature over the Western Ghats has significantly One of the unique aspects of the Kerala flood was the ~7 days long rainfall before the day of the flood. escalated. Thermodynamic situations favor intense and deep precipitation over the Western Ghats, Surface winds due to monsoonal flow excited a water surge, which inundated low-lying areas. Many which makes the region prone to flash floods at present and in the future [34]. Additionally, landslide factors played a synergic role in making it the worst historical flood of Kerala. However, it is known and storm surges are other possible reasons for the KFE. The floodwater remained turbidly muddy. that LPS over AS and BoB initiates rainfall over central India [49]. Over the Indian region, multiple One of the unique aspects of the Kerala flood was the ~7 days long rainfall before the day of the low-pressure systems may play an essential role in influencing extreme rainfall events [50], and KFE flood. Surface winds due to monsoonal flow excited a water surge, which inundated low-lying areas. was one such case. At present, the ultimate goal of weather forecasts should be dedicated to extreme Many factors played a synergic role in making it the worst historical flood of Kerala. However, it is rainfall events, which always pose a challenging task [5,14], because every new flood is unique. known that LPS over AS and BoB initiates rainfall over central India [49]. Over the Indian region, Predicting such events may be possible with the improvement of medium-range modeling. The multiple low-pressure systems may play an essential role in influencing extreme rainfall events [50], reliability of weather forecasts for such extreme rainfall and a long duration of active spells of the and KFE was one such case. At present, the ultimate goal of weather forecasts should be dedicated monsoon should be prioritized, rather than just criticizing the operation of dams during to extreme rainfall events, which always pose a challenging task [5,14], because every new flood is unprecedented extreme rainfall. unique. Predicting such events may be possible with the improvement of medium-range modeling. Due to unidirectional and robust wind, it is possible that there was a more significant sea surface The reliability of weather forecasts for such extreme rainfall and a long duration of active spells of the height anomaly during the KFE around the Kerala state, which may have blocked the outflow of monsoon should be prioritized, rather than just criticizing the operation of dams during unprecedented rivers. The ecological and environmental losses are too numerous to count owing to flood damage. extreme rainfall. The state, Kerala, was affected by drought at the end of August 2019, followed by flooding. Therefore, Due to unidirectional and robust wind, it is possible that there was a more significant sea surface the region was marred by two natural disasters, almost two weeks apart. Such striking features of height anomaly during the KFE around the Kerala state, which may have blocked the outflow of rivers. meteorology are intriguing and challenging for the scientific community to focus on. The ecological and environmental losses are too numerous to count owing to flood damage. The state, The construction of new reservoirs and wetland serves the purpose of flood mitigation and ensuring an availability of water during dry seasons. A multipurpose and hybrid system of water

Atmosphere 2020, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 740 18 of 21

Kerala, was affected by drought at the end of August 2019, followed by flooding. Therefore, the region was marred by two natural disasters, almost two weeks apart. Such striking features of meteorology are intriguing and challenging for the scientific community to focus on. The construction of new reservoirs and wetland serves the purpose of flood mitigation and ensuring an availability of water during dry seasons. A multipurpose and hybrid system of water resources may be one of the resolutions for handling hydropower generation, flood control, and water availability options. This work does not include the influence of remote teleconnection (e.g., ENSO) on heavy monsoonal rainfall over Kerala [51]. However, such connections are outside of the scope of the present research work. Weather forecasters of extreme events are advised to look at the aspect of the interaction among LPS, dry-air intrusion, and offshore troughs over the Indian region for extreme rainfall events. An increase in global temperature can influence the fundamental atmospheric variables, e.g., pressure, wind circulation, evaporation, and many other variables. It is difficult to pinpoint the locations of heavy rainfall events, yet preferred locations fall under and nearby the path of low-pressure systems. Orographic regions of heavy rainfall are other favored locations of extreme precipitation. Further research work is required to understand this theory of interactions in heavy rainfall, maybe through modeling.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4433/11/7/740/s1, Figure S1: INSAT-3D satellite cloud imagery in thermal imagery channel valid at 0300 UTC (a) 14 August 2018, (b) 15 August 2018, (c) 16 August 2018 and (d) 17 August 2018 respectively. Figure S2 Wind speed (shaded; 1 ms− ) and direction at 200 hPa level valid on 00 UTC of (a) 13 August (b) 14 August (c) 15 August and (d) 16 August (e) 17 August and (f) 18 August 2018 derived from ERA Interim reanalysis at 25 km resolutions. Figure S3 Vorticity 6 1 field ( 10 s− ) valid on 12UTC of (a) 14 August (b) 15 August 2018 at 600 hPa levels derived from ERA Interim × 2 2 reanalysis at 25 km resolutions. Figure S4 (a) Daily climatology of kinetic energy (m s− ) over the Arabian Sea (AS: 50◦ E–72.5◦ E; 5◦ N–15◦ N; black colour), Bay of Bengal (BOB: 82.5◦ E –97.5◦ E; 5–15◦ N; red colour) and Indian landmass (INL: 72.5◦ E –82.5◦ E; 5–15◦ N; blue colour) at 850 hPa level are derived from ERA-Interim Reanalysis 1 on 00 UTC of 8–21 August 2018. Similarly (b) depicts net tropospheric moisture (NTM; gm kg− ) and (c) mean tropospheric temperature (MTT; K) respectively. Figure S5 (a) Off-shore trough (hPa) along the west coast average 2 (8◦–10◦ N; 76◦–77◦ E) and (b) total precipiatable water content (kg m− ) derived from ERA Intrim reanalysis at 25 km resolutions. Author Contributions: V.K. conceived and fumulated the study. V.K., P.K.P. and H.-P.C. prepared the figures. All other authors (T.S. and S.V.B.R.) along with V.K. discussed the results and prepared the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors acknowledge TAMU Kingsville, TX, USA and SV University for the support. Furthermore, the authors gratefully acknowledge various datasets, e.g., ERA-Interim reanalysis fields, IMD datasets, INSAT-3D, IMD + TRMM 3b42 merged analysis, and GrADS package. Conflicts of Interest: The authors declare no conflicts of interest.

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