A diagnostic study of extreme precipitation over Kerala during August 2018

The state of Kerala, located on the west coast of India, experienced a record 100‐year flood that resulted in major landslides from unprecedented prolonged and extremely heavy rainfall (50–480 mm·day−1) during August 1–19, 2018, causing extensive damage and about 500 causalities. Rainfall observations indicate that the heavy rainfall occurred over two spells (August 7–10 and 14–18) in association with an offshore trough, and a depression over the Bay of Bengal (BOB). High‐resolution 38‐year climatology data (5 km) and the ERA‐Interim reanalysis dataset show a strong low‐level jet over the Arabian Sea and a depression over the BOB with a southwestward tilt during the heavy rainfall. Very high‐resolution (2‐km) mesoscale model simulations suggest that this high convective instability due to the strong westerly jet along with the formation of offshore vortex, the transport of mid‐tropospheric moisture under the presence of conducive vertical shear of horizontal wind, and transport of mid‐tropospheric moisture from the BOB are the major factors (as shown in the schematic diagram) behind the extreme heavy rainfall over Kerala.


| INTRODUCTION
The Indian southwest summer monsoon (ISM) is characterized by distinct spells of rainfall that occur in association with the synoptic features of the monsoon trough, off-shore troughs along the west coast of India, movement of monsoon lows and depressions, upper/ mid-tropospheric cyclonic circulations, and more (Rao, 1976). The interaction between the steep topography of Western Ghats (WG) and moist winds of the ISM triggers deep convective systems, which trigger rainfall between 10 and 100 mmÁday −1 (Sahany et al., 2010;Guhathakurta et al., 2015). The existence and position of an offshore trough over the west coast of India, the formation of depression over the Bay of Bengal (BOB), and its north-west propagation towards central India further enhances moisture convergence and leads to heavy rainfall (Rao, 1976;Francis and Gadgil, 2006). The state of Kerala, located in the southern WG, sits at the tipping point of the monsoon onset and receives seasonal rainfall of about 2,000 mm (Krishnakumar et al., 2009). The rainfall characteristics over Kerala are unique because of isolated structures with steep slopes separated by a wide mountain gap, called Palghat gap (Tawde and Singh, 2015).
From August 1 to 19, 2018, Kerala experienced exceptionally heavy rainfall that lead to extensive flooding and landslides, and caused about $5 billion worth of damage and more than 500 causalities (Mishra et al., 2018). This unprecedented and extreme rainfall generated a record 100-year flood that forced officials to open nearly 80 dams at once, creating havoc, landslides, and significant damage to crops and livestock (CWC report, 2018;Sudheer et al., 2019;Mishra et al., 2018). Unlike the historical and localized flood events of Mumbai in 2005 (Vaidya and Kulkarni, 2007), Utharakand in 2013 (Vellore et al., 2015), and Chennai in 2015 , the entire state of Kerala was inundated with heavy rainfall spells that lasted for several days. The India Meteorological Department (IMD) reported that the extreme rainfall over Kerala in the month of August, 2018 was the highest recorded over the past 78 years.
The formation and intensification mechanisms associated with the extremely heavy rainfall events over the west coast were studied using global and regional weather prediction models (e.g., Litta et al., 2007;Vaidya and Kulkarni, 2007;Bhanu Kumar et al., 2012), these emphasized the advantage of high-resolution regional models for capturing mesoscale features of heavy rainfall (e.g., Bhaskarrao and Hari Prasad, 2006;Bohra et al., 2006;Vellore et al., 2015;Raju et al., 2015;Umakanth et al., 2016). Diagnostic analysis of previous studies revealed that the presence of thermodynamically favorable conditions, such as strong convective instability, a conducive vertical shear of horizontal wind, a moisture-laden mid-troposphere, and the formation F I G U R E 1 (a) Time series of 24-hr accumulated rainfall (mmÁday −1 ) from IMD AWS stations during August 1-19, 2019, (b) Spatial distribution of daily rainfall (mmÁday −1 ) obtained from IMD satellite and rain gauge merged product during August 7-19, 2018 of a mid-tropospheric cyclonic system, were possible factors leading to the occurrence of localized heavy precipitation events (Litta et al., 2007;Vaidya and Kulkarni, 2007;Srinivas et al., 2018).
This study investigates the factors and physical mechanisms that lead to the extremely heavy rainfall over Kerala in August 2018 using stations and gridded observations and global and high-resolution regional reanalysis datasets. To analyze the exceptional conditions that prevailed during the month of August 2018, we used the WRF high-resolution reanalysis along with the global ECMWF reanalysis data (ERA-Interim). Furthermore, a very high resolution (2-km) cloud resolving WRF model simulation is used for the diagnostic analysis of the extremely heavy rainfall event that occurred on August 14-17, 2018. The paper is organized as follows. Section 2 describes the data and methodology. Section 3 details the observed rainfall characteristics and associated synoptic and diagnostic analysis of heavy rainfall, Section 4 provides a summary and conclusions.
A 38-year (1980-2018) high-resolution reanalysis (Viswanadhapalli et al., 2017;Langodan et al., 2017aLangodan et al., , 2017bBindu et al., 2018;Dasari et al., 2019;Viswandhapalli et al., 2019), generated using the Weather and Research Forecasting (WRF) model (Skamarock et al., 2008) and ERA-Interim (ERA-I) reanalysis (Dee et al., 2011), was used to study the prevailing anomalous atmospheric circulation patterns over Kerala in August 2018. Generally, global reanalyses provide acts as a good source of information for studying severe weather conditions. However, recent studies (e.g., Kishore et al., 2016;Žagar et al., 2018;Nikhil et al., 2019) indicated that the global reanalyses may not represent the mesoscale features and associated precipitation characteristics of extreme events. Particularly for extremely heavy rainfall events over complex terrain regions, such as the Western Ghats, the mesoscale signatures are either smoothed out or misrepresented in the global reanalyses. These studies further suggested that the ERA-I data better reproduces the precipitation characteristics than other global reanalyses as it is generated by an advanced assimilation system incorporating all available conventional and satellites radiance datasets. We have therefore used the high resolution WRF regional reanalysis as it has the advantage of accurately representing the mesoscale flow features over complex terrains and incorporates the signature of extreme events through the frequent initialization and assimilation of data (Lo et al., 2008;Viswanadhapalli et al., 2017).
For a more detailed diagnostic analysis of heavy rainfall between August 14 and 17, 2018, we further used very high-resolution (2-km) WRF model outputs configured with three two-way nested domains resolutions of 18, 6, and 2 km, with the innermost domain of 2 km focused over the regions of heavy rainfall in Kerala for the diagnostic analysis. The adopted physics of this model were based on previous studies of heavy precipitation events over the Indian region (Srinivas et al., 2013Srikanth et al., 2016;Attada et al., 2018;Reshmi Mohan et al., 2018). The WRF model was initialized at 0000 UTC of

| RESULTS AND DISCUSSION
The spatio-temporal variability of rainfall at 10 AWSs across Kerala and the IMD satellite-merged rainfall estimates are first analyzed. The analysis of synoptic meteorological conditions and the anomalous atmospheric conditions associated with the Kerala heavy rainfall event are then studies based on a regional high-resolution reanalysis. A diagnostic analysis of the event using the cloud resolving (2-km) mesoscale model simulations is finally presented.

| Observed rainfall distribution and synoptic meteorological conditions
The time-series of daily accumulated rainfall observations during the study period ( The meridional cross-section (averaged within the latitude band of 8-13 N) of the wind (calculated as in Fig. 3) from WRF and ERA is displayed in Figure S4. It shows stronger anomalous winds in the lower atmosphere between 925 and 850 hPa, indicating stronger westerly-to-southwesterly monsoon flow over the AS and the west coast of India during the second spell of heavy precipitation. The maximum northwesterly winds (4 ms −1 ) are found between 850 and 700 hPa over the Indian Peninsula. The core of the maximum winds extended vertically up to 500 hPa over the BOB, suggesting an increased cyclonic flow over Peninsular India and the BOB during the heavy rainfall episode. The WRF reanalysis dataset (not the ERA-I dataset) shows relatively stronger monsoonal flow over the AS and cyclonic circulation over the BOB. An enhanced vertical extension of intense winds near the west coast, an intensification of the East African jet, and a westward shift of the winds associated with the cyclonic circulation over the BOB was also noticeable in the WRF reanalysis, compared to the ERA data. The wind flow of August 2018 suggests that both enhanced monsoonal circulation and the location of cyclonic circulation over the BOB played a major role in the prolonged occurrence of heavy rainfall over Kerala.

| Diagnostic analysis of the heavy rainfall event using mesoscale model simulations
The previous analysis of the observations and high-resolution datasets shows that the extreme precipitation during August 1-19, 2018 over Kerala was associated with a monsoon low-pressure trough, an intense offshore trough over the west coast of India, and a deep depression over the BOB. To investigate the major factors contributing to the formation, intensification, and sustainment of the rainfall, we carried out a diagnostic analysis using the very high resolution (2-km) WRF model simulations.

| Analysis of moisture transport and vorticity
The availability of moisture and its source regions is crucial to the development and intensity of convective systems (Brimelow and Reuter, 2005;Huang and Cui, 2015). The anomalous low-level moisture transport (calculated as in Fig. 3) during August 14-17, 2018, based on the very high-resolution (2-km) model simulations, shows a strong moisture transport (>20 kgÁm −1 Ás −1 ) over the south-eastern AS (Fig. 4) which comes from the cross-equatorial flow and the southern Red Sea through the Gulf of Aden. The contribution of the moisture from F I G U R E 3 Anomalies in horizontal winds (ms −1 , shown in vectors) and geopotential heights (in m; color shading) at 925, 850, 450, and 150 hPa from ERA-I and WRF reanalysis datasets. The anomalies in the monthly fields are computed by subtracting monthly mean parameters of August 2018 from 38-year's climatology the depression over the BOB was concentrated mainly over the southern Indian Peninsula and the Central and Southern BOB. The inflow of dry desert air from the eastern AS towards the west coast of India is seen in the lower and mid-tropospheric levels (850-700 hPa) enables to maintain the conditional instability. Previous studies (e.g., Sabin et al., 2013;Parker et al., 2016) indicated that the intruded dry air at mid-tropospheric levels enhances the low level cold and moist winds. The presence of strong low-level westerly jet cools the lower temperatures and increases the moisture content, lead to enhanced θ e and reduced convective inhibition levels. The intrusion of mid-level dry air, on the other hand, increases the CAPE, which provides the ideal conditions to increase the convective instability due to the low CIN and high CAPE values along with a negative gradient of θ e . The cyclonic circulation over the BOB extending southwest to the northeast towards Kerala coast resulted in enhanced moisture transport (<7 kgÁm −1 Ás −1 at 450 hPa) in the upper levels along the west coast. Therefore, the three major factors that played a critical role in the formation of Kerala heavy rainfall event were the strong low-level westerly jet on the hilly topography, the subsequent mechanical uplifting of air and moist convection, and the advection of dry continental winds at mid-tropospheric levels (700 hPa). These factors maintained static stability while the transport of moisture from the BOB facilitated the prolonged occurrence precipitation over the southwest coast.
The strength of the low-level trough and its location can be identified from the geopotential height, and we use vorticity to quantitatively assess the intensity of the low-pressure system. The spatial distribution of relative vorticity and geopotential clearly show a strong offshore trough off the coast of Kerala (Fig. S5) associated with a strong low-level convergence with the positive vorticity. A cyclonic circulation at midtropospheric levels is clearly seen at 450 hPa, which confirms the monsoon depression over Odisha and adjoining BOB, with a vertical tilt towards the north coastal Andhra Pradesh. The cyclonic flow extended up to 450 hPa (approximately 6.5 km), enhancing the low-level convergence and further strengthening the low-level monsoon winds. A weak anti-cyclonic vorticity was also noticeable above 450 hPa. The spatial distribution of winds and relative humidity clearly indicate strong westerly winds (approximately 40 ms −1 ) at low levels, with a saturated atmosphere (RH > 95%) over Kerala and coastal Karnataka. Northwesterly winds with relatively dry air were apparent in the layer of 700-600 hPa, compared to the conditions in the lower levels (1,000-700 hPa). At 450 hPa, north-to-northwesterly winds with relatively high humidity (RH > 90%) extended from the BOB's cyclonic flow to coastal Karnataka and northern Kerala. The wind, vorticity, and humidity analyses at different levels clearly illustrate an altered flow pattern and moisture accumulation caused by the enhanced low-level convergence and vorticity.

| Analysis of mesoscale characteristics
A vorticity budget analysis (e.g., Dasari et al., 2017;Srinivas et al., 2018) was conducted to examine the life cycle of the convective system and its associated dynamical features using the very high resolution WRF simulations (2-km). The time-height section of the area located between 11 to 12 N and 75 to 76 N, averaged for vorticity budget analysis is presented in Fig. 5 and shows that the vertical The time-height vertical section of equivalent potential temperature (θ e ), horizontal winds, potential temperature (θ), and vertical wind over central

| SUMMARY AND CONCLUSIONS
This study investigated the anomalous atmospheric conditions that lead to the extreme rainfall events over the state of Kerala (southwestern India) from August 1 to 19, 2018 using high-resolution regional and global reanalysis datasets. High resolution (2-km) cloud resolving WRF mesoscale model simulations were further analyzed to investigate the dynamics and thermo-dynamical factors behind the heavy rainfall. The main conclusions are as follows.
1. Station observations showed the heavy rainfall occurred in two spells (August 7-10 and 14-18) with the presence of an offshore trough and a depression over the BOB.
2. The spatially rainfall distributions observed from IMD gridded data suggests that the first spell was concentrated mainly over the northern mountainous region of Kerala. The rainfall then spread all over Kerala during the second spell with maxima over central and south-central Kerala.
F I G U R E 6 Time-Pressure cross-sections of (a) equivalent potential temperature (K), (b) relative humidity (%), (c) winds (ms −1 , wind speeds are shaded), (d) potential temperature perturbations (K, contours), (e) vertical velocities (ms −1 ) averaged for Kerala (11-12 N and 75-76 E) 3. The high-resolution WRF reanalysis and ERA-interim data suggest an increased low-level westerly jet accompanied by increased upper level easterly jet during the extreme rainfall episodes. The presence of strong and stationary cyclonic circulation over the BOB that extended up to the mid-troposphere might have played an important role in the prolonged occurrence of heavy rainfall over Kerala. 4. A strong low-level westerly jet along with the formation of the offshore vortex, the transport of mid-tropospheric moisture under the presence of conducive vertical shear of horizontal wind, and the transport of mid-tropospheric moisture from the BOB are the major factors contributing to the extreme rainfall. The formation of wind confluence near surface over Kerala, the shear zones in mid and upper troposphere along with the enhanced upper air circulation are the major factors for the exceptionally heavy rainfall over Kerala. The results of the present study are highly useful for operational forecasting purposes as high resolution models are regularly employed by the national weather forecasting centers across the globe. The model diagnostics on the location of troughs, unusual circulation patterns, such as the low-level and mid-tropospheric circulations, meso-vortices and unusual low-level convergence/upper air divergence features will be highly useful for effective prediction of the exceptionally heavy precipitation events.