Role of Eastern Ghats Orography and Cold Pool in an Extreme Rainfall Event Over Chennai on 1 December 2015

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Role of Eastern Ghats Orography and Cold Pool in an Extreme Rainfall Event over Chennai on 1 December 2015

JAYESH PHADTARE Centre for Atmospheric and Oceanic Sciences, and Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India

(Manuscript received 15 December 2016, in ﬁnal form 27 January 2018)

ABSTRACT

Chennai and its surrounding region received extreme rainfall on 1 December 2015. A rain gauge in the city recorded 494 mm of rainfall within a span of 24 h—at least a 100-yr event. The convective system was stationary over the coast during the event. This study analyzes how the Eastern Ghats orography and moist processes localized the rainfall. ERA-Interim data show a low-level easterly jet (LLEJ) over the adjacent ocean and a barrier jet over the coast during the event. A control simulation with the nonhydrostatic Weather Research and Forecasting (WRF) Model shows that the Eastern Ghats obstructed the precipitation-driven cold pool from moving downstream, resulting in the cold pool piling up and remaining stationary in the upwind direction. The cold pool became weak over the ocean. It stratified the subcloud layer and decelerated the flow ahead of the orography; hence, the flow entered a blocked regime. Maximum deceleration of the winds and uplifting happened at the edge of the cold pool over the coast. Therefore, a stationary convective system and maximum rainfall occurred at the coast. As a result of orographic blocking, propagation of a low pressure system (LPS) was obstructed. Because of the topographic b effect, the LPS subsequently traveled a southward path. In a sensitivity experiment without the orography, the cold pool was swept downstream by the winds; clouds moved inland. In the second experiment with no evaporative cooling of rain, the cold pool did not form; flow, as well as clouds, moved over the orography.

1. Introduction latitude of Chennai, the mean plateau height is about 750 m. During winter monsoon, winds impinge on the A large part of southern India receives most of its Eastern Ghats from the eastern side. annual precipitation during October–December. Dur- A strong El Niño–Southern Oscillation (ENSO) event ing these months, the intertropical convergence zone inﬂuenced the 2015 Northern Hemisphere winter. (ITCZ) lies over the northern ﬂank of the equatorial ENSO conditions are favorable for winter monsoon Indian Ocean; northeasterly winds prevail over the Bay over India (Rasmusson and Carpenter 1983; Ropelewski of Bengal (BoB) and the south Indian peninsula (Fig. 1). and Halpert 1987; Zubair and Ropelewski 2006; Kumar This period of the year is known as ‘‘winter monsoon’’ or et al. 2007). The winter monsoon of 2015 was highly ‘‘northeast monsoon’’ (Rajeevan et al. 2012). During active. Three heavy rainfall incidences occurred over this period, low pressure systems (LPSs) often develop Chennai during this season: 7–9 November, 14–16 over the eastern equatorial Indian Ocean and make November, and 30 November–2 December (Chakraborty landfall at the east coast of India by moving north- 2016). Synoptic- and planetary-scale analysis of this sea- westward due to the planetary b effect (Chan 2005). son can be found in Chakraborty (2016).Here,thefocus Some of the LPSs grow into tropical cyclones (TCs). is on the extreme rainfall of 1 December 2015. During this Unlike the Western Ghats mountain range—which event, the precipitating system remained stationary over lies more or less along the entire west coast of India— Chennai. As a result, the accumulated rainfall crossed the Eastern Ghats mountain range is about 200 km away the 350-mm mark within 24 h at several weather sta- from the eastern coastline and is discontinuous (Fig. 1). tions in the city, with 494 mm being the highest at The slope of the Eastern Ghats is also gentler. Along the Tambaram station—at least a 100-yr rainfall event (Narasimhan et al. 2016; van Oldenborgh et al. 2016). Corresponding author: Jayesh Phadtare, [email protected] On 2 December, Chennai was declared a ‘‘disaster

DOI: 10.1175/MWR-D-16-0473.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/08/21 06:05 PM UTC 944 MONTHLY WEATHER REVIEW VOLUME 146

Moist processes such as latent heating and evaporative cooling during precipitation can alter the Fr of the unsaturated flow and affect the orographic blocking (Smith and Lin 1982; Chu and Lin 2000; Miglietta and Buzzi 2001; Jiang 2003; Miglietta and Buzzi 2004; Chen and Lin 2005; Frame and Markowski 2006; Reeves and Lin 2007; Reeves and Rotunno 2008; Miglietta and Rotunno 2009, 2010, 2014). With the idealized numerical models, these studies showed that due to the non- linear nature of moist processes, the precipitating system can become quasi stationary on the upwind side at a range of Fr. Chances of severe flooding are higher in these cases that produce localized heavy precipitation. Chen and Lin (2005) suggested a method to predict this interaction by characterizing conditionally unstable FIG. 1. Map showing 950-hPa winds’ climatology, orography, and flows on the basis of its upstream Fr and convective other geographical features mentioned in the study. Shading in- dicates height of the orography from ETPO5 data (300-, 500-, 750-, available potential energy (CAPE). With an idealized 1000-, and 1200-m contours are plotted); vectors indicate ERA- two-dimensional numerical model, they showed that the Interim winds’ (18 grid spacing) climatology at 950 hPa (October– evaporative cooling in the subcloud layer is related to December 2006–15). the upstream CAPE values. In addition to the upstream CAPE values, Miglietta and Rotunno (2009, 2010) var- zone’’ by the state government. Rain-related incidents kil- ied the parameters related to the mountain shape led at least 250 people (https://earthobservatory.nasa.gov/ (height and width) and atmospheric sounding and pro- IOTD/view.php?id587131). duced regime diagrams in these parameter spaces to Although Chennai is about 200 km away from the predict the behavior of conditionally unstable flows Eastern Ghats, observations elsewhere show that blocked by an orography. Essentially, they show that precipitation enhancement can occur away from the for a given mountain shape, the precipitation rate and mountains in the upwind direction in orographically pattern around the mountain depend on the advective blocked flow cases (Houze et al. 2001; Xu et al. 2012; time scale ta and the convective time scale tc, provided Viale et al. 2013). Mountain–valley breeze and the that the mountain is high enough to trigger convection. associated local convection is the simplest way in In real-world cases, winds often have a considerable which the orography can induce precipitation. How- vertical shear; Miglietta and Rotunno (2012, 2014) ever, in many cases, interaction between the mean showed that in the idealized models, the precipitation background winds and moist processes complicates rates increase and match the observed ones when the the orographic influence on precipitation (Houze winds have vertical shear (jet across the orography and 2012). Moist low-level jets (LLJs) are often cited calm winds above) instead of a uniform profile. Reeves as one of the primary causes of heavy orographic and Lin (2007) showed that the behavior of moist flows precipitation (Lin et al. 2001). The precipitation pat- can be entirely different when a preexisting convective tern in the vicinity of the orography depends on system approaches the orography along with the flow. the Froude number Fr 5 [U/(NH)] of the LLJ, where The southwest coast of Taiwan is a good example U is the mean wind speed normal to the orography, where the mechanisms discussed before are at work. N is the Brunt–Väisälä frequency of atmosphere During the East Asian monsoon, the lower-tropospheric 1/2 [(g/uyo)(›uy/›z)] (where g is the gravity, uy is the southwesterly monsoonal winds flow over the Central virtual potential temperature, and ›uy/›z is the vertical Mountain Range (CMR) of Taiwan, which is about gradient of uy), and H is the height of the orography 100 km inland. As a result, heavy rainfall events often (Smith 1979; Pierrehumbert 1984; Lin and Wang occur over the southwest coast of Taiwan. This is one of 1996). Note that NH gives the speed of the downslope the most studied regions for the orographic influence on density currents from the mountain. For Fr , 1, winds precipitation (Chen et al. 1991; Akaeda et al. 1995; Chen lack the kinetic energy to climb the mountain height, and Li 1995; Li et al. 1997; C.-S. Chen et al. 2005; G. T.-J. and the flow is blocked; precipitation is restricted to Chen et al 2005; Zhang et al. 2003; Davis and Lee 2012; the upwind side of the mountain. For Fr . 1, winds Xu et al. 2012). Similarly, copious rainfall over the climb the mountain barrier, and precipitating systems west coast of India in the summer monsoon season is move to the lee side. also a result of an interaction of westerly monsoonal

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flow with the Western Ghats mountains along the coast at 6-hourly {at 0000, 0600, 1200, and 1800 UTC [India (Grossman and Durran 1984; Smith 1985; Ogura and standard time (IST) 5 UTC 1 0530]} intervals. The Yoshizaki 1988; Xie et al. 2006). In a two-dimensional vertical structure of the atmosphere over Chennai dur- modeling study, Ogura and Yoshizaki (1988) showed ing the event is studied from the radiosondes released at that the inclusion of ocean surface fluxes in the model the local weather station. Soundings were obtained from localizes the maximum rainfall amount over the coast. the University of Wyoming web portal (http://weather. Thus, it seems that moist processes are crucial in the uwyo.edu/upperair/sounding.html). Infrared (IR) im- localization of rainfall. Over the east coast of peninsular ages from the geostationary satellite Kalpana-1 (located India, the topographical settings are similar to those of over 748E) are used to observe the convective organi- the west coast of Taiwan. However, the mean altitude of zation of the system. The pixel size of these IR images is the Eastern Ghats (;750 m) is not as high as the CMR about 8 km at subsatellite point, and images are avail- of Taiwan (;2000 m). Its moderate altitude and distant able at half-hourly intervals. These images were ob- location from the coast might be the reasons why the tained from the Meteorological and Oceanographic influence of Eastern Ghats on the east coast rainfall still Satellite Data Archival Centre (MOSDAC), Indian Space remains unexplored. Research Organization (ISRO). (Images are archived at This paper analyzes the orographic influence of the http://www.mosdac.gov.in/.) Rainfall data from the Eastern Ghats on the 1 December 2015 extreme rainfall National Aeronautics and Space Administration event over Chennai and its surrounding region. The (NASA)’s Integrated Multisatellite Retrievals for immediate aim of this case study is not to improve the GPM Core Observatory satellite (IMERG) (Huffman severe weather forecast over this region. Improvements et al. 2015) are used for showing the rainfall accumu- in numerical weather models and a comprehensive lation on 1 December 2015. And the 3B42 product of analysis of climatology of extreme rainfall events are the Tropical Rainfall Measuring Mission (TRMM) necessary for that purpose (Schultz 2010). Individual (Huffman and Bolvin 2014), also provided by NASA, case studies such as this collectively can lay the foun- is used for showing the climatological rainfall over the dation for such studies. Moreover, as stated before, the South Asian region. IMERG 3IMERGHH is available at influence of the Eastern Ghats on the impinging easterly half-hourly intervals with 0.18 grid spacing, and TRMM flow has not been studied before. This case study pres- 3B42 gives a daily accumulated rainfall with 0.258 grid ents an analysis of this interaction. A better un- spacing. derstanding of the flow dynamics over any mountainous b. Precipitation region is essential for improving the weather forecast over that region. Thus, even though the primary aim of Figure 2a shows the climatological rainfall during the this paper is to explain the stated extreme rainfall event winter monsoon months over Southeast Asia from the over Chennai, understanding gained from this analysis TRMM 3B42 daily precipitation data. The mean pre- can be useful for improving the general weather forecast cipitation rate over the east coast of India during the 2 over this region. This case study uses data from a re- season is around 8–10 mm day 1. Figure 2b shows analysis product, satellites, and the local soundings. A the IMERG accumulated precipitation between 0000 nonhydrostatic numerical model is used for carrying UTC 1 December and 0000 UTC 2 December 2015. The out experiments, which ratify the orographic blocking maximum rainfall accumulation is about 400 mm, and it mechanism. Section 2 gives a detailed description of the lies along the coast just to the south of Chennai. Notice datasets used in this study and analyzes the 1 December that the rainfall accumulation contours run along the rainfall event with these datasets. Section 3 presents the coast and the Eastern Ghats orography. The location of numerical model setup and the results of the modeling maximum rainfall and its north–south organization experiments. Section 4 concludes the study. suggest a significant influence of the local topography. Rainfall amounts decrease as one approaches the orography. Similar features are also seen in the clima- 2. Event description tological rainfall in Fig. 2a. Figure 3 shows rainfall time series at the Nungam- a. Data bakkam station in Chennai from 1200 UTC 30 November For the presentation of meteorological fields such to 1200 UTC 2 December. It shows that there were as pressure, winds, and the total column water vapor some instances (1200 and 1500 UTC) at which signifi- (TCWV), the European Centre for Medium-Range cant rainfall was recorded on 30 November. At 0100 Weather Forecasts (ECMWF) interim reanalysis (ERA- UTC 1 December, rainfall picked up and lasted until Interim; Dee et al. 2011) is used. These fields are available about 0000 UTC 2 December. There was no break in the

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2006; Kumar et al. 2007). According to the Bureau of Me- teorology, Australia (http://www.bom.gov.au/climate/mjo/), the Madden–Julian oscillation (MJO) activity during 27 November–1 December 2015 was significantly strong and was situated over the Maritime Continent, that is, phase 4 (Wheeler and Hendon 2004). During the pas- sage of MJO, the enhanced convection over the eastern equatorial Indian Ocean often forms LPSs that propa- gate northwestward, making landfall over the east coast of India. Thus, both factors ENSO and MJO were favorable for the enhancement of convective activity over southern India and the surrounding ocean during this extreme precipitation event. A preliminary dynamic analysis of the synoptic weather pattern with respect to the 1 December extreme rainfall event is presented in this subsection. ERA-Interim data are used for this purpose. A thermodynamic analysis from the local sounding data will be presented in next subsection (section 2d). Figure 4 shows 850-hPa winds, geopotential contours, and moisture flux in shading from the 18 ERA-Interim data. On 28 November, an easterly wind surge carrying moisture flux was observed over the eastern BoB. This low-level easterly jet (LLEJ) entered the bay from the South China Sea (SCS). On subsequent days, the LLEJ moved westward and prevailed over the east coast of India on 29–30 November. This jet supplied moisture to the east coast of peninsular India. Simultaneously, an LPS development was underway near the southeast coast of Sri Lanka on 28–29 November. A well-organized LPS was located over that region on 30 November. Thereafter, like most of the LPSs that develop over this region, it started moving northwestward. On 1 December, the LPS was seen over Sri Lanka. Although the LPS became better organized after the wind surge event, it FIG. 2. (a) TRMM precipitation climatology for October– did not transform into a depression [India Meteorolog- December 1998–2014; (b) iMERG accumulated precipitation for ical Department (IMD) classifies an LPS as a ‘‘de- 0000 UTC 1 Dec–0000 UTC 2 Dec 2015. pression’’ when the system has two or three closed isobars at 2-hPa interval and wind speed from 17 to 27 kt 2 rainfall during this period. Total rainfall accumulation (about 9–14 m s 1) at sea level]. Intensification and from 0000 UTC 1 December to 0000 UTC 2 December is propagation of preexisting quasi-stationary synoptic 310 mm at this station. disturbances after northeasterly wind surge events over the SCS are reported in Chang et al. (1979) and Chang c. Synoptic–dynamics analysis and Lau (1980). A dynamic analysis of the interaction Before moving into the synoptic analysis, the between the easterly wind surge event and the LPS in planetary-scale atmospheric conditions will be reviewed this case is out of the scope of this study. Rather, the briefly. The Northern Hemisphere winter of 2015 was focus of this study is on the mesoscale dynamics of the under the influence of strong ENSO conditions. During stationary precipitating system formed over the coast on the ENSO phase, there are anomalous easterly winds 1 December. It is hypothesized that the stationary sys- over the BoB; this leads to anomalous moisture con- tem was a result of an interaction between the enhanced vergence over the Indian peninsula. Thus, positive low-level moist winds and the Eastern Ghats orography. ENSO phases give surplus winter monsoon rainfall ac- Development of convection during this period is cumulations over this region (Zubair and Ropelewski depicted in Fig. 5. IR brightness temperature from the

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FIG. 3. Rain gauge time series from the Nungambakkam station in Chennai. geostationary satellite Kalpana-1 is taken as a proxy for main convective envelope was still far off the coast. By the convection. Convection developed over the BoB at 1 December, the main convective system was over the the location of wind surge. It moved westward along coast, where it remained stationary for around 24 h. As with the wind surge. On 30 November, some convective the day progressed, the convection acquired a linear activity was seen along the east coast of India, but the north–south organization along the coast (Fig. 5d). On

21 21 FIG. 4. ERA-Interim winds, geopotential contours, and moisture ﬂux in shading (g kg ms ) at 850-hPa level with 18 grid spacing. (a) 0000 UTC 28 Nov, (b) 0000 UTC 29 Nov, (c) 0000 UTC 30 Nov, and (d) 0000 UTC 1 Dec 2015.

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FIG.5.Kalpana-1 satellite IR images: (a) 0000 UTC 29 Dec, (b) 0600 UTC 30 Nov, (c) 0000 UTC 1 Dec, (d) 1300 UTC 1 Dec, (e) 1200 UTC 2 Dec, and (f) 0000 UTC 3 Dec 2015. A cutoff of IR brightness temperature #221 K is applied to all images.

2 December, the convection was over a coastal region 1 December. At 950 hPa, LLWs from the BoB were well south (about 118N) of Chennai, and then moved deﬂected southward over the coast during this period. eastward away from the coast. Winds over the coast resembled a barrier jet formed To analyze the interaction of winds with the orogra- due to the orographic blocking (Parish 1982; Chen and phy, the low-level winds (LLWs) at 950 hPa are con- Smith 1987). The direction of deﬂection is likely a sidered. Figure 6 shows 950-hPa winds and TCWV result of the semigeostrophic balance within the barrier from 0.758 ERA-Interim data from 28 November to layer (Pierrehumbert and Wyman 1985). Thus, it is

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FIG. 6. ERA-Interim 950-hPa winds and TCWV in shading (both with 0.758 grid spacing). (a) 0000 UTC 28 Nov, (b) 0000 UTC 29 Nov, (c) 0000 UTC 30 Nov, and (d) 0000 UTC 1 Dec 2015. LLWs were blocked by the Eastern Ghats orography. hypothesized that the deflection of LLWs along the vortex travels a clockwise path around the orography. coast was due to the Eastern Ghats orography. On Diabatic processes strengthen the vertical coupling in 2 29 November, the TCWV increased to about 60 kg m 2 the cyclonic vortices, and the motion of the cyclone is with the advection of moisture flux from the bay, as seen linked to the lower-level circulation (Hsu 1987). Thus, in Fig. 4b. The effect of this large-scale moistening on obstruction at the orographic levels is also felt above the local thermodynamic processes is assessed in section that. This is probably the first study that shows the to- 2d. Thereafter, conditions for the orographic blocking pographic blocking of cyclonic vortex over India. are discussed in section 2e. d. Thermodynamics In Figs. 4d and 6d, cyclonic circulation of the LPS on 1 December is seen over Sri Lanka. The LPS re- Figure 8 shows the atmospheric soundings from the mained quasi stationary on 2–3 December north of it, radiosondes released from Chennai on 30 November and then went around the peninsula by moving south- (Figs. 8a,b) and 1 December 2015 (Figs. 8c,d). The most ward (Fig. 7). Blocking and southward deflection of TCs striking feature of these profiles is that the entire depth by the topography have been reported along the CMR in of the atmosphere is moistened near the saturation. Taiwan (Lin et al. 2005) and Sierra Madre in Mexico Moistening of the mid- and upper troposphere is re- (Zehnder 1993). Because of the planetary b effect, a garded as one of the dominant precursors of deep con- general tendency of a cyclonic vortex embedded in the vective outbreak in the tropics (Brown and Zhang 1997; weak mean flow is to move northwestward. When the Sherwood 1999). When the atmosphere has near- lower-level peripheral winds of vortex are blocked by saturation vertical profile, dilution of buoyancy and the orography on the western side, the topographic cloud liquid water in convective cores due to entrain- b effect (Carnevale et al. 1991; Zehnder 1993; Kuo et al. ment is less; thus, the rainfall efficiency and probability 2001) dominates over the planetary b effect, and the of extreme precipitation in such conditions is very high

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FIG. 7. ERA-Interim 850-hPa winds and geopotential contours (both with 18 grid spacing). (a) 0000 UTC 3 Dec, (b) 0000 UTC 4 Dec, (c) 0000 UTC 5 Dec, and (d) 0000 UTC 6 Dec 2015. Gray dots roughly show the location of the center of the LPS. Clockwise movement of the LPS around the peninsula is in consensus with the topographic b-effect theory.

(Doswell et al. 1996). As far as the orographic blocking et al. 2004; Peters and Neelin 2006; Holloway and Neelin is concerned, the cold pool produced in such a moist 2009). Throughout the event, PWAT was more than 2 subcloud layer is relatively weak. It is swept downstream 60 kg m 2. by the background winds. In the presence of down- On 1 December (Figs. 8c,d), southward deflection of stream orography, a piling up of cold pool and stationary easterly LLW is clearly seen—LLWs within the lowest convective systems is most likely in such situations. In 1 km become northeasterly from the easterlies above case of drier profiles, strong cold pools can drive the that level (reanalysis wind fields agree with this). At precipitation systems in the upwind direction (Miglietta 1200 UTC 30 November, such blocking was not seen and Rotunno 2010). Near-saturation atmospheric pro- (Fig. 8b). files prevailed from 29 November (not shown) due Note that at 0000 UTC 1 December, the subcloud to the moisture transport by the enhanced easterly layer was very moist but was still subsaturated at all winds (Figs. 4b, 6b). Precipitable water (PWAT) on 29 levels. There was a scope for the evaporation of rain- 2 2 November increased to 63 kg m 2 from 32 kg m 2 on an drops during precipitation. At 1200 UTC, there were earlier day. Observations show that beyond a critical deep clouds and rainfall over the sounding station; value of PWAT, rainfall increases sharply (Bretherton as a result, the sounding shows saturated profile below

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FIG. 8. Soundings from Chennai: (a) 0000 UTC and (b) 1200 UTC 30 Nov 2015; and (c) 0000 UTC and (d) 1200 UTC 1 Dec 2015. Solid line shows the dry-bulb temperature, dashed–dotted line shows the dewpoint temperature, and dashed line shows the pseudoadiabat.

2 2 600 hPa right to the surface. This saturation is likely a U ; 10 m s 1, N ; 0.012 s 1 (mean values calcu- result of evaporation of cloud and raindrops. As said lated within 1-km altitude from the sounding of 1200 before, moist subcloud layer gives a weak cold pool, as UTC 30 November), and H ; 750 m, which gives Fr ; 1.1. evaporative cooling is weak. Therefore, a strong signa- Reeves and Lin (2007) showed that at similar Fr values, ture of cold pool is not seen in the soundings at the coast. maximum precipitation is produced over the upslope of Prior to the commencement of 1 December rainfall, mountain when there is no preexisting convective system 2 moderate convective instability (CAPE ; 1000 kJ kg 1) in the impinging flow. But when a preexisting convective was present in the sounding of 1200 UTC 30 November. system approaches a mountain along with the flow, the During the rainfall event, there was hardly any CAPE convective system can become stationary at several available over the coast. Nevertheless, LLWs from the hundred km upstream, and maximum precipitation is warm BoB can provide significant instability (warm and produced at the location of stagnation. The 1 December moist air) for the convection to persist for a long time. case presented here falls into the latter category. A dynamic increase in the N values (and hence, low- e. Orographic blocking ering of Fr) due to the formation of the cold pool is cited Prior to the arrival of the convective system, an esti- as the reason for the stagnation of the convective system mate of the Fr 5 [U/(NH)] can be made by substituting by Reeves and Lin (2007). In the model simulations of

Unauthenticated | Downloaded 10/08/21 06:05 PM UTC 952 MONTHLY WEATHER REVIEW VOLUME 146 this event presented in the next section, it was found that the cold pool not only stratifies the lowest levels, but also slows down the impending winds well ahead of the orography by providing a denser air front. Both phenomena result in the reduction of Fr. 2 For the 1200 UTC 1 December sounding, U ; 7ms 1, 2 and N ; 0.013 s 1; this gives Fr 5 0.7. Thus, the flow was in the blocked regime during the stagnation of the convective system. These values of estimated Fr on 1200 UTC 30 November and 1200 UTC 1 December are in consensus with the flow pattern observed in section 2d (Figs. 8b,d). In the next section, the mechanism of orographic blocking is discussed in detail. The role played by moist processes in localizing the convection is also analyzed. Experiments with a nonhydrostatic numerical model are performed for these objectives.

3. Numerical model experiments FIG. 9. Nested domains for WRF-ARW simulations. The dashed a. Model description box shows the region from which the orography was removed in the NOTOPO run. The Advanced Research (ARW) version of the Weather Research and Forecasting (WRF) Model (WRF-ARW; Skamarock et al. 2008) was used for the microphysics, and Yonsei University (YSU) scheme experiments. The model was run in a nonhydrostatic (Hong et al. 2006) for boundary layer processes. These mode. The simulation done with a single domain having schemes were used in both domains. The simulation be- 10-km horizontal grid spacing could not produce the gins at 0000 UTC 30 November and ends at 0000 observed intensity of rainfall. Thus, two nested domains UTC 3 December 2015. (Fig. 9) were used. The outer domain has 10 km, while The grid spacing of the finer domain lies in the ‘‘gray the inner domain has 3.33-km horizontal grid spacing. zone’’ (3–8 km) of cumulus parameterization (CP) The nesting was a two-way nesting; that is, the fine grid (Gerard et al. 2009). At this grid spacing, the motion of domain feedback affected the coarse grid domain solu- convective cells is not fully resolved. On the other hand, tion. With this setup, the simulated rainfall was closer to some of the assumptions made for parameterizing con- the observed rainfall. The aim of the study is to prove vection are valid only at grids coarser than 10 km. Thus, the orographic blocking of winds and the resulting cloud using CP schemes in the gray zone is dubious. Never- stagnation. Model output with 10-km grid spacing is theless, there are several cases in the literature where sufficient for this purpose. Thus, only output of 10-km usage of some form of CP has improved the model domain was used for the analysis (since two-way nesting performance in the gray zone (Deng and Stauffer 2006; was employed, the coarse structure of the rainfall in the DuVivier et al. 2017; Lean et al. 2008; Roberts and Lean two domains was similar). Terrain following 30 vertical 2008; Sun et al. 2014). Mahoney (2016) validated several sigma levels in the atmosphere were used in the simu- CP schemes in the gray zone for simulating an extreme lation. However, for comparing the model output with rain event over Colorado. This study showed that the the observed fields, the model output was reproduced on BMJ scheme simulated the extreme rainfall locations pressure levels. Initial and 6-hourly boundary conditions and intensities satisfactorily. In the present case, explicit for the simulation were provided from the National convection in the 3.33-km domain did not simulate the Centers for Environmental Prediction (NCEP) Final observed rainfall correctly—the rainfall intensity was (FNL) operational global analyses data. The horizontal weaker, and it was placed over ocean. However, testing grid spacing of FNL data is 18318. These data are the model for its predictability is not the subject of this provided at the surface and at 26 pressure levels paper. The reasons for this model behavior are not in- from 1000 to 10 hPa. The following parameterization vestigated here. CP is used in the finer domain for all schemes were used: Betts–Miller–Janjic´ (BMJ) scheme model experiments. (Janjic´ 1994, 2000) for cumulus convection, WDM5 To substantiate the role of the orography in the flow microphysics scheme (Lim and Hong 2010) for cloud blocking, the WRF-ARW model was run with an

Unauthenticated | Downloaded 10/08/21 06:05 PM UTC APRIL 2018 P H A D T A R E 953 identical setup for the following cases: 1) control run with the actual orography (CTL) and 2) without the peninsular orography (NOTOPO). The region from which the orography was removed for the NOTOPO runisshowninFig. 9 by the dashed box. For this experiment, the land elevation over the dashed box was made zero for both domains. The differences between the two simulations are presented in this section ﬁrst. The role of evaporative cooling and the cold pools is relatively subtle. To explain this role, another model experiment, 3) with the actual orography but with no evaporative cooling of raindrops (NOEVAP), is done. Results of this experiment are presented at the end of the section. b. Precipitation Figure 10 shows rainfall accumulation during 0000 UTC 1 December–0000 UTC 2 December for the CTL (Fig. 10a) and the NOTOPO (Fig. 10b) runs. Compared to the rainfall accumulation observed by the Core Ob- servatory satellite (Fig. 2), the CTL run simulates the location of maximum rainfall accumulation and the intensity correctly, but underestimates its areal spread. In the NOTOPO run, the rainfall accumulation has reduced substantively. The maximum rainfall accumulation in the NOTOPO case is about 200–250 mm, compared to 400 mm in the CTL run. Further, note that the localization of rainfall along the coast is broken down in the NOTOPO case. This suggests that the orography has played a vital role in localizing the convection during the event. Figure 11 shows the time series of accumulated precipitation averaged over the boxes in Fig. 10.On1 FIG. 10. Accumulated rainfall of WRF-ARW simulation during 0000 UTC 1 Dec–0000 UTC 2 Dec 2015 for (a) CTL and December, precipitation picks up sharply in the IMERG (b) NOTOPO runs. Rectangular box around precipitation area observation. Until 0000 UTC 1 December, the mean shows the region over which precipitation is averaged for pro- rainfall accumulation is just 50 mm, which rises to ducing the time series in Fig. 11. 350 mm at 0000 UTC 2 December. In the CTL run, precipitation also picks up on the same day. Neverthe- c. Cloud systems less, the pickup is not as sharp as in the IMERG observation. In the NOTOPO run, such sharp pickup on To show the propagation of the cloud systems in the 1 December is absent. Hence, the record heavy pre- model simulations, cloud liquid water path (LWP) is cipitation on 1 December was assisted by the orography. plotted. LWP is the total weight of the liquid water On 30 November, the CTL and the NOTOPO pre- droplets in the atmospheric column above a unit surface cipitation accumulation is nearly equal. In section 2c,it area. Figure 12 shows the LWP in the CTL and NOTOPO was mentioned that the convective system was still over runs. The evolution of cloud systems in the CTL run ocean on 30 November. The western edge of the system (Figs. 12a,b) is similar to the observations in Fig. 5.At was near the coast, and it gave some rainfall over the 0000 UTC 1 December, the cloud system is anchored coast. It is likely that the orographic assistance for the over the coast at the same location as seen in Fig. 5c. The 30 November rainfall was minimal. Note that the me- cloud system in the CTL run remained stationary over soscale numerical model takes an initial few hours the coast throughout the day. Note that maximum (6–12 h) for the spinup. Dynamical ﬁelds after 12 h are values of LWP are located right over the coast. In the considered for the analysis in this study. Any dynamical NOTOPO run, at 0000 UTC 1 December, the location analysis of 30 November rainfall is not intended. of the cloud system is similar to that in the CTL run

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FIG. 11. Time series of the accumulated precipitation spatially averaged over a rectangular box shown in Fig. 10.

(Fig. 12c). But the LWP values are nearly half of those in NOTOPO run. By 1200 UTC 1 December (Fig. 12d), the CTL run. The subsequent evolution of cloud systems clouds formed far inland. Clouds covered a large inland is entirely different—cloud systems moved inland in the area from the east coast. Maximum LWP values are

FIG. 12. Cloud LWP on 1 Dec 2015 in CTL run at (a) 0000 UTC and (b) 1200 UTC, and in NOTOPO run at (c) 0000 UTC and (d) 1200 UTC.

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FIG. 13. Winds and virtual potential temperature uy at 950 hPa on 1 Dec 2015 in CTL run at (a) 0000 UTC and (b) 1200 UTC, and in NOTOPO run at (c) 0000 UTC and (d) 1200 UTC. The cold pool is blocked by the orography in the CTL run. produced far inland, whereas LWP values are reduced Figure 14 shows the evolution of rainfall along the coast 0 substantively over the coast. and the perturbation potential temperature (uy) inland. The rainfall is averaged over the same box over the coast d. Dynamics 0 shown in Fig. 10,anduy is averaged over a similar box but Figure 13 shows 950-hPa winds and the virtual po- shiftedtothewestby0.58 (shown in Fig. 13a over the coast tential temperature uy from the CTL run. In the CTL labeled as N) because the winds pushed the cold pool to the run, winds are similar to the ERA-Interim data; LLWs west and changed the stratification along the orography. 0 over the BoB are deflected southward, and a barrier jet It shows that the negative uy (cold pool) evolves along is formed along the coast (Figs. 13a,b). In the NOTOPO with the rainfall. From 0000 UTC 1 December to 0000 run (Figs. 13c,d), the deflection of LLWs is far less than UTC 2 December, the cold pool sits over the coast. The in the CTL run. Thus, it is clear that in the CTL run, the cooling of the subcloud layer can be due to the evaporative orography is blocking the LLW. In the CTL run, uy in cooling because there is no other source of cooling (uy the barrier region is about 38C lower than that over the values are higher over the BoB, so cooling by cold air ad- open BoB. This is due to the precipitation-induced cold vection is ruled out). It was shown earlier that the soundings pool of the convective system. On the westward side, the over the coast (Fig. 8) showed near-saturation profiles, and cold pool is blocked by the orography of the Eastern evaporative cooling can be weak in such cases. However, it Ghats. Thus, the cold pool gets accumulated in the up- is later shown that the evaporative cooling of raindrops was wind direction over the coastal plains. In the NOTOPO essential for the subcloud layer cooling. Another simulation run, the cold pool is swept away inland by the LLW. by turning off the evaporative cooling of raindrops in the

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0 FIG. 14. Evolution of (a) rainfall and (b) perturbation virtual potential temperature uy (only negative values are shown) over the coast in the CTL run. See section 3d for the details. microphysics scheme was done to prove this point. Results radiations mixing up the boundary layer. On 1 De- of this experiment are discussed in the next subsection. cember, the value of N does not show any diurnal 2 Prior to that, the evolution of N and Fr during the event in variation; it remains constant at 0.013 s 1. This value the CTL run is presented. is in consensus with that observed from the sounding Figure 15 shows the evolution of N and Fr, where N is in section 2e.TheN values increased with the cold 0 computed over the same region where uy was computed pool deepening. in Fig. 14. The values are averaged within a layer of Orography-induced pressure perturbations slow ;1 km (900 hPa) from the surface. On 30 November, N down the ﬂow well ahead of the orography. Determining shows a diurnal variation. During the night, the value of the true kinetic energy of the ﬂow impinging on the N is quite high, and it reduces during the day with solar mountain, hence, is not a straightforward task. Therefore,

FIG. 15. Evolution of (a) Brunt–Väisälä frequency N, (b) zonal winds U, and (c) Froude number (Fr) in the CTL run. The solid lines show U and Fr over the box BB, and the dotted lines show U and Fr over the box U at the coast shown in Fig. 13a.

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0 FIG. 16. Evolution of (a) rainfall and (b) perturbation virtual potential temperature uy (only negative values are shown) over the coast in the NOTOPO run. See section 3d for the details. calculation of Fr is often somewhat ambiguous in the real- (Markowski and Richardson 2011, p. 346). The ambiguity world case studies; it is unclear how far upstream one increases when the cold pool is present ahead of the should move to get the value of U to calculate the Fr orography. The calculation of Fr assumes uniform

FIG. 17. Zonal winds (shading and dashed contours), perturbation virtual potential tem- 0 perature uy (thick contours, only negative values are shown), and vertical velocity (thin contours) averaged over 118–138N during 0000 UTC 1 Dec–0000 UTC 2 Dec 2015 in (a) CTL and 2 (b) NOTOPO runs. Contours of vertical velocity start from 20.4 Pa s 1 with an interval of 2 20.1 Pa s 1.

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FIG. 18. Winds and geopotential contours (in m) at 850 hPa (a),(c) at 0000 UTC 2 Dec and (b),(d) at 0000 UTC 3 Dec 2015 in (a),(b) CTL and (c),(d) NOTOPO runs. The LPS is blocked by the orography in the CTL run. stratification ahead of the orography. But the cold pool naive to assume that the U considered for determining provides a denser front and lowers U ahead of the orog- Frcoast is free from any orography-induced pressure raphy exclusively (Fig. 13). Keeping this in mind, two perturbations and subsequent deceleration. But con- values of Fr are calculated here: The first by considering sidering the box sufficiently away from the orography the U value over the open BoB (Frbb), and the second keeps its effect minimal.) along the coast (Frcoast). The solid line in Fig. 15 shows the On 1 December, when the value of N is high around 2 2 mean values of U over box BB shown in Fig. 13a, averaged 0.013 s 1, the U at the coast varies from 6 to 8 m s 1, 2 within the lowest 1-km layer from the surface. The whereas U over BB is around 10–12 m s 1. The bottom dotted line shows similar values of U averaged over a panel of Fig. 15 shows values of Fr calculated with the box just ahead of the coast (shown in Fig. 13a, labeled two speeds. The value of Frbb remains around 1.2 during as U). The idea is to show how the cold pool de- the event, but that of Frcoast varies around 0.7–0.9. In celerates the flow ahead of the orography and lowers fact, the cold pool farther inland is denser than that over the Fr. This comparison is meaningful as long as the the box where U is calculated along the coast. Hence, the flow is channelized toward the orography from the two flow would face further deceleration as it enters the boxes. Figure 13 shows that this was true on 1 December. denser cold pool inland, and the effective Fr will be

Around 0000 UTC 2 December, an LPS is along the much less than Frcoast. Thus, the presence of the cold coast, and two different ﬂow patterns prevail over pool ahead of the orography plays a crucial role in the the two boxes. In such case, the above comparison transformation of ﬂow from the unblocked to the would be absurd. (As stated before, it would be very blocked regimes. In the next subsection, it is shown that

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FIG. 19. Simulated soundings at 1200 UTC 1 Dec 2015 in (a),(c) CTL and (b),(d) NOEVAP runs at (a),(b) the coast (138N, 808E) and (c),(d) the foothills (138N, 79.28E; location is shown in Fig. 20a). in the absence of the cold pool, flow moves over the deceleration as they approach the cold pool. Maximum orography undeflected. deceleration of zonal winds in the subcloud layer and the 0 Figure 16 shows the evolution of uy with the rainfall in core of the updraft are at the front of the cold pool, just the NOTOPO run. The regions chosen for averaging ahead of the coast. Figure 17b shows a similar plot, but both the variables are the same as in Fig. 14. As the cold for the NOTOPO run. In this case as well, the cold pool pool was swept downstream, the pile of cold air is is found to be extending over the coastal ocean. How- weaker/shallower than that in the CTL run. Figure 17a ever, in the absence of the orography, the cold pool is shows a height–longitude cross section along 128N, with shallower, and the stratified jet faces little deceleration. fields meridionally averaged over 18 latitude on each Updraft in this case is less vigorous. Notice that in both side and temporally averaged over a time period starting runs, the cold pool is stronger over land and becomes from 0000 UTC 1 December to 0000 UTC 2 December. weak over ocean. Xu et al. (2012) suggest that over the 0 Shading shows zonal velocity, thick contours show uy, warm ocean, surface fluxes replenish the boundary layer and thin contours show vertical velocity. The cold pool is and feed the convection near the coast. Xu et al. (2012) piled up over the orography and extends over the ocean. also showed that the storm became quasi stationary by The updraft is located along the coast. The locations of the ‘‘back building’’ mechanism, in which new cells are the maximum updraft velocity and the observed maxi- produced repetitively at the same location along the mum rainfall are in consensus. Zonal winds face coast. It is speculated that similar back building of

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FIG. 20. Cloud LWP in the NOEVAP run. (a) 0000 UTC, (b) 1200 UTC, and (c) 1800 UTC 1 Dec; and (d) 0000 UTC 2 Dec 2015. Asterisk (*) in (a) shows the location for inland sounding shown in Fig. 19. convective cells must have occurred in this case. the flow ahead of the orography and lowers the Fr. To Ground-based radar data are required to prove this. prove this point, the microphysics scheme was modified Figure 18 shows 850-hPa winds and geopotential dur- to turn off the evaporative cooling of the raindrops in ing the model experiments. Because of the orographic this experiment. Though raindrops were allowed to blocking in the CTL run, the LPS along the east coast evaporate, the associated latent heat absorption was remains stationary throughout the run (Figs. 18a,b), made zero. Latent heat transformations due to all other whereas in the NOTOPO run, the LPS moves west- microphysical processes were present. Evaporatively ward, crossing the flat peninsula (Fig. 18c), and on 0000 cooled cold pools reduce the subcloud layer entropy and UTC 3 December, it is seen over the west coast of India give negative feedback to the convection. When the (Fig. 18d). Most of the intense LPSs that approach the evaporative cooling is turned off, this negative effect Indian east coast move over the moderate peninsular vanishes, and convective instabilities grow much more orography. However, the LPS in this particular case had rapidly. To limit these instabilities, the simulation in this comparatively weaker winds that were blocked by the experiment was started at 1800 UTC 30 November and orography. Hence, the westward movement of this LPS ended at 0000 UTC 2 December. was obstructed by the orography. Figure 19 shows the differences between the soundings at the coast (Chennai) and at an inland location [shown e. Evaporative cooling in Fig. 20b by an asterisk (*)] in the CTL and NOEVAP It was speculated in the earlier experiment that the experiments during the event. Simulated sounding over evaporatively cooled cold pool presents a denser front to Chennai in the CTL run (Fig. 19a)at1200UTC

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FIG. 21. Winds and virtual potential temperature uy at 950 hPa in the NOEVAP run at (a) 0000 UTC, (b) 1200 UTC, and (c) 1800 UTC 1 Dec; and (d) 0000 UTC 2 Dec 2015.

1 December is similar to the observed sounding in stagnation. Low-level flow along the orography in the Fig. 8d. The same sounding in the NOEVAP experi- NOEVAP run is analyzed next. ment shows that the lowest layer is warmer by about Similar to Fig. 13, Fig. 21 shows 950-hPa winds and uy, 38–48C. The winds at the lower levels are not deflected but for the NOEVAP run. The flow in this experiment southward. A sounding from the CTL run at the same shows a significant diurnal variation. Air with lower uy time but from a location near the foothills shows a rel- values is seen in the nocturnal and morning hours atively drier lower troposphere and a cold layer within (Fig. 21a) along the foothills of the mountain range. This the lowest 1 km (Fig. 19c). When we see the same cooling can be due to the downslope katabatic flow from sounding from the NOEVAP run (Fig. 19d), again there the mountains or by local radiative cooling. During is a warming at the lowest layer by 38–48C. Therefore, it those hours, 950-hPa flow from the BoB is deflected is clear that the cooling in the subcloud layer in the CTL southward, which forms a barrier jet along the coast. run was due to the evaporative cooling of raindrops. The Similar early-morning barrier winds along the coast saturated profile above 850 hPa in Fig. 19d is due to the were seen in the reanalysis data (Fig. 6). But during the presence of cloud above that location. Figure 20 shows day, as solar radiations heat the land, flow from the BoB the evolution of clouds in NOEVAP run. The clouds heads toward the orography undeflected (Figs. 21b,c). cross the orography on 1 December 2015. This proves At 0000 UTC 2 December, low-level cooling is not seen, that the surface cold pool, formed due to the evapora- unlike at 0000 UTC 1 December. This can be due to the tion of raindrops, was essential for the coastal cloud widespread cloudy sky on 1 December. Clouds block the

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FIG. 22. Evolution of (a) Brunt–Väisälä frequency N, (b) zonal winds U, (c) Froude number (Fr) in the NOEVAP run. The solid lines show U and Fr over the box BB, and the dotted lines show U and Fr over the box U at the coast shown in Fig. 13a. terrestrial outgoing longwave radiations, and hence, (Markowski and Richardson 2011, p. 347). As a result, radiative cooling is weak in cloudy conditions. The the precipitating clouds were stagnated over the coast— cloud-free regions (around 168N, 768E) show cooling at much ahead of the orography. 0000 UTC 2 December. The diurnal variation in the flow blocking becomes evident in Fig. 22. Figure 22 is similar 4. Summary and conclusions to Fig. 15, but for the NOEVAP run. The variation of N On 1 December 2015, Chennai and its surrounding from high values in the night and morning to low values region received a record heavy rainfall. The pre- during the day is quite evident. When cooler and denser cipitation system during the event was stationary over air prevails over the land, winds face greater de- the coast. This study analyzes how the Eastern Ghats celeration. As a result, in the night and morning hours, orography and cold pool localized the precipitation Fr is less than 1, and the flow in this period is in the system. An orographic blocking mechanism was pro- blocked regime (Fig. 21a). During daytime, when the posed by looking at the data from ERA-Interim, satellite lower layer warms and is less stratified, winds face less observations, and local soundings. The nonhydrostatic deceleration, and the Fr values are greater than 1; the WRF-ARW numerical model was used for further anal- flow is unblocked. ysis and to prove the hypothesis. The following are the Remember that similar diurnal variation in the major conclusions drawn from this study: stratification of the subcloud layer was seen in the CTL run on 30 November (Fig. 15)—an increase in N from 1) Moist LLEJs prevailed over the adjacent ocean 1200 UTC 30 November to 0000 UTC 1 December. during the event. However, the values of N on 1 December remain high 2) Winds at the low level were blocked by the Eastern throughout the day. The NOEVAP experiment proves Ghats and a barrier jet formed along the coast. that this was due to the formation of the cold pool, which 3) A detailed analysis of the blocking mechanism with is absent in the NOEVAP run. Therefore, stratification the WRF-ARW model reveals that the cold pools and deceleration of the winds by the surface cold produced by the evaporative cooling on 1 December pool ahead of the orography is the reason that values were essential for the flow blocking. The cold pool of Fr remained low during 1 December and the flow stratified the subcloud layer and decelerated the was blocked upstream by the Eastern Ghats orography. winds ahead of the orography. Thus, the flow This phenomenon is known as ‘‘diabatic damming’’ entered a blocked regime.

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4) The cold pool piled up ahead of the orography. It was REFERENCES u0 ;2 stronger inland ( y 3 K) and weaker over the Akaeda, K., J. Reisner, and D. Parsons, 1995: The role of mesoscale ocean. As the warm and moist marine air came in and topographically induced circulations in initiating a flash the cold pool region along the coast, it was uplifted at flood observed during the TAMEX project. Mon. Wea. Rev., the edge of the stationary cold pool. Hence, convec- 123, 1720–1739, https://doi.org/10.1175/1520-0493(1995)123,1720: . tion became stationary over the coastal region. TROMAT 2.0.CO;2. Bretherton, C. S., M. E. Peters, and L. E. Back, 2004: Relationships 5) As a result of orographic blocking, the northwest- between water vapor path and precipitation over the tropical ward propagation of the LPS was seized. Later, the oceans. J. Climate, 17, 1517–1528, https://doi.org/10.1175/ LPS moved in a southward direction along the 1520-0442(2004)017,1517:RBWVPA.2.0.CO;2. peninsula. 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