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Extreme Rainstorms that Caused Devastating Flooding across the East Coast of Peninsular Malaysia during November and December 2014

OOI SEE HAI National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, Kuala Lumpur, Malaysia

AZIZAN ABU SAMAH National Antarctic Research Center, Institute of Postgraduate Studies, and Institute of Ocean and Earth Sciences, University of Malaya, Kuala Lumpur, Malaysia

SHEEBA NETTUKANDY CHENOLI National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, Kuala Lumpur, Malaysia

KUMARENTHIRAN SUBRAMANIAM Malaysian Meteorological Department, Petaling Jaya, Selangor, Malaysia

MUHAMMAD YUNUS AHMAD MAZUKI National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, Kuala Lumpur, Malaysia

(Manuscript received 2 September 2016, in ﬁnal form 6 January 2017)

ABSTRACT

During the early boreal winter (northeast) monsoon (November–December), cold air frequently bursts out from intense Siberian highs toward the Chinese coast in response to the development and movement of a 500-hPa trough. The resultant strong low-level northwesterlies turn into northeasterlies across the South China Sea as ‘‘cold surges.’’ On interacting with the near-equatorial trough, mesoscale convective systems form north of the trough, normally giving rise to heavy downpours and severe flooding, mainly along the coastal stretch in the east coast states of Peninsular Malaysia. In November 2014, a 1-week-long episode of heavy downpours, producing more than 800 mm of rain, occurred along the coastal stretch of northeastern Peninsular Malaysia. However, during December 2014, two episodes of extreme rainfall occurred mostly over inland and mountainous areas of the east coast of Peninsular Malaysia, in particular across its northern sector. These two unusual events, which lasted a total of 11 days and delivered more than 1100 mm of precipitation, resulted in extreme and widespread flooding, as well as extensive damage, in many inland areas. Analysis shows that the stronger wind surges from the South China Sea due to very intense cold-air outbreaks of the Siberian high developed under ENSO-neutral conditions. In addition, the mesoscale convective systems that developed across the northeastern Indian Ocean (near northern Sumatra) in response to the propagation of a 500-hPa short-wave trough across the Indian subcontinent toward China were the combined factors for these unusual extreme rainfall and flooding events along the east coast of Peninsular Malaysia.

1. Introduction occurred along the coastal stretch of northeastern Peninsular Malaysia. However, from mid-December Heavy rain with its associated flooding is an annual onward, two episodes of extreme rainfall caused wide- occurrence along the east coast of Peninsular Malaysia spread flooding in Kelantan, Terengganu, and Pahang (Fig. 1) during the northeast monsoon. In November on the east coast of Peninsular Malaysia. The unusual 2014, an episode of heavy rainfall and ensuing flooding situation was that these long-lasting extreme rainfall events were concentrated over the catchment areas in Corresponding author e-mail: Sheeba Nettukandy Chenoli, the upper reaches of the Kelantan and Pahang River [email protected] basins instead of near the lower reaches, as normal. A

DOI: 10.1175/WAF-D-16-0160.1 Ó 2017 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 10/07/21 11:52 PM UTC 850 WEATHER AND FORECASTING VOLUME 32

oscillation. The relationship between the rainfall variability over the Maritime Continent region and ENSO has been studied by Hendon (2003), McBride et al. (2003), and Tangang and Juneng (2004). These studies reveal that the winter monsoon is weak during El Niño years and is likely to be strong during La Niña years. Recently, Wu and Leung (2009), as well as Tubi and Dayan (2012), studied the impact of ENSO on the Si- berian high, the East Asian winter monsoon, and the modulation of rainfall over southern China and Hong Kong. Unlike the results from the above-mentioned previous studies, the cases examined in this paper, as revealed in section 3, show that the December 2014 episodes of unusual cold surge–induced extreme rainfall FIG. 1. Topography of Malaysia (Peninsular Malaysia and the states of Sarawak and Sabah). occurred mostly over inland and mountainous areas in the northern sector of the east coast of Peninsular Malaysia and were due to the influences of the intense mountainous hydrological station (Gunung Gagau) in Siberian high during an ENSO-neutral year along with Pahang recorded 624 mm of torrential rain from 15 to developing mesoscale convective systems in the north- 19 December and 1395 mm from 20 to 24 December eastern Indian Ocean. 2014. These rainfall amounts were claimed to have re- turn periods of 40 and 1000 yr, respectively (Sun Daily, 2. Data and methods 20 January 2015). The extreme rainfall situation was aggravated by the low water-holding capacity of the Surface and upper-air reanalysis data at 0000 UTC lower reaches, excessive logging and land clearing, as from the European Centre for Medium-Range well as sediment buildup in river areas. In fact, exces- Weather Forecasts (ECMWF) are used. These so- sive land clearing caused serious erosion and swift called ERA-Interim (Dee et al. 2011) data have a spa- drainage of water, leading to widespread flooding. River tial resolution of 0.75830.758 in latitude and longitude. levels during these two episodes far exceeded those of We complement the reanalysis data with a wide range of the 1967 and 2004 floods. At the height of the flood, satellite observations (Rahimi et al. 2015). We use more than 66 500 people were evacuated, at least 21 monthly, daily, and 3-hourly Tropical Rainfall Measur- people died, and 250 000 lost their homes. Estimated ing Mission (TRMM) precipitation at 0.258 resolution; damages cost about 1 billion Malaysian ringgits (RM), the data were obtained from the Goddard Earth Sci- almost U.S. $300 million (source: Malaysian National ences Data and Information Services Center (GES- Security Council). DISC) Interactive Online Visualization and Analysis The available records show that the above flood- Infrastructure (Giovanni). TRMM data (Bookhagen associated extreme meteorological events during 2010) are merely used to complement the precipitation December 2014 are the first to occur in Peninsular analysis in data-sparse land and sea areas throughout the Malaysia. Thus, understanding the features and pro- region. Furthermore, selected daily/hourly gridded cesses of these rare episodes is essential in advancing the Multifunctional Transport Satellite-2 (MTSAT-2) in- scientific knowledge for better flood mitigation to min- frared channel 1 (IR1) data, which are available from the imize the socioeconomic impacts in the future (Valipour Center for Environmental Remote Sensing (CEReS), 2012, 2016). This is the core objective and approach of Chiba University, Chiba, Japan, are also utilized. These this paper. Several recent studies carried out for the data, which have a resolution of 0.048, cover an area Malaysia region show that there has been an increase in from 608Sto608N and from 858 to 2058 longitude. The the frequency and intensity of extreme rainfall events measured blackbody temperatures (TBBs) of these in- (Suhaila et al. 2010; Zin et al. 2010). In addition, a few frared images are color enhanced by suppressing those previous case studies of extreme rainfall over Peninsular TBBs warmer than 2208C based on the convection in- Malaysia by Tangang et al. (2008) and Juneng et al. dex, as described by Chang et al. (2005). The coldest (2007) were mainly focused on the extreme precipitation cloud tops are associated with colors ranging from blue events caused by the interaction of northeasterly cold to orange to pink. The colder the cloud tops are, the surges with the Borneo vortex, as well as the impact on higher the clouds and, most likely, the heavier the the events by phenomena such as the Madden–Julian thunderstorms. For example, near the strongest IR

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TABLE 1. List of El Niño, La Niña, and neutral events in the index from Chang et al. (2005), which is chosen as the tropical Pacific based on the ONI. averaged 925-hPa meridional wind between 1108 and 8 8 El Niño years La Niña years Neutral years 117.5 E along 15 N(Fig. 5a, described in more detail below). By adapting the index definition from Chang et al. 1982 1983 1981 1986 1984 1985 (2005), we further define the easterly surge index [zonal 1987 1988 1989 wind surge due to strengthening or equatorward move- 1991 1995 1990 ment in the subtropical ridge of the northwestern Pacific 1994 1998 1992 (Raman et al. 1978) as a result of a Siberian high out- 1997 1999 1993 break] as the averaged 925-hPa zonal wind between 7.58 2002 2000 1996 8 8 2004 2007 2001 and 15 N along 120 E. A surge event occurs when either 21 2006 2010 2003 one of these indices exceeds 8 m s . The surge intensity is 2011 2005 classified into weak, moderate, and strong categories for a 2008 surge index between 8 and 10, 10 and 12, and greater than 2 2012 12 m s 1 [as adopted from Chang et al. (2005)], re- 2013 2014 spectively. Cold and/or easterly surges are necessary but not a sufficient condition to ensure the impingement and sustenance of surge-induced heavy rainfall events toward gradients at the leading edge of an enhanced image is the the east coast of Peninsular Malaysia. location of heavy rain with cumulonimbus clusters. TBBs from 2708 to 2808C indicate that the cloud top 3. Analysis and discussion has penetrated the tropopause level (above 100 hPa or a. Basic climatological features around 53 000 ft) in this region. December features a strong northeast monsoon dur- During November, the surface Siberian high over the ing the early boreal winter and the above-stated extreme northern Asiatic landmass (Fig. 2a) becomes a stable meteorological events in 2014 also occurred in this feature after its gradual buildup since September (Tick month. ERA-Interim mean December data in terms of and Abu Samah 2004). The overlying 500-hPa trough mean sea level pressure and 925-hPa winds from 1981 to aligns along the coast. Development of this trough fa- 2014 are hence used to assess the influence of ENSO on vors the intensification of the Siberian high near Lake the strength of the Siberian high, which triggers the in- Baikal, causing a couple of cold-air outbreaks that ad- cursion of cold surges toward the equatorial South vance toward the coast. Generally, the resultant low- China Sea (SCS). Composited anomalies are obtained level strong northwesterlies flow toward the East China based on the latest standard climatological period coast, turn into northeasterlies as ‘‘cold surges,’’ and of 1981–2010. The oceanic Niño index (ONI), which penetrate seaward across the South China Sea and to- is available online (http://www.cpc.noaa.gov/products/ ward equatorial Southeast Asia. This is made evident analysis_monitoring/ensostuff/ensoyears.shtml), is used by the ‘‘cold tongue’’ of sea surface temperature (SST) to identify El Niño, La Niña, and neutral events in the aligned parallel to the coast of Vietnam with an isoline tropical Pacific, as tabulated in Table 1. of 288C stretching as far south as 68N. Higher SSTs are To identify heavy/extreme rainfall periods and their consequently found off the coast of the Malaysian distributions, precipitation data from meteorological states of Sarawak and Sabah. The seaward portion of stations on the east coast of Peninsular Malaysia (Fig. 1) the near-equatorial trough is over the warmer SST re- are used: Kota Bahru [68100N, 1028170E, 4.6 m above gion, which is an important force in the generation of mean sea level (MSL)], Kuala Krai (58320N 1028120E, low-level convergence (Lindzen and Nigam 1987). The 68.3 m MSL), Kuala Trengganu (58230N 1038060E, 5 m near-equatorial trough in the equatorial SCS is ori- MSL), Kuantan (38470N 1038130E, 15 m MSL), and ented almost zonally around 58N. Interactions among Mersing (28270N 1038500E, 43.6 m MSL). Only the Kuala the strong northeasterlies with the trough give rise to Krai station (some missing data during December 2014 strong cyclonic shears and the consequent development are noted) is representative of the inland hilly area, of mesoscale or synoptic-scale convective systems while the others are coastal stations. The climatological north of the trough (Ooi et al. 2011). High mean base period for the reanalysis data is chosen as 1981– monthly rainfall totals of about 600 mm (Fig. 2b)are 2010 while that for TRMM is from 1998 to 2013. found along the coastal stretch of northeastern Penin- To detect all of the surge-induced events due to the sular Malaysia. cold-air outburst from the Siberian high moving toward By December, the 500-hPa trough has become more the equatorial South China Sea, we adopt the cold surge pronounced (Fig. 2c). Mean sea level pressures over

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FIG. 2. Climatological plots of mean sea level pressure (gray and blue shaded; hPa, with contours of 1008, 1010, 1020, 1025, 1030, and 1035 hPa shown as short black dashes), 500-hPa geopotential height (thick dark contours at 8-dm intervals), 925-hPa wind (barbs and kt), and SST (magenta; 8C) for (a) November and (c) December as well as (b),(d) their respective TRMM precipitation totals (gray and color shaded; mm). central Asia and China have increased from November, the cold surges and the associated monsoonal rainfall in leading to the intensified north–south and east–west the equatorial South China Sea region (Ooi 1999). This pressure gradients. As shown in Fig. 3a, a strong Siberian is clearly reflected by the anomalous mean sea level high or a split of this high moves southward and/or pressure and 925-hPa wind patterns during the strong eastward (Chan 2005), giving rise to more pulses of early winter monsoon month of December (Figs. 3b–d). northeast/east wind surges. Over the South China Sea, As shown in Fig. 3b, a generally negative mean sea level low-level northeasterly flow is stronger and persistent. pressure anomaly west of Lake Baikal within the domain Colder SSTs with an isoline of 278C are located off the of 408–608Nand808–1208E (as indicated by the rectan- southeastern coast of Vietnam. The near-equatorial gular box) implies that the Siberian high is weak during El trough is now anchored around 38N. Rainfall has in- Niño conditions. Correspondingly, the cold surges trig- creased in the SCS over the area of strong low-level gered by the Siberian high toward the equatorial SCS are northeasterly winds. Higher mean monthly rainfall of weak, as shown by the anomalous south-southwesterlies about 800 mm extends southward along the east coast of acrosstheSCS.DuringLaNiña conditions (Fig. 3c), the Peninsular Malaysia (Fig. 2d). Siberain high is strong, particularly in the vicinity and east El Niño–Southern Oscillation (ENSO) has an influ- of Lake Baikal. Hence, cold surges are strong, as revealed ence on the intensity of the Siberian high (Wu and by the light anomalous southerlies across the SCS. Under Leung 2009; Tubi and Dayan 2012) and thus modulates ENSO-neutral conditions (Fig. 3d), even though the

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FIG. 3. (a) Climatological mean sea level pressure difference during December (color shaded; hPa) from the preceding November superimposed with climatological mean 925-hPa winds (barbs and kt). The red rectangular box indicates domain of the Siberian high. Climatological anomalies in the mean sea level pressure and 925-hPa winds in December during (b) El Niño, (c) La Niña, and (d) ENSO-neutral periods. (e) The anomalies in the mean sea level pressure and 925-hPa winds during the December 2014 ENSO-neutral period.

Siberian high is intense, the signiﬁcant positive mean sea by the presence of some weak anomalous southwesterlies level pressure anomaly west of Lake Baikal is noted to across the SCS. align more zonally. In other words, the cold surges are The Arctic Oscillation (AO) is also known to be related weaker than those during La Niña conditions but are to the Siberian high intensity (Tubi and Dayan 2012). stronger than those during El Niño conditions, as supported To evaluate the statistical signiﬁcance of this and the

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FIG. 4. Time series of (a) regressed Siberian high anomalies (hPa; FIG. 5. (a) Time cross section of the 0000 UTC averaged 925-hPa red solid line) in December and residuals between its observed and meridional wind along 15.08N and between 110.08 and 117.58E regressed anomalies (blue dots) and (b) regressed Peninsular Ma- (black line) and zonal wind between 7.58 and 15.08N along 120.08E laysia rainfall anomalies (mm; red solid line) in December and re- (blue line), as cold and easterly surge indices, respectively, during siduals between its observed and regressed anomalies (blue dots). November and December 2014. Rectangular boxes indicate three ñ ñ Letters E (red) and L (blue) indicate El Ni o and La Ni a years, respective periods of extreme rainfall episodes. (b) Time variation respectively, while the remaining years are ENSO-neutral periods. of daily rainfall amount (mm; 0800–0800 local time) at selected meteorological stations along the east coast of Peninsular Malaysia above-mentioned observed anomalies with respect to the during November and December 2014. December rainfall anomalies in Peninsular Malaysia, multiple linear regression is performed on the area- negative during El Niño years and positive during La Niña averaged mean sea level pressure anomalies within the years. In addition, the regressed area-averaged rainfall Siberian high domain (SHA; obtained from the rectangular anomalies (PMA) within the Peninsular Malaysia domain box in Fig. 3) using ONI and the Arctic Oscillation index (defined as 2.58–7.58N, 102.58–105.08E, and obtained from (AOI; available online at http://www.cpc.ncep.noaa.gov/ the CMAP dataset) with SHA and SCA (area average of products/precip/CWlink/daily_ao_index/ao_index.html)as the mean sea level pressure within the southern China proxies for two independent variables for ENSO and AO, domain, i.e., 208–308N, 1008–1208E) as proxies give rise to respectively. The small p values of 0.33 and 0.42 for the following relationship: ONI and AOI, respectively, of the linear regression SHA 520.42 3 ONI 2 0.24 3 AOI 1 0.25, show that the PMA 5 19.27 3 SHA 1 19.58 3 SCA 1 25.06. influence of ENSO is statistically more significant than that of AO even though the regression represents only about Apart from the increased variance of 32% (multiple 22% of the variance, as indicated by the multiple R R 5 0.32) from this regression, the small p value of 0.17 (Fig. 4a). Figure 4a also reveals that SHA is generally from SHA as compared to that of SCA (i.e., 0.42) strongly

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FIG. 6. Hydrographs for (a) Sungai Lebir at Tualang, Kuala Kerai, and (b) Sungai Kelantan at Pasir Mas from 12 Nov to 31 Dec 2014. The green, yellow, and red lines denote normal, alert, and danger flooding levels, respectively. [Source: Drainage and Irrigation Department, Malaysia.] indicates that PMA is greatly influenced by SHA (Fig. 4b). the east coast of Peninsular Malaysia to be from 13 to This is clearly visible in both the charts shown in Fig. 4. 20 November 2014. The flooding peaked at both locations around the alert level close to 19 November, in- b. Identification of heavy/extreme rainfall periods dicating that the rainfall was normal. Rainfall data A moderate-to-strong easterly surge occurred from 17 from a few meteorological stations that are widely to 19 November 2014 when it initiated its gradual climb spaced can only be used to assess the duration and areal from 11 November (Fig. 5a) with pulses of weak cold extent but not the intensity of this episode. surges on 13 and 19 November. Further examination of By December 2014, four moderate-to-strong cold daily rainfall totals (Fig. 5b) and hydrographs at Kuala surges occurred intermittently together with five Kerai and Kota Bahru (Fig. 6) led to the identification of strong easterly surges preceding and/or trailing them the first episode of surge-induced heavy rainfall along (Fig. 5a). Using the same approach as above (Figs. 5b

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FIG. 7. Mean sea level pressure (gray and blue shaded; hPa, with contours of 1008, 1010, 1011, 1020, 1025, 1030 and 1035 hPa shown as short black dashes), 500-hPa geopotential height (thick black contours with 8-dm intervals), and 925-hPa wind (barbs and kt) at 0000 UTC on (a) 13, (b) 14, (c) 15, and (d) 16 Nov 2014. and 6), the complex mixed surges occurred between 11 reflect the extreme intensity of the rainfall but also its and 19 December, as well as between 20 and 23 De- prolonged duration. cember 2014, and helped to identify the respective c. Case studies periods of 14–19 December and 20–24 December 2014 as the second and third episodes of the surge-induced 1) EPISODE 1: 13–20 NOVEMBER 2014 extreme rainfall along the east coast of Peninsular Malaysia. Two significant flood peaks above the dan- A pronounced 500-hPa trough from Lake Baikal with ger level at both Kuala Kerai and Kota Bahru (Fig. 6) advecting cold air had reached 358N off the China coast around 18 and 24 December, respectively, not only on 13 November 2014 (Fig. 7a). The trough then moved

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FIG.8.AsinFig. 7, but for (a) 17, (b) 18, (c) 19, and (d) 20 Nov 2014. east-northeastward with gradual weakening around decreased to around 1030 hPa subsequently. In response 14 November. A cutoff low of mild cold air from to the progression of the 500-hPa trough, the 1016- and northern China approached 408N along the coast on 1012-hPa isobars show an alternating pattern of a south- 16 November and then disappeared shortly on its ward push and a northward retreat along 1108Efrom13to eastward track. 20 November 2014 (Figs. 7 and 8). Strong northeasterly 2 Underneath the upwind side of the 500-hPa trough is wind bands of 20–30 kt (where 1 kt 5 0.51 m s 1)at the region favored for surface anticyclonic development. 925 hPa persisted south of 208Nuntil58N. In general, the A large Siberian high with a center pressure of about strong wind band was aligned from northeast to south- 1040 hPa was detected southwest of Lake Baikal on west. The near-equatorial trough was initially around 38N 12 November (not shown). The pressure of the center and later dipped to 18N on 19 November (Fig. 8c)asa

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FIG. 9. Enhanced MTSAT-2 IR1 TBBs (color shaded; 8C) with superimposed 925-hPa wind streamlines (a)–(h) from 13 to 20 Nov 2014. result of a mild cold-air outburst along the Chinese coast convective development there (Figs. 9e–h). This arose on 16 November (Fig. 7d). Along 958E (over the Indian from the tightening of the pressure gradient in the Bay of Ocean) and north of the equator, a 15–25-kt easterly/ Bengal on 18 and 19 November in response to the mild east-northeasterly wind band appeared between 58 and development of a 500-hPa short-wave trough across the 108N from 17 to 20 November, indicating transient Indian subcontinent (Figs. 8b,c).

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21 FIG. 10. (a) Time–height cross section of 0000 UTC speciﬁc humidities (g kg ) at (left) 158N, 1158E; (center) 108N, 1108E; and (right) 58N, 1058E. (b) Time series of mean sea level pressure (hPa) at 28,48, and 68N along 1048E. (c) Time–height cross section of 2 2 0000 UTC area-averaged relative vorticity (at 61 3 10 5 s 1 intervals) between 28 and 68N and 1028 and 1048E for November 2014. Rectangular box indicates period of extreme rainfall.

Figure 10a shows vertical profiles of 0000 UTC spe- deep moist convection (Doswell 1982). During the pe- cific humidities during November 2014 at 158N 1158E riod of this heavy rain episode (indicated by the rect- (close to the source of cold surge), 108N 1108E (close to angular box in the figure), high moisture content 2 southern Vietnam) and 58N 1058E (close to Peninsular (greater than 16 g kg 1 below 900 hPa) increased from Malaysia), respectively. Specific humidity is used as a the north toward the south, and its depth of availability tracer of moisture and its differing time–height sections also increased from 700 to above 400 hPa. Time series of here can reflect the abundance, depth, and advection of the mean sea level pressure close and parallel to the moisture with respect to time at the selected locations. rainband (Fig. 10b) shows that the pressure was lowest Significant moisture advection is an important forcing on 16;17 November, and the rainfall was noted to be for not only convective initiation but also for generating heavier (Fig. 5b). This implies that the release of latent

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FIG. 11. Daily TRMM precipitation (mm) from 13 to 20 Nov 2014.

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FIG. 12. Mean sea level pressure (gray and blue shaded; hPa, with contours of 1008, 1010, 1011, 1020, 1025, 1030, 1035, and 1040 hPa shown as short black dashes), 500-hPa geopotential height (thick black contours with 8-dm intervals), and 925-hPa wind (barbs and kt) at 0000 UTC on (a) 14, (b) 15, (c) 16, and (d) 17 Dec 2014. heat, which warms the atmosphere, was the principal by the weak anticyclonic shear above 300 hPa, is not a factor controlling the mean sea level pressure. As shown crucial factor in the development of mesoscale con- in Fig. 10c, cyclonic vorticity penetrated above 300 hPa vective systems. during the initial period of the episode but dropped to Figure 11 shows the daily TRMM precipitation data below 600 hPa after 17 November. In addition, signiﬁ- from 13 to 20 November 2014. The major rainbands cant cyclonic vorticity with core values of more than 2 3 were generally lodged along and off the coastal areas. 2 2 10 5 s 1 occurred below 600 hPa initially and was re- The trough was initially located around 38N from 13 to 2 2 duced to about 1 3 10 5 s 1 after 17 November. Fig. 10c 15 November (Figs. 7a–c). The interaction of the cy- also indicates clearly that upper divergence, as represented clonic shear north of the trough with the surface

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FIG. 13. As in Fig. 12, but on (a) 18, (b) 19, (c) 20, and (d) 21 Dec 2014.

discontinuities along the coast led to the coastal align- 108N in the neighboring Indian Ocean (Figs. 9f–h) pro- ment of the convective cloud band. The rainband in- vided the momentum to cause the rainband in Penin- tensiﬁed and broadened on 18 November 2014 in sular Malaysia to be advected inland. response to a mild cold-air outbreak from the southern 2) EPISODE 2: 14–19 DECEMBER 2014 China coast on 16 November 2014. It became zonal and penetrated farther inland on 19 and 20 November 2014. At 500 hPa, a pronounced northeast–southwest-oriented Apart from the weakening and splitting of the 925-hPa cold core trough moved southwestward, passed Lake Bai- winds along 1058E into northeasterly and northerly di- kal on 15 December (Fig. 12b), deepened, and advected rections north of the trough after 17 November (Fig. 8a), farther south to near 308N(Fig. 12c)on16December; the transient convective development between 58 and it then moved out from along the Chinese coast and

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FIG. 14. Enhanced MTSAT-2 IR1 TBBs (color shaded; 8C) with superimposed 925-hPa wind streamlines: (a)–(f) from 14 to 19 Dec 2014. weakened gradually on 17 December (Fig. 12d)be- Siberian high toward the SCS and western North Pa- fore its subsequent east-northeastward track toward ciﬁc, the tight pressure gradient between the 1020- and the sea. 1012-hPa isobars shows pulsating ﬂuctuations dur- A large intense Siberian high with center pressure of ing this period (Figs. 12 and 13a,b). A strong more than 1040 hPa was noted southwest of Lake northeasterly/east-northeasterly wind band of 20–45 kt Baikal on 14 December and moved southeastward to- at 925 hPa appeared, particularly across the SCS, and the ward southern China on 16 December with a sub- 20-kt winds reached as far south as 58N. Owing to the sequent reduction in core intensity and size. In initial strong momentum, the near-equatorial trough response to the progression of the 500-hPa trough and was initially around 18N and later shifted slightly the resultant outburst of intense cold air from the northward to 28N on 19 December. A 500-hPa cutoff low

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21 FIG. 15. (a) Time–height cross section of 0000 UTC specific humidities (g kg ) at (left) 158N, 1158E; (center) 108N, 1108E; and (right) 58N 1058E. (b) Time series of mean sea level pressure (hPa) at 28N, 48N, and 68N along 1048E. (c) Time–height cross section of the 0000 2 2 UTC area-averaged relative vorticity (at 61 3 10 5 s 1 intervals) between 28 and 68N and 1028 and 1048E for December 2014. Rectangular box indicates period of extreme rainfall. was visible over the northwestern part of the Indian winds to 20–30 kt over the Bay of Bengal. The resultant subcontinent on 14 December (Fig. 12a). The prominent increased cyclonic wind shear close to the northern near- short-wave trough thus formed and then moved east- equatorial trough across the Indian Ocean caused the in- southeastward before weakening eventually over In- tensification of convective activity off of northern Sumatra dochina on 19 December (Fig. 13b). The ridge behind (Fig. 14). the trough intensified the surface high over the northern In comparison with the November rainstorm reaches of the Indian subcontinent, leading to the (Fig. 10a), the time–height section of specific humidity tightening of the pressure gradient (as reflected by the during the period of this episode shows that moisture white 1016- and 1018-hPa contour bands) and strength- was already abundant and available throughout the ening of the 925-hPa northeasterly/east-northeasterly troposphere from the South China Sea toward

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FIG. 16. Daily TRMM precipitation (mm) from 14 to 24 Dec 2014.

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FIG. 17. As in Fig. 12, but for (a) 22, (b) 23, (c) 24, and (d) 25 Dec 2014.

Peninsular Malaysia (Fig. 15a). As shown in Fig. 15b, the episode. This was partly due to the more southward lo- mean sea level pressure was the lowest on 16 December cation of the near-equatorial trough. In addition, apart near 68N, implying that extreme rainfall was conﬁned from the impact of the convective activities off of mainly toward the northern part of Peninsular Ma- northern Sumatra on 17 December 2014, the consecu- laysia’s east coast. Though the cyclonic vorticity pene- tive strong cold and easterly surges (Fig. 5a) provided trated up to 300 hPa (Fig. 15c), its core value is only sufﬁciently strong northeasterly wind momentum to 2 2 1.0 3 10 5 s 1 in the lower troposphere. overcome the effects of coastal surface discontinuity and During the period from 14 to 19 December 2014, the advected the rainstorm farther inland, with the maxi- broad rainbands (Fig. 16) were noted to penetrate more mum precipitation core lying along the coast and its inland and southerly as compared with the November immediate sea area.

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FIG. 18. As in Fig. 14, but from 20 to 25 Dec 2014.

3) EPISODE 3: 20–24 DECEMBER 2014 progression of the 500-hPa trough, this more intense Siberian high moved south-southeastward and burst out After the second episode, another prominent trough at toward the SCS before its subsequent eastward move- 500 hPa with intense cold air advected from Lake Baikal ment to the western North Pacific on 22 December moved toward the Chinese coast, reaching slightly south (Fig. 17a). In this episode, 20–45-kt northeasterly/ of 308N on 21 December 2014 (Fig. 13d), before its sub- easterly winds at 925 hPa persisted not only in the west- sequent east-northeastward track toward the sea. ern North Pacific but also in the SCS. Winds of 30–35 kt A large intense Siberian high with a center pressure of reached as far south as 58N in the SCS while the near- more than 1040 hPa was detected southwest of Lake equatorial trough hovered around 28N. A significant Baikal on 20 December (Fig. 13c). In response to the feature to note is that the intense Siberian high also

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FIG. 19. (left) Three-hourly (from top to bottom, left to right) enhanced MTSAT-2 IR1 TBBs (8C) and (right) accumulated TRMM precipitation (mm) (from top to bottom, left to right) at each specific time (UTC; starting from 0000 UTC or 0800 local time) on 23 Dec 2014. burst out from the south-southwestward simultaneously, throughout the troposphere from the South China Sea heading toward the Andaman Sea on 22 December and toward Peninsular Malaysia (Fig. 15a). In Fig. 15b, this strengthening the northeasterlies there. The increased episode shows a deeper drop in the mean sea level cyclonic shear in the southern Andaman Sea thus caused pressure on 22 and 23 December. The lowest pressure the already intensified convective activity off northern occurred over the central and southern parts of the east Sumatra during episode 2 to develop into a cyclonic coast of Peninsular Malaysia, implying that heaviest disturbance and move slowly westward (Fig. 18). rainfall amounts were confined there. Two strong and As in episode 2, the time–height section of specific deep centers of cyclonic vorticity (on the order of more 2 2 humidity during the period of this episode shows than 2.0 3 10 5 s 1) existed and, in particular, pene- that moisture was already abundant and available trated above 100 hPa on 22–23 December (Fig. 15c)in

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FIG. 20. (left) TRMM precipitation (mm) for (a) November 2014 and (b) December 2014, as well as (right) their respective anomalies. (Climatological base period used for computing anomaly is 1998–2013.) association with the occurrence of very deep and intense to be irregular in space and time while the enhanced convective activity. MTSAT-2 TBB satellite imagery shows increased con- Figure 16 also shows the daily TRMM precipitation vection at night due to the nocturnal radiational cooling data from 20 to 24 December 2014. The rainbands were of cloud tops (McBride and Gray 1980), reaching a broader, penetrated farther inland, extended more maximum during the early morning. southward, and lasted from 21 to 23 December, as 4) PRECIPITATION DURING THE EPISODES compared with the second episode. During this episode, the consecutively stronger cold and easterly surges During November 2014, high monthly precipitation (Fig. 5a) provided the greater wind momentum neces- totals of more than 800 mm were noted along the north- sary to overcome the effects of coastal surface discon- eastern coast and its neighboring sea area of Peninsular tinuity to advect the rainstorm farther inland, where it Malaysia’s east coast (Fig. 20a, left). The anomaly for the became lodged against the mountain ranges located November rainstorm (Fig. 20a, right) shows a signiﬁcant near the west coast of Peninsular Malaysia. Meanwhile, increase of more than 500 mm. During December 2014, the developing and westward-moving cyclonic distur- very high monthly precipitation totals of more than bance off of northern Sumatra provided the additional 1400 mm were noted inland as well as along the coastal momentum to cause the rainstorm to remain quasi- region and its neighboring sea area (Fig. 20b, left), ex- stationary over the inland area. Consistent with the tending as far south as 28N.TheanomalyoftwoDecember lowest mean sea level pressure and deep and strong rainstorms (Fig. 20b, right) shows a very signiﬁcant in- cyclonic vorticity, extensive and maximum precipitation crease in precipitation of more than 800 mm. occurred on 23 December 2014. Three-hourly enhanced MTSAT-2 TBB satellite images (Fig. 19, left) and the 4. Summary and concluding remarks associated 3-hourly accumulated TRMM precipitation (Fig. 19, right) revealed the complex cloud intensity The extreme rainstorms investigated in this study changes and distribution pattern variations. The distri- occurred during 2014, which was an exceptional ENSO- butions of varying intense precipitation cores are noted neutral year. The anomaly of the Siberian high showed

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FIG. 21. Composited 0000 UTC mean sea level pressure (gray shaded; hPa, with contours of 1008, 1009, 1010, 1020, 1025, 1030, 1035, and 1040 hPa as short black dashes) during (a) 13–20 Nov and (b) 14–24 Dec 2014, and (c) the difference between (a) and (b) south of 208N. that the high was very intense during December 2014. cyclonic circulation southeast of Sri Lanka. Hence, as The cold air burst out strongly south-southeastward shown in Fig. 22, the prevalence of very strong north- across the SCS and headed southeastward toward the easterlies due to very strong and consecutive cold and western North Pacific. The anomalous light southerlies easterly surges not only provided the necessary momen- across the SCS indicate that the corresponding cold tum to overcome the coastal discontinuity effect but also surges were very strong. strong to very strong cyclonic shear north of the near- The composite average mean sea level pressure equatorial trough, for the development and sustenance of (Fig. 21) reveals that the core intensity of the Siberian intense and deep mesoscale convective rainstorms over high during the December rainstorm period was about the inland areas of the east coast of Peninsular Malaysia. 1040 hPa as compared with that of 1030 hPa in Novem- The developing and westward-moving cyclonic distur- ber. The south-southeastward outburst of the intense cold bance off of northern Sumatra provided the impetus for air from the Chinese coast gave rise to tighter pressure the intensification of the rainstorms to be located farther gradients both in the SCS and the western North Pacific. inland, contributing to the rapid and high river levels due The impact of a 500-hPa short-wave trough over the In- to the more confined drainage basin. These results are dian subcontinent also resulted in a tighter pressure clearly supported by the anomalous southeasterlies gradient over the Bay of Bengal. This and the south- across Peninsular Malaysia and toward the cyclonic cir- southwestward outburst of the Siberian high toward the culation southeast of Sri Lanka. Andaman Sea of the Indian Ocean led to stronger Malaysia and the neighboring equatorial South China northeasterlies and an eventual development of a Sea are located in the western part of the Maritime

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related to improved qualitative forecasting, warnings, and awareness.

Acknowledgments. ERA-Interim reanalysis data were provided by the ECMWF while TRMM and MTSAT-2 IR1 data were obtained from Giovanni and CERES, respectively. The data analysis supporting this project was carried out using the Grid Analysis and Display System (GrADS) software from OpenGrADS. We thank Dr. Peter Braesicke of Karlsruhe Institute of Technology, Karlsruhe, Germany, for his constructive feedback. This research study has been funded by the Fundamental Research Grant Scheme (FRGS) FP049- 2013 and Trans Disciplinary Research Grant (TRGS) Project 417 TR001A-2015, as well as Institute of Ocean and Earth Sciences Research Grant (IOES) IOES- 2014B of the Ministry of Higher Education, Malaysia. This work has also been strongly supported by the Vice Chancellor of the University of Malaya and the Director General of the Malaysian Meteorological Department.

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