DOMINANT FACTORS OF JAKARTA S THREE LARGEST FLOODS

J. Hidrosfir Indonesia Vol.3 No.3 Hal. 105-112 Jakarta, Desember 2008 ISSN 1907-1043 DOMINANT FACTORS OF JAKARTA S THREE LARGEST FLOODS Edvin Aldrian Peneliti Utama Bidang Meteorologi dan Klimatologi Badan Pengkajian dan Penerapan Teknologi Abstract In year 2002, 2007 and 2008 Jakarta experienced heaviest flood in early February from rain that precipitate in several days ahead. Those rainfalls take place in early morning for few consecutive days. The extreme early morning rainfall occurs due to three dominant phenomena the Madden Julian oscillation wet phase, the cold surge and a vortex in the southwest of Java. The cold surge from north compressed the air mass over the north of Jakarta Bay, picked up as much as water vapor over it and brings to the bay. Especially at night during intense land breeze, south ward movement of compressed and moist air collided frontally with the land breeze and caused strong uplifting at the coast. The latter phenomenon created strong convection at the Jakarta metropolitan areas at early morning and caused extreme rainfall events. Those daily rainfall rate are almost and exceed the monthly rainfall values. Analises of data from satellite observation, rain gauge, wind and radar imagery prove the mechanism of extreme weather in early morning that cause heavy flood over Jakarta. The incoming of those three phenomena could be predicted in advance with quite a certainty. The MJO is a feature that has 50 days oscillation so that MJO wet phase could be predicted in advance up to 2 weeks. The cold surge that comes from Siberia has index of sudden drop of pressure in Hongkong before propagate to Southeast Asia, thus could be predicted in advance about one week ahead. Lastly the vortex development in southwest of Java could be predicted up to 1 days in advanced. Those information on the three major or dominant weather feature will be a good early warning for heavy flood of Jakarta especially in early February. Key words: flood, weather extreme, cold surge, MJO, diurnal pattern 1. INTRODUCTION In the history of the capital city of Jakarta, there were heaviest floods that occurred mostly on early February that are most devastating. The flood of 1996, 2002, 2007 and 2008 are the heaviest that killed 10, 25, 54 and 9, respectively (various sources). The flood, beginning on February 2, 2007 was a result of heavy rain, deforestation in areas south of the city, and waterways clogged with debris. Moreover, the flood 2007 affected 80 separate regions in and around Jakarta, and over 70,000 homes were flooded, resulting in the displacement of some Korespondensi Penulis : Telp (021) 316 8828; edvin@webmail.bppt.go.id Dominant Factors...J. Hidrosfir Indonesia Vol. 3 (3) : 105-112 105

200,000 people. There was a high level of illness, with 1,066 patients treated by hospitals due to diarrhea and 329 due to dengue fever. A total of approximately 190,000 people have fallen ill due to floodrelated illnesses. The flood has caused Rp 8 trillion (US$879.12 million) in losses. During the 2008 flood more than 40 locations across Jakarta were inundated by flood waters reaching heights of more than one meter. The flooding forced 360 people in West Jakarta, around 2,000 in East Jakarta, and 600 others in Central Jakarta to abandon their homes and take shelter in school buildings, houses of worship, and other safer areas made available by the Jakarta city administration (Data are compiled from various newspaper sources). Jakarta is prone to flood due to poor landscape and land cover as well as poor flood management. In a period between 1990 and 1995 there were 15 flood days recorded (23 Jan 1990, 21 Dec 1991, 24 Jan 1992, 28 Feb 1992, 16 Mar 1992, 23 Apr 1992, 3 Dec 1992, 10 Jan 1993, 8 Jan 1994, 25-26 Mar 1995, 12-14 Oct 1995, 15 Nov 1995), while between 1996 to 2000 there were 9 times of flood days (9-14 Jan 1996, 2 Apr 1996, 15 Oct 1996, 13-14 Jan 1997, 12 May 1998, 26-28 Jan 1999), then from 2001 to 2005 there were 21 times of flood days (6 Feb 2001, 14-15 Jan 2002, 23 Jan 2002, 28 Jan 2002, 29 Jan 2002, 30 Jan 2002, 1-2 Feb 2002, 12 Jan 2004, 17 Feb 2004, 21 Apr 2004, 28 May 2004, 12 Jul 2004, 29 Nov 2004, 12 Dec 2004, 21 Jan 2005, 23 Jan 2005, 6 Mar 2005, 16 Jun 2005, 15 Jul 2005) and lastly up to March 2008 there were 11 flood days (17 Jan 2006, 20 Apr 2006, 31 Jan 2007, 1-5 Feb 2007, 1-2 Nov 2007, 1 Feb 200). Data are compiled from various newspaper sources. The latter three major floods of 2002, 2007 and 2008 occur almost at similar time at the end of January and early February. Climate condition are not the same, 2007 is a weak El Nino year while 2002 and 2008 are not. In order to understand mechanism during those heavy floods, observed data from the three flood episode are investigated focusing on the common and dominant weather patterns that lead to heaviest and extreme rainfall that cause severe flood on the following days. Whenever there are common and dominant feature that govern the weather pattern during those three events, then the mechanism of the feature will be studied for further predictability of future incoming similar event. The predictability of weather pattern are inevitable to prevent devastating impact of heavy flood and to prepare for a flood early warning system for the greater Jakarta metropolitan area. 2. METHODS This study uses data from several observation methods. The data comprises of radar, satellite, and rain gauge observation. The satellite observation includes the surface wind observation, cloud observation. Meanwhile the cloud observation comprises of the geostationary satellite observation and reanalyses based on satellite data as the outgoing longwave radiation (OLR) at certain areas. The rain gauge data has hourly data during specific dates before and after flood days. 3. RESULTS The results of hourly rain gauge observation (Table 1) exhibits dominant of peak of the rainy season pattern where the convective rainfall span during the early morning of local time. The rain gauge data are observed in the Pondok Betung station (106 45' 00"E- 06 15' 20.8"S) southwest of Jakarta. During the mid of rainy season, ocean is as warm as the land and the 106 Aldrian E. 2008

Table 1. Hourly rainfall rate at Pondok Betung during flood events 2007 and 2008 probability of convection extend into night and not during the afternoon hour as normally does (Kusumayanti, 2008). The afternoon to early evening shower is due to the sea breeze intensity during that time of the day, which changed into early morning convection. Strangely that the intensity of those morning rainfalls are too heavy during the 2007 and 2008 flood which reach up to 341.8 mm/day and 209 mm/day, respectively. Those values exceed the monthly average over that station. Observation in other stations also indicate exceeding normal monthly average such as Cengkareng in 2008 that has 317mm out of 340mm of monthly average and Tangerang in 2008 that has 262mm out of 246mm of monthly average. Why there are too heavy rains in late night and early morning like those. Also strangely evident from data the normal afternoon time is exempted from rains. Thus there are strong upside down mechanism that govern the abnormal diurnal pattern of daily rainfall. Another observing apparatus during the flood is the weather radar image as shown in Fig 1 for the example of flood case 2007. The radar image will give reflectivity measurement of rainfall rate. Although the given values are not absolute quantitatively, however the qualitative visual image could give explanation on the weather pattern. Based on the analyses from weather patterns observed from radar image during the Jakarta floods, 2007 there are diurnal pattern that shift between the coastal area in early morning to the mountainous area south of Jakarta in the afternoon. The diurnal shift of rainfall pattern is clearly seen from the radar image. Similar conditions are also observed during the 2002 and Dominant Factors...J. Hidrosfir Indonesia Vol. 3 (3) : 105-112 107

Figure 1. Radar image during flood event 2007 Figure 2. Hourly TBB gradient of the Jakarta metropolitan areas during major three flood events that indicate early morning convection and rainfalls 2008 flood event. The weather radar is located in the Pondok Betung area. The hourly gradient MTSAT satellite observation from infrared channel or the temperature black body (TBB) data in Fig. 2 shows similar pattern of cloud development and cloud dissipation (convective rainfall). The hourly gradient data are produced by subtracting the current data from data of the previous hour and averaging over a certain box of area. The chosen box is located surrounding the metropolitan capital of Jakarta. The three flood episodes have similar diurnal pattern for the timing of heavy rainfall in early morning. Strong diurnal pattern that is observed in the coastal city of Jakarta does not exist in the southern mountainous area (not shown). Another application of MTSAT satellite data are observation of the weather 108 Aldrian E. 2008

condition during the flood events. The satellite data are available for every 6 hour from Feb 1 to Feb 4 (Fig. 3). From the satellite image there are some interesting features revealed. First, there are appearance of cold surge from the north that strongly suppress air southward that directly hit Jakarta from the north. Secondly, there are the appearance of vortex in southwest of Java Island during that day. The combination of both the cold surge and vortex shall lead to intensive north south air mass pressure to the Jakarta Bay from the north and strong soak up to the southwest by the vortex. This mechanism indicates a strong weather feature combine with the diurnal pattern that intensifies during the flood. Moreover from the same satellite observation, there is indication of strong incoming Madden Julian Oscillation from the west to west Java area and north south surge from the South China Sea, Karimata strait to the Jakarta Bay. The latter phenomenon known as the cold surge usually hits this region from October to February and rarely cross the equator. When the surge crosses the equator, it will suppress air mass southward, block ITCZ northward movement (Aldrian and Utama, 2007) and bring consequent extreme weather in the south. Satellite observation up to Feb 1 indicated strong surge by clear or no cloud condition along the path of the surge. Madden Julian Oscillation (MJO; e.g. Madden and Julian, 1972) is a dominant component of intraseasonal (20-90 day) variability in the tropical atmosphere. Over the past two decades, a number of studies have investigated roles of the MJO in rainfall variabil- Figure 3. Satellite image of MTSAT geostationary satellite observation for every 6 hour observation from 1 February to 4 February 2007 during the flood event. Dominant Factors...J. Hidrosfir Indonesia Vol. 3 (3) : 105-112 109

Figure 4. MJO episodes during the flood events 2007 and 2008. Dashed (solid) contour indicate dry (wet) phase. ity over the tropics to higher latitudes. There are still few studies mention the locality of MJO effect on Indonesian climate. The observations of MJO activity during the three major flood indicate dry MJO phase precede the flood event and follow by a burst wet phase of MJO during the few days before flood. Fig. 4 illustrate the case of 2007 and 2008. Over the maritime continent, the MJO will eastward propagate and oscillate in 50-days period. Persistence phenomena occur during the three major floods in 2002, 2007 and 2008. The floods occur during the incoming wet phase of MJO in the maritime continent especially the west Java area. The next phenomenon that occurs persistently during the three major flood is the cold surge that cross the equator line and cause compress air mass near Jakarta bay and allow a lot of water vapor intake/supply in the bay that fuelled heavy rainfall. A model study (not shown here) indicate a strong uplifting in the north coast of Jakarta due to this effect. The cold surge pressure creates a cold pool advection near the coast and blocking by topography south of Jakarta. A cold surge is indicated by meridional flow at near surface layer, example 850mb level as shown in Fig. 5. At the end of January, the surge already passes the equator and presses the air mass southward. A depth study of cold surge and its influence on the maritime continent could referred to Aldrian and Utama (2007). A combination of strong phenomena that lead to an extreme weather event and flood the Jakarta metropolitan area occurs persistently during flood of 2002, 2007 and 2008. Those phenomena are diurnal rainfall pattern due to land sea breeze, Madden Julian Oscillation wet phase episode, a vortex in southwest of Java and the strong cold surge episode. The early morning rainfall with extremely high rainfall rate occurs due to a combination of cold night land breeze and warm frontal southward surge. Both phenomena collide to each other in frontal direction (see Fig. 6). The land breeze over Jakarta Bay is strong due to the presence of three mountains in the south. If the south cold surge does not exist, in normal condition the northwest monsoon will have wind flow to the southeast direction over the capital city and does not create frontal collision with the land breeze. North wind (cause by cold 110 Aldrian E. 2008

Figure 5. Meridional wind at 850mb height from the NCEP/NCAR reanalyses data, strong surge indicate by heavy wind above 6 m/s southward surge) coming toward the coast frontally collide with land breeze causes strong uplift that create large convective clouds in the coast at early morning. Compress air mass from the north bring more than adequate water vapor supply to the Bay and also the incoming MJO wet phase event worsen the situation. Strong frontal wind from offshore picked up moisture from the cloud cluster offshore from the previous MJO episode. Additionally the vortex located southwest of Java also strengthened the impact by pulling the air mass of west java to its location and thereby increase southward flow of the air mass from Learning the north. from In the short, experience, the heavy the flood three event dominant Jakarta feature is of caused weather by that extremely cause high heavy rate flood morning over rainfall Jakarta due metropolitan to a combination areas are of MJO still wet predicted. phase, The cold cold surge surge and a is vortex a long in term southwest spatially of Java. and temporally because it propagate from Siberia down to east Asia to southeast Asia. The incoming cold surge is defined as a pressure drop over Hongkong a week before reaching the equator. The Madden Julian Oscillation has a cycle of 50 days, so the incoming wet phase could be predicted well in advance up to 2 weeks. Lastly the vortex over southwest Java could be predicted one day in advance. With all this information a flood early warning system could be established by monitoring those three weather feature in January for flood in early February. 4. CONCLUSIONS A simultaneous combination of local phenomena of land breeze, cold surge, MJO wet phase and a vortex in southwest Java causes heavy flood phenomena over Jakarta in early February 2002, 2007 and 2008. Observation from several data of satellite, radar, rain gauge and wind observation support the combine feature described above. A well and combine monitoring that lead to a flood early warning system for Jakarta in January will be a recommendation from this study because the incoming threat could be predicted in advance one day, one week or two weeks for the vortex, cold surge and MJO wet phase. Dominant Factors...J. Hidrosfir Indonesia Vol. 3 (3) : 105-112 111

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