NOTES AND CORRESPONDENCE. An Observational Study of the Extremely Heavy Rain Event in Northern Vietnam during 30 October 1 November 2008

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1 Journal of the Meteorological Society of Japan, Vol. 89A, pp , DOI: /jmsj.2011-A23 NOTES AND CORRESPONDENCE An Observational Study of the Extremely Heavy Rain Event in Northern Vietnam during 30 October 1 November 2008 Peiming WU, Yoshiki FUKUTOMI Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kanagawa, Japan and Jun MATSUMOTO Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kanagawa, Japan Department of Geography, Tokyo Metropolitan University, Hachioji, Japan (Manuscript received 21 May 2010, in final form 5 November, 2010) Abstract An extraordinarily heavy rain event occurred in northern Vietnam from 30 October to 1 November The three consecutive days of extremely heavy rain resulted in the worst flooding event in Hanoi, the capital of Vietnam, and its environs in 24 years. Results from analysis of the Japan Meteorological Agency Climate Data Assimilation System (JCDAS) re-analysis data show that a synoptic-scale tropical wave disturbance formed over the South China Sea and traveled northwestward over the eastern coast of the Indochinese Peninsula, brought the extremely heavy rainfall to northern Vietnam. In the mid-latitudes, a belt of surface high extended southeastward from western Mongolia to the East China Sea, causing a persistent northeasterly monsoonal flow on its southeastern edge along the southern coast of China. The northeasterly flow worked together with the tropical disturbance, creating a strong low-level wind convergence in northern Vietnam. Examination of the structure of the tropical disturbance revealed that the meridional wind structure and cyclonic vorticity of the disturbance extended vertically to approximately 300 hpa, with the maximum fluctuations occurring at the hpa layer and shows nearly vertical with height. A remarkable feature of the atmosphere is the large increases in the equivalent potential temperature in the upper troposphere higher than about 500 hpa, with strong southerly winds greater than 15 m s 1 in the hpa layers for the three days in question. In conclusion, the tropical disturbance, which worked together with the persistent Asian winter monsoon, caused the extreme rainfall event. 1. Introduction Vietnam is located on the eastern margin of the Indochinese Peninsula (Fig. 1). Almost every year heavy rainfall occurs somewhere in the country Corresponding author: Peiming Wu, Research Institute for Global Change (RIGC), Japan Agency for Marine- Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa , Japan. pmwu@jamstec.go.jp , Meteorological Society of Japan and cause severe flooding and great damage. Such unusually heavy rainfall event occurred in Hanoi, the capital of Vietnam, and its environs, between 30 October and 1 November Daily rainfall of 514.2, and mm were observed at Ha Dong station for 30, 31 October and 1 November 2008, respectively (Fig. 2), setting a new record for daily rainfall in northern Vietnam. The total amount of rain recorded at most observation points over the three days in the Hanoi metropolitan area was more than 500 mm. The three consecutive days

2 332 Journal of the Meteorological Society of Japan Vol. 89A Fig. 1. Topography of Vietnam and surroundings, location of Hanoi and some upper-air observation stations (circled) in the region. Shading denotes terrain elevation. of heavy rain resulted in the worst flooding in the Hanoi metropolitan area for 24 years. The weather in the Indochinese Peninsula and its surrounding areas is influenced by many types of atmospheric disturbance both from the tropics and from the mid-latitudes, with various temporal and spatial scales. The intraseasonal variations of precipitation in some areas of the peninsula during the summer monsoon season are associated with westward-propagating cyclonic disturbances (Yokoi and Satomura 2005). Relevant phenomena include typhoons (tropical cyclones), which occur in the northwestern Pacific Ocean regularly all year round, particularly during the boreal summer and autumn from June to November. As Vietnam lies in the path of typhoons that originate from the northwestern Pacific and from the South China Sea, both in summer and autumn they can cause widespread heavy rains over an extended period of time. The origin, structure and propagating characteristics of tropical disturbances over the northwestern Pacific have been examined extensively by many researchers (e.g. Chang 1970; Reed and Recker 1971; Lau and Lau 1990; Takayabu and Nitta 1993; Sobel and Bretherton 1999; Dickinson and Molinari 2002). Tropical synoptic-scale disturbances with periods between 3 and 10 days include tropical depression (TD)-type disturbances, also referred to as easterly waves. The wavelength of a TD-type disturbance is km, with a westward phase speed of approximately 7 8 m s 1 at around 150 E over the Pacific, and slower at more westerly longitudes near the Philippines (e.g. Nitta and Takayabu 1985; Tam and Li 2006). Convective variations in the tropical northwestern Pacific are closely associated with TD-type disturbances (e.g. Takayabu and Nitta 1993; Dunkerton and Baldwin 1995; Molinari et al. 2007). As most previous studies of tropical disturbances in the western Pacific region mentioned above have performed during the boreal summer season,

3 February 2011 P. WU et al. 333 Fig. 2. Time series of daily rainfall observed at Ha Dong Meteorological Observatory ( N, E) in northern Vietnam for 1 October to 30 November In Vietnam, daily rainfall is defined as the total amount of rain during the period from 1900 LT (1200 UTC) of the previous day through 1900 LT of the day. there were only few examinations to date of how westward-propagating tropical synoptic-scale disturbances influence the weather in Southeast Asia. Yokoi and Matsumoto (2008) investigated the synoptic-scale atmospheric conditions over the South China Sea causing heavy rainfall in central Vietnam in November Their results showed that a cold surge northerly wind anomaly in the lower troposphere, and a southerly wind anomaly over the central South China Sea played key roles in the heavy rainfall event that occurred in central Vietnam during the transition phase between the boreal summer and winter monsoon seasons. However, the seasonal march of precipitation in northern Vietnam di ers greatly from that in central Vietnam. While most part of the annual rainfall in central Vietnam concentrates in the three months from September to November, the rainy season of northern Vietnam is in the summer monsoon season during June September (Yokoi et al. 2007). In almost every year, extremely heavy rain event occurs in central Vietnam in October and November. In contrast, usually heavy rainfall in northern Vietnam occurs during the summer monsoon season. Also, no typhoon or tropical cyclone was reported in the region during the period 30 October 1 November 2008 when the extremely heavy rain occurred in Hanoi of northern Vietnam. The purposes of the present study were therefore to discover the factors that brought about the 30 October/1 November 2008 heavy rainfall event in northern Vietnam by investigating the atmospheric circulation both in the tropics and at the midlatitudes using data from satellite observations, upper air soundings and objective re-analysis. The rest of this paper is organized as follows: Section 2 describes the data analysis in this study. The origin, horizontal and vertical structure of a synoptic-scale tropical wave disturbance are presented in section 3. The occurrence of a persistent northeasterly monsoonal flow during the heavy rain event is discussed in section 4, and occurrence of strong upper tropospheric southerly winds over northern Vietnam is presented in section 5. Finally, section 6 is a summary. 2. Data and analysis In this study, the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS) products were used to examine the origin and movement of the tropical synoptic-scale distur-

4 334 Journal of the Meteorological Society of Japan Vol. 89A bance that caused the heavy rain in northern Vietnam. JMA conducted a Japanese 25-year reanalysis (JRA-25) in collaboration with the Central Research Institute of Electric Power Industry (CRIEPI) and produced high quality meteorological datasets (Onogi et al. 2007) to be used in seasonal prediction models and climate research. The dataset contains global data on a 2:5 2:5 latitude longitude grid and 17 pressure levels from the surface to the middle stratosphere every six hours. Although the JRA-25 analysis covers only the 25-year period from 1979 to 2004, JMA also operates a real-time climatic assimilation system known as the JCDAS which o ers to users under the same conditions as the JRA-25 products. We used the JCDAS data for October to November 2008 when the extremely heavy rain occurred in northern Vietnam. The data analyses in this study followed the approach of Fukutomi and Yasunari (2005; 2009). First, all the atmospheric variables were averaged to a daily mean before analysis. Anomaly time series of all variables were calculated by removing the first three harmonics of the seasonal cycle from the original 365-day time series. The anomaly time series were then filtered using a Butterworth filter (Kaylor 1977), with half-power points at two and ten days. This is a relatively broad filter and it can e ectively isolate the synoptic-scale fluctuations. Finally, the vorticity was computed using a spherical harmonic transform method. Apart from the JCDAS re-analysis data, the daily precipitation obtained from the Tropical Rainfall Measurement Mission (TRMM) satellite 3B42 product, Quick Scatterometer (QuikSCAT) sea surface wind data, infrared (IR) images from a geostationary meteorological satellite and routine rawinsonde sounding data were also used in part of this work. 3. Origin, horizontal and vertical structure, and movement of a synoptic-scale tropical wave disturbance 3.1 Precipitation from satellite and horizontal winds at the 850 hpa level Figure 3 shows the daily precipitation data obtained from the Tropical Rainfall Measurement Mission (TRMM) satellite 3B42 product and the horizontal winds at the 850 hpa level from the JCDAS re-analysis data for the period 25 October 1 November The rainfall estimates were obtained from the 3-h interval B42 dataset version 6 TMI/IR merged rainfall product (Hu man et al. 2007). During the three days October, rain fell in a wide area from the Philippine Sea to the southern part of the South China Sea. The horizontal extent of the rain area then decreased after 28 October 2008, with rain occurring on 27 and 28 October along the east coast of central and southern Vietnam. The rain area then migrated in a northwesterly direction towards northern Vietnam. Heavy rainfall was also observed in that period at surface weather stations in northern Vietnam, as mentioned previously. The wind distribution data for the 850 hpa level (Figs. 3a 3h) on 25 October to 1 November 2008 show that easterly winds prevailed throughout that time in most areas south of about 20 latitude. From 25 to 27 October, the easterly winds were relatively strong in the area from the Philippine Sea to the northern South China Sea. In the subsequent three days, the easterly wind continued in the region, although its strength was less. Note that from 28 October, a cyclonic disturbance which had developed near the Nansha Islands over the South China Sea was moving to the northwest, its horizontal extent covering almost all of the southern South China Sea and the Indochinese Peninsula. As it continued to move in the northwesterly direction, a relatively strong southeasterly wind developed over the northern South China Sea and northern Vietnam from 29 October to 1 November, and on 31 October and 1 November an easterly wind again prevailed over the same area south of approximately the 20th parallel as on October. It is noteworthy that although the tropical wave brought the unusually heavy rainfall event to northern Vietnam, the disturbance finally did not develop to a tropical cyclone. 3.2 Origin and horizontal structure of a synopticscale tropical wave disturbance In order to track the tropical disturbance during its development and northward movement, Fig. 4 shows a sequence of vorticity anomalies at the 850 hpa level for the 10 days from 24 October to 2 November using the 2 10-day bandpass-filtered winds. From 24 to 26 October, southerly anomalies and an anticyclonic circulation over the South China Sea was traveling westward, on 27 October moving further to the west of the Indochinese Peninsula. Then, the anticyclonic circulation was followed by a cyclonic disturbance centered near 5 N, 113 E (to the north of Borneo) which, in the next

5 February 2011 P. WU et al. 335 Fig. 3. Daily rainfall obtained from the TRMM satellite 3B42 dataset and horizontal winds at the 850 hpa level from the JCDAS re-analysis data for 25 October 1 November three days, moved northwestward, approaching central and northern Vietnam. The strength of the vorticity of the disturbance showed little change during that period. By 31 October, it recurved toward the northeast in northern Vietnam. Two days later, that is, by 2 November, the cyclonic disturbance center had moved away from the Indochinese Peninsula to Guangdong in southern China

6 336 Journal of the Meteorological Society of Japan Vol. 89A Fig. 4. Evolution of 2-day to 10-day bandpass-filtered 850-hPa wind and vorticity anomalies for 24 October 2 November Solid contours and shaded areas indicate cyclonic anomalies. Dashed contours represent anticyclonic anomalies (zero contour omitted). and gradually weakened, its influence on the weather in Vietnam had therefore become weaker. Summarizing these results, a synoptic-scale tropical wave disturbance originating over the South China Sea on 27 October traveled northwestward over the east coast of the Indochinese Peninsula, causing the three consecutive days of heavy rain in northern Vietnam.

7 February 2011 P. WU et al. 337 Fig. 5. Profiles of (a d) the anomalies of relative vorticity, (e h) anomalies of meridional wind, and (i l) equivalent potential temperatures in the meridional plane along E (as shown in Fig. 4a) for the 4 days from 28 to 31 October Positive anomalies are shown as solid contours and negative anomalies are shown as dotted contours. Contour intervals are 0: s 1,0.5ms 1, and 3 K. 3.3 Vertical structure and movement of a synopticscale tropical wave disturbance In order to further determine the vertical structure of the tropical wave disturbance, we plotted the vertical cross section of the relative vorticity, meridional wind and equivalent potential temperature using the 2 10-day bandpass-filtered reanalysis data. Figure 5 shows the vertical structure

8 338 Journal of the Meteorological Society of Japan Vol. 89A in the meridional plane along E (these longitudes traverse Hanoi in northern Vietnam) for the 4 days from 28 to 31 October On 28 and 29 October, when the disturbance made its approach to northern Vietnam, the cyclonic vorticity appeared a little earlier at the middle level than at the low level. This is probably due to a slightly westward tilt with height of the disturbance. The cyclonic vorticity extended vertically up to approximately 300 hpa, with the maximum vorticity occurring at the hpa layer and shows nearly vertical with height in the north-south direction. So, this was a rather deep disturbance, since most tropical waves across the northwestern Pacific are confined to the lower troposphere, generally extend vertically not higher than 500 hpa. (e.g., Chang 1970; Takayabu and Nitta 1993; Tam and Li 2006; Serra et al. 2008). On 28 and 29 October, before the disturbance arrived at the east coast of the Indochinese Peninsula, Vietnam is a broad area with northerly wind anomalies in the middle and low levels of the troposphere. On 30 October, when the disturbance approached to northern Vietnam, a broad area with southerly wind anomalies occurred up to about 400 hpa to the south of 20 N, with the maximum perturbations occurring at the hpa layer. With the approach and passage of the disturbance over northern Vietnam, increases in the equivalent potential temperatures occurred up to about 700 hpa from 29 to 31 October. Meanwhile, on 30 and 31 October a warm core structure in the upper levels of the troposphere was clearly evident, as the equivalent potential temperatures increased from 500 to 200 hpa in the area just above northern Vietnam. This structure is also seen in other convectively coupled tropical waves in the northwestern Pacific region (Lau and Lau 1990; Tam and Li 2006; Serra et al. 2008). The upper warm core is attributable to warming by the latent heat of condensation in cloud formation and increases of moisture in the atmosphere associated with the disturbances. In the present study, the warm core characteristics of the wave disturbance is apparent even from the lower resolution of the JCDAS reanalysis data, owing to the strong intensity and long duration of convection during the extremely heavy rain event in northern Vietnam. It is well known that the movement of tropical cyclones and easterly waves is usually steered by the surrounding flow throughout the troposphere. These tropical disturbances generally move westward tending slightly towards the north and are steered primarily by easterly winds on the equator side of a subtropical ridge and the planetary vorticity advection. In the present case, the synoptic-scale tropical wave disturbance that formed over the South China Sea was situated on the southwestern edge of the western Pacific subtropical ridge. The disturbance moved northwestward around the perimeter of the subtropical ridge after it formed on 27 October 2008, as described previously. The track of the disturbance approximately followed the contour line of the 500 hpa level 5880 m geopotential height (not shown). The movement of the disturbance was therefore steered primarily by the southeasterly winds on the western edge of the western Pacific subtropical ridge at the mid-level. Only about three days after it formed, the disturbance appeared over northern Vietnam on 30 October Thereafter, as it had crossed the subtropical ridge axis, the disturbance recurved to travel northeastward under the influence of the upper westerly wind stream north of Vietnam. During its course along the east coast of the Indochinese Peninsula, the tropical disturbance brought the extremely heavy rain to northern Vietnam. 4. Occurrence of a persistent northeasterly monsoonal flow over the northern South China Sea and its role in formation of the heavy rain The Asian winter monsoon, usually between November and March (Ding 1994), has a strong influence on the weather in the Indochinese Peninsula and surroundings. To investigate the role of the Asian winter monsoon in the extremely heavy rain event, this section examines the atmospheric circulation in the mid-latitudes at the time of the heavy rain event. The average daily sea-level pressure distributions (isobars) for the period 25 October 1 November 2008 are shown in Fig. 6. On 25 and 26 October (Figs. 6a and 6b), a remarkable feature of the weather system in the mid-latitudes was a surface high situated over the Altay Mountains in western Mongolia, centered at 50 N, 95 E. The high-pressure area extended southeastwards to the east coast of China. On October (Figs. 6c 6e), a split in the high pressure area moved toward the east and on 30 October 1 November when the heavy rain occurred in northern Vietnam, the belt of high pressure oriented NW SE direction covered all of mainland China, the East China Sea and the western region of the Japanese Archipelago. This

9 February 2011 P. WU et al. 339 Fig. 6. Average daily sea-level isobars for 25 October to 1 November Grey shaded areas represent surface air pressures > 1014 hpa.

10 340 Journal of the Meteorological Society of Japan Vol. 89A Fig. 7. Sea surface wind vectors measured by NASA s Quick Scatterometer on the QuikSCAT satellite for: (a) 27 October, (b) 28 October, (c) 29 October and (d) 30 October caused a pressure gradient to form over the northern South China Sea, located on the southern edge of the surface high-pressure zone. It is notable that there was little change in this surface pressure pattern in the mid-latitudes for the whole period of the heavy rain event. The largest pressure gradient along the southern coast of China was found on October, and cold temperature anomalies near the surface up to about 800 hpa occurred in southern China in these two days (not shown). However, on the later days cold temperature anomalies were not apparent. Strong surface heat-fluxes probably weakened the cold air anomalies, since it was fine weather and solar radiation was strong during daytime in southern China in the period. The sea surface winds measured by NASA s QuikSCAT satellite for October 2008 are shown in Fig. 7. At 1048 UTC on 27 October 2008 (Fig. 7a), strong northeasterly winds with speeds of approximately m s 1 were observed over a wide area of the northern South China Sea, from the Taiwan Strait to the Gulf of Tonkin east of northern Vietnam. The areas of strong wind were consistent with the presence of a large pressure gradient at sea level as shown in Fig. 6c, in other words, the pressure gradient and the resultant strong northeasterly air flow were caused by the belt of high pressure in the mid-latitudes that extended NW SE from the area near the Altay Mountains in western Mongolia to the East China Sea (Fig. 6). Corresponding to the distribution of the pressure gradient, the northeasterly winds over the coastal seas from the Taiwan Strait to the

11 February 2011 P. WU et al. 341 northern South China Sea persisted for the period October, while the surface winds over the central and northern parts of China were light, consistent with the low pressure gradient in that region (Figs. 6d 6f ). A strong surface easterly or northeasterly flow existed over the northern South China Sea during the heavy rain event. In contrast to a midwinter easterly surge associated with rapid eastward migration of a split mid-latitude anticyclone (e.g., Wu and Chan 1995; Chan and Li 2004), the belt of high pressure that lay over the Altay Mountains extended southeastwards as far as the East China Sea throughout the heavy rain event. The influence of a typical easterly surge in the middle of winter tends to be short-lived due to the continued eastward movement of the anticyclone away from the longitudes of the South China Sea but, because the belt of surface high pressure pattern in the mid-latitudes changed little in the period 30 October 1 November 2008, the persistent northeasterly flow along the southern coast of China was maintained. This persistent northeasterly monsoonal flow worked together with the tropical disturbance, enhancing the low-level wind convergence in northern Vietnam. As will be described in the following section, a relatively strong low-level easterly airflow accompanied by great increases in atmospheric moisture occurred in northern Vietnam one day before and during the heavy rain event. The atmospheric water vapor measured by the Advanced Microwave Scanning Radiometer (AMSR-E) on the Aqua satellite indicated that the precipitable water vapor during the period was greater than 60 mm along the eastern coast of Vietnam over the South China Sea (not shown). Therefore, the persistent northeasterly monsoonal airflow acted to transport a large amount of moisture from the South China Sea to the northern part of the peninsula during the heavy rain event. The persistent low-level northeasterly airflow from the Asian winter monsoon joined with the upper southerly winds from the tropical disturbance to create a strong vertical wind shear over northern Vietnam, that might be a favorable condition for the formation of severe long-lived convection. Summarizing the above results of this section, the persistent northeasterly Asian winter monsoonal flow was a major factor in the formation of the extremely heavy rainfall in northern Vietnam during the period of 30 October to 1 November Occurrence of strong southerly winds in the upper troposphere over northern Vietnam This section investigates the vertical structure of the tropical wave disturbance that caused the extremely heavy rain in northern Vietnam, and examines the upper tropospheric circulation and its influence on the heavy rain. Profiles of horizontal wind, relative humidity and equivalent potential temperature vs. time at the station for the period 25 October 3 November 2008 are shown in Fig. 8. Above 900 hpa level, southerly winds prevailed throughout the period. Strong southerly winds at speeds greater than 10 m s 1 were observed in the hpa layers. The maximum meridional winds appeared in the hpa layer from 30 October to 1 November, when the extremely heavy rain fell in northern Vietnam. It is noteworthy that strong southerly winds >15 m s 1 were observed in the hpa layers during the three consecutive days in question. Below 700 hpa level, strong easterly winds were observed, with maximum wind speed being at the hpa layer, one day before and during the heavy rain event. The maximum zonal winds occurred at a lower altitude than the upper strong meridional winds. As the tropical disturbance passed over northern Vietnam, there were large increases in the equivalent potential temperature at the altitudes of about 500 to 200 hpa at the station, with a warm, moist boundary layer near ground level. In addition, the humidity profile shows that a rapid increase of moisture in the entire troposphere above Hanoi commenced on 29 October 2008, one day before the heavy rainfall event. As shown in Fig. 3, heavy rainfall occurred when the tropical disturbance moved over the east coast of Vietnam. Therefore, the increases in the equivalent potential temperature aloft observed at Hanoi were attributable to warming by the latent heat of condensation in cloud formation and increases in moisture in the atmosphere. Past studies of summertime synoptic-scale disturbances in the western North Pacific have shown that their average vertical structure is characterized by a warm anomaly at intermediate levels (from 500 up hpa) accompanied by a cold boundary layer within the trough (e.g. Chang et al. 1970; Lau and Lau 1990). The pronounced surface cooling is likely to be due to convective downdrafts transporting low-entropy air from mid-levels into

12 342 Journal of the Meteorological Society of Japan Vol. 89A Fig. 8. Profile of horizontal wind, humidity and temperature at Hanoi as a function of time for 25 October 3 November 2008 obtained from upper-air soundings. (a) Shaded areas indicate the meridional wind component; contours indicate equivalent potential temperature (K); (b) Shaded areas indicate zonal wind components, and contours indicate relative humidity (%). the boundary layer (Serra et al. 2008). Note that, unlike typical summertime synoptic-scale disturbances in the western Pacific, in the present case there was no low-level cold core within the disturbance that traveled over the east coast of the peninsula, although the increases in the equivalent potential temperature at the upper levels were similar to typical wave disturbances. This implies that convective downdrafts in the disturbance were not as significant in the case being studied. An upper anticyclone is generally characterized by mostly light winds within a region of higher pressure, particularly in the area near the highpressure center. During the heavy rain event, the northern part of the Indochinese Peninsula was situated beneath the western edge of the upper-level ridge, so the geopotential height contours over the region were scattered (the upper weather chart not shown) and the corresponding upper winds were therefore expected to be weak. However, as shown previously, the upper-air observations at Hanoi show strong southwesterly winds greater than 15 m s 1 at the hpa layer over northern Vietnam for the three days of the heavy rain event (Fig. 8). Latent heat release associated with the formation of cloud typically creates the warm core of a tropical wave disturbance, resulted in the development of an upper tropospheric anticyclone. However, the upper-air analysis did not indicate a closed anticyclonic circulation over the region of northern Vietnam. In fact upper tropospheric winds are southerly and strong over the northern part of the Indochinese Peninsula for the three days of heavy rain. That is to say, the upper tropospheric high was presumably reinforced locally over the region due to its unusually warm core of the tropical disturbance, and the upper tropospheric southerly winds along the western edge of the upper anticyclone over Hanoi might have been strengthened by convection associated with the tropical disturbance. It is reasonable to consider that the strong upper southerly winds over northern Vietnam during the extremely heavy rain event were principally caused by the e ect of latent heat release of condensation from the development of convection. When upper level winds become stronger, upper level divergence would be increased, which in turn might lead to further development of convection. Therefore, the positive feedback mechanism between the strong southerly winds in the upper troposphere and convection associated with the tropical disturbance could also have played an important role in the formation of the heavy rain event. 6. Summary In order to find the causative factors of the extraordinary heavy rain and flooding event in the Hanoi metropolitan area of Vietnam between 30 October and 1 November 2008, we have examined the atmospheric circulations both in the tropics and

13 February 2011 P. WU et al. 343 the mid-latitudes by using data from satellites, upper-air soundings and objective re-analysis. A synoptic-scale tropical wave disturbance which originated over the South China Sea north of Borneo moved northwestwards over the east coast of the Indochinese Peninsula. In its course, the tropical disturbance caused heavy rainfall in Hanoi and its environs. Meanwhile, in the mid-latitudes a belt of surface high pressure situated near the Altay Mountains in western Mongolia extended southeasterly to central China and the East China Sea. The anticyclone caused a persistent northeasterly monsoonal flow located at its southeastern edge along the southern coast of China, which enhanced the low-level wind convergence, and transported a large amount of moisture from the South China Sea to northern Vietnam. Moreover, the low-level northeasterly monsoonal flow combined with the upper southerly winds created a strong vertical wind shear over the region. The meridional wind structure and cyclonic vorticity of the disturbance extended almost vertically with height up to about 300 hpa, with the maximum fluctuations at the hpa layer. An increase in the equivalent potential temperatures and moisture near the surface up to about 700 hpa were observed with the passage of the disturbance. Meanwhile, strong southerly winds greater than 15 m s 1 in the hpa layers appeared at the three consecutive days of extremely heavy rain, which were principally caused by the e ect of latent heat release from the development of convection. The increased upper-level winds caused upper-level divergence, in turn might have strengthened the convective activity in the region. In conclusion, the cooperative e ect of the synoptic-scale tropical disturbance and the persistent northeasterly Asian winter monsoon was the main cause of the extreme heavy rain event. Acknowledgments This study was supported by the research project, Data Integration & Analysis System, funded by the National Key Technology, the Ministry of Education, Culture, Sports, Science and Technology. We are greatly grateful to two anonymous reviewers for many helpful comments. We are also grateful to Drs. Keisuke Mizuno, Manabu D Yamanaka and Mr. Hideyuki Kamimera of the Japan Agency for Marine-Earth Science and Technology, for their useful comments and support on this study. The daily rainfall data at Ha Dong Meteorological Observatory in northern Vietnam are provided by the National Hydrometeorological Service of Viet Nam (NHMS). The QuikScat data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. Data are available at The balloon sounding data are provided by University of Wyoming. Data are available at weather.uwyo.edu. References Chan, J. C. L., and C. Li, 2004: The East Asia Winter Monsoon. C.-P. Chang (ed.) East Asian Monsoon, World Scientific, Chang, C.-P., V. F. Morris, and J. M. Wallace, 1970: A statistical study of easterly waves in the western Pacific: July December J. Atmos. Sci., 27, Dickinson, M., and J. Molinari, 2002: Mixed Rossby gravity waves and western Pacific tropical cyclogenesis. Part I: Synoptic evolution. J. Atmos. Sci., 59, Ding, Y. H., 1994: Monsoon over China. Kluwer Academic Publisher, 419pp. Dunkerton, T. J., and M. P. Baldwin, 1995: Observation of 3 6-day meridional wind oscillations over the tropical Pacific, : Horizontal structure and propagation. J. Atmos. Sci., 52, Fukutomi, Y., and T. Yasunari, 2005: Southerly surges on submonthly time scales over the eastern Indian Ocean during the southern hemisphere winter. Mon. Wea. Rev., 133, Fukutomi, Y., and T. Yasunari, 2009: Cross-equatorial influences of submonthly scale southerly surges over the eastern Indian Ocean during Southern Hemisphere winter, J. Geophys. Res., 114, D20119, doi: /2008jd Hu man, G. J., R. F. Adler, S. Curtis, D. T. Bolvin, and E. J. Nelkin, 2007: Global rainfall analysis at monthly and 3-hr time scales. In: Measuring Precipitation from Space: EURAINSAT and the Future. V. Levizzani, V. P. Bauer, and J. Turk, (eds.), Kluwer Academic, Kaylor, R. E., 1977: Filtering and decimation of digital time series. Institute of Physical Science Technology Tech. Note BN 850, University of Maryland, College Park, 42 pp. Lau, K.-H., and N.-C. Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances. Mon. Wea. Rev., 118, Lum, C. Y., 1976: 500 mb Troughs Passing over Lake Baikal and the Arrival of Surges at Hong Kong (Tech. Note 31). Royal Observatory, Hong Kong, 22pp. Molinari, J., K. Mombardo, and D. Vollaro, 2007: Trop-

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