DSpace at VNU: Synoptic-Dynamic Analysis of Early Dry-Season Rainfall Events in the Vietnamese Central Highlands

AMERICAN METEOROLOGICAL SOCIETY Monthly Weather Review EARLY ONLINE RELEASE This is a preliminary PDF of the author-produced manuscript that has been peer-reviewed and accepted for publication Since it is being posted so soon after acceptance, it has not yet been copyedited, formatted, or processed by AMS Publications This preliminary version of the manuscript may be downloaded, distributed, and cited, but please be aware that there will be visual differences and possibly some content differences between this version and the final published version The DOI for this manuscript is doi: 10.1175/MWR-D-15-0265.1 The final published version of this manuscript will replace the preliminary version at the above DOI once it is available If you would like to cite this EOR in a separate work, please use the following full citation: van der Linden, R., A Fink, T Phan-Van, and L Trinh-Tuan, 2016: SynopticDynamic Analysis of Early Dry-Season Rainfall Events in the Vietnamese Central Highlands Mon Wea Rev doi:10.1175/MWR-D-15-0265.1, in press © 2016 American Meteorological Society Manuscript (non-LaTeX) Click here to download Manuscript (non-LaTeX) MWR-D-150265_Revision3.docx Synoptic-Dynamic Analysis of Early Dry-Season Rainfall Events in the Vietnamese Central Highlands Roderick van der Linden1 Institute for Geophysics and Meteorology, University of Cologne, Cologne, Germany Andreas H Fink Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany 10 11 Tan Phan-Van and Long Trinh-Tuan 12 Department of Meteorology, Vietnam National University Hanoi University of Science, 13 Hanoi, Vietnam Corresponding author address: Roderick van der Linden, Institute for Geophysics and Meteorology, University of Cologne, Pohligstr 3, 50969 Cologne, Germany E-mail: rvdlinde@uni-koeln.de 14 Abstract 15 The Central Highlands are Vietnam’s main coffee growing region Unusual wet spells 16 during the early dry season in November and December negatively affect two growing cycles 17 in terms of yield and quality The meteorological causes of wet spells in this region have not 18 been thoroughly studied to date Using daily rain gauge measurements at nine stations for the 19 period 1981–2007 in the Central Highlands, four dynamically different early dry-season 20 rainfall events were investigated in depth: I Tail end of a cold front; II Tropical Depression- 21 type disturbance; III Multiple tropical wave interaction; and IV Cold surge with Borneo 22 Vortex 23 Cases I and IV are mainly extratropically forced In case I, moisture advection ahead of 24 a dissipating cold front over the South China Sea led to high equivalent potential temperature 25 in the southern highland where this air mass stalled and facilitated recurrent outbreaks of 26 afternoon convection In this case, the low-level northeasterly flow over the South China Sea 27 was diverted around the southern highlands by relatively stable low layers On the contrary, 28 low-level flow was more orthogonal to the mountain barrier and high Froude numbers and 29 concomitant low stability facilitated the westward extension of the rainfall zone across the 30 mountain barrier in the other cases In case III, an eastward travelling equatorial Kelvin wave 31 might have been a factor in this westward extension too The results show a variety of 32 interactions of large-scale wave forcings, synoptic-convective dynamics and orographic 33 effects on spatio-temporal details of the rainfall patterns 34 Introduction 35 In about the last 20 years, Vietnam grew to one of the leading producers and exporters 36 of coffee in the world In 2013, Vietnam’s contribution to the worldwide Robusta (coffea 37 canephora) production was about 40% with an export share of about 23% (USDA FAS 2014), 38 accounting for about 2% of Vietnamese export revenues (GSO of Vietnam 2014) The main 39 coffee growing region of Vietnam are the Central Highlands, spanning from about 11 to 40 15.5°N and 107 to 109°E and being the southwestern part of the Southeast Asian Annamese 41 Cordillera1 (Figure 1) The Central Highlands are aligned parallel to the coast, are subdivided 42 into a northern and southern part exceeding 2000 m in elevation and the Dak Lak plateau in 43 between (Figure 1) The dry season in the highlands commences in November, as can be seen 44 in Figure of Nguyen et al (2013) Their climate region S2 corresponds to the highland 45 region considered here November also heralds the start of the coffee bean harvest In the 46 November-December period, a return of substantial rainfall negatively impacts the yields in 47 two ways: firstly, the rains lead to flowering of the buds, whilst the ripening coffee beans of 48 the precedent growing cycle are still on the bush (Alvim 1960; Crisosto et al 1992) To avoid 49 damage to the flowers, the beans are harvested prior to the optimum time Secondly, the 50 subsequent harvest is impacted since the buds are actually in need of a resting period during 51 the dry season Besides impacts on the cultivation of coffee and other agricultural products, 52 heavy rainfall bears a risk of flooding and landslides 53 The large socio-economic impacts of wet spells over the Central Highlands in the early 54 dry season lead us to thoroughly analyze the synoptic-dynamic causes of such events While 55 no such study covering the Vietnamese Central Highlands for this season is known to the 56 authors, several studies (Yokoi and Matsumoto 2008; Wu et al 2011; Wu et al 2012; Chen et 57 al 2012a; Chen et al 2012b; Chen et al 2015a; Chen et al 2015b) investigated the causes of 58 extreme rainfall events along Vietnam’s central and northern coast (i.e., climate regions S1 In Vietnamese: Truong Son 59 and N4 in Nguyen et al 2013) Contrary to the Vietnamese Central Highlands where an 60 extended rainy season occurs between May and October, the peak of the rainy season in these 61 regions is October-November and daily rainfall totals exceeding several 100 mm are not 62 uncommon (Yokoi and Matsumoto 2008) 63 A well-known cause of rainfall in the South China Sea (SCS1) area are northeasterly 64 cold surges during the November-April winter monsoon season They are mainly controlled 65 by planetary-scale dynamics of the northern hemispheric mid-latitudes, penetrate the Tropics, 66 lead to a surge in low-level northeasterlies over the SCS, and enhance convection over the 67 Maritime Continent including the near equatorial SCS However, tropical influences on cold 68 surges have been shown in the literature too; Jeong et al (2005) and Chang et al (2005) 69 found an interaction between cold surges and the Madden-Julian Oscillation (MJO; Madden 70 and Julian 1972), and Zhang et al (1997) and Chen et al (2004) showed that cold surges are 71 also influenced by El Niño–Southern Oscillation (ENSO) This prominent type of tropical- 72 extratropical interaction has been studied in detail, with many studies emerging after the 73 Global Atmospheric Research Program (GARP)/First GARP Global Experiment Winter 74 Monsoon Experiment in 1978/1979 (e.g., Chang et al 1979; Chang and Lau 1980; Johnson 75 and Chang 2007, and references therein) However, the bulk of the studies concentrated on 76 East Asia, the SCS, the Maritime Continent and the December-February (DJF) period (e.g., 77 Johnson and Zimmermann 1986; Wu and Chan 1995; Chang et al 2005; Ooi et al 2011; Park 78 et al 2011; Koseki et al 2014) During the DJF period, the northerly wind enhancement 79 associated with SCS cold surges reaches at least the equator and is often associated with the 80 formation of the Borneo Vortex (Chang et al 2005) Juneng and Tangang (2010) found that 81 the Borneo vortices intensified during the DJF 1962–2007 period and that the centers of the 82 vortices moved northwestward closer to the southeastern coast of Vietnam Ooi et al (2011) In Vietnamese notation the SCS is frequently referred to as the Vietnam East Sea (e.g., Phan et al 2015) 83 describe a January 2010 case in which the Borneo Vortex moved northwestward and 84 developed into a tropical depression affecting southern Vietnam However, Yokoi and 85 Matsumoto (2008) highlighted differences in cold surges occurring in October-November and 86 January-February Basically, early-season cold surges tend to stall in the central SCS at about 87 10°N, where at this time of the year the ITCZ is located, whereas winter cold surges reach the 88 equator 89 Yokoi and Matsumoto (2008) and Wu et al (2011) point to a role of westward 90 propagating tropical wave disturbances for heavy rainfall events along the north central 91 Vietnamese coast These low-level disturbances are alternatively termed easterly waves or 92 tropical depression (TD)-type disturbances They are known to be involved in tropical 93 cyclogenesis in the Western Pacific (Frank and Roundy 2006) In the classical wavenumber- 94 frequency diagram based on Outgoing Longwave Radiation (OLR), they correspond to 2–6 95 day westward propagating so-called TD-type disturbances (Kiladis et al 2006), which have 96 wavelengths of 2500–3500 km (Kiladis et al 2009) The latter notation is used in the present 97 study Wu et al (2012) argued that the concurrent occurrence of the convectively active part 98 of the MJO and a TD-type disturbance led to an extreme rainfall event in central Vietnam in 99 early October 2010 Yokoi and Matsumoto (2008) claim that the TD-type disturbances 100 occurred as a result of a Rossby wave response to a large-scale convection anomaly over the 101 Maritime Continent However, multiple tropical wave interactions of the MJO, Convectively 102 Coupled Equatorial Waves (CCEWs; Wheeler and Kiladis 1999), and TD-type disturbances 103 on rainfall events in the Indochina Peninsula have hitherto not been studied 104 Therefore the present paper will employ both classical synoptic-dynamic and tropical 105 large-scale wave analyses to study the evolution of early dry-season rainfall events in the 106 Vietnamese Central Highlands The study aims at selecting an, in terms of dynamic forcings, 107 as diverse as possible sample of anomalous rainfall events in the period 1981–2007 It shall 108 contribute to an improved understanding of the chain of atmospheric processes that ultimately 109 lead to rainfall in Vietnam’s most important coffee-growing region In Section 2, data and 110 methods are described Section discusses the four selected rainfall events and Section 111 provides a summary and discussion of results 112 113 Data and Methods 114 Daily rainfall totals from fifteen stations operated by the Vietnamese National Hydro- 115 meteorological Service (NHMS) in the Central Highlands and adjacent coastland were used 116 (Figure and Table 1) In addition, the APHRODITE Monsoon Asia V1101 gridded rainfall 117 product that is based on station measurements was utilized in the 0.25° × 0.25° latitude- 118 longitude resolution (Yatagai et al 2012) Station data availability before 1981 and the end 119 year of the APHRODITE product restrict the investigations period to November-December 120 1981–2007 The 24-hour period of daily rainfall in station and APHRODITE data is 1200– 121 1200 UTC (1900–1900 LT) The calendar date is assigned to the date of the end of the 24- 122 hour period The three-dimensional wind components, mean sea-level pressure (MSLP) and 123 surface pressure, geopotential, temperature, specific humidity, and potential vorticity at 124 standard pressure levels were obtained from the ERA-Interim reanalysis (Dee et al 2011) at a 125 horizontal resolution of 0.75° × 0.75° and a temporal resolution of six hours Additional 126 surface charts including fronts were provided by the NHMS Six-hourly NCEP/NCAR 127 reanalysis MSLP data (Kalnay et al 1996) at a 2.5° × 2.5° resolution were used to calculate a 128 long-term time series of the Siberian High (SibH) intensity after Jeong et al (2011) The SibH 129 intensity is the mean DJF MSLP in the region 40–65°N, 80–120°E that is standardized with 130 respect to the mean and standard deviation for 1949/50–2013/14 The corresponding intensity 131 of the Aleutian Low was assessed using the North Pacific (NP) Index (Trenberth and Hurrell 132 1994) The NP index is the mean monthly sea level pressure averaged over the region 30– 133 65°N and 160°E–140°W 134 To describe the evolution of deep convection, three-hourly Gridded Satellite (GridSat)- 135 B1 climate data record intercalibrated IR brightness temperature data in the 11 µm window 136 channel (Knapp et al 2011) at a resolution of × km2 were employed Finally, daily NOAA 137 Interpolated OLR (Liebmann and Smith 1996) in the latitude belt 0–15°N was used at a 2.5° × 138 2.5° resolution to filter for the MJO, Kelvin and Equatorial Rossby (ER) waves with the 139 wavenumber-frequency filter after Wheeler and Kiladis (1999) A 2–10-day Lanczos band- 140 pass filter (Duchon 1979) that was applied to NOAA OLR data is used to determine activity 141 of TD-type disturbances (Wu et al 2011) 142 The daily rainfall time series of the nine stations located in the Central Highlands region 143 (Figure 1) were searched for dates matching the following criteria: Measurements were 144 available for at least three out of the nine stations; Dates were selected if rainfall amounts 145 greater than or equal to 10 mm day-1 were recorded at three ore more stations If only records 146 from between three and five stations were available, this criterion was relaxed to two stations 147 with more then 10 mm day-1 The 10 mm day-1 threshold has been taken after informal 148 interviews with coffee farmers in the Central Highlands by the third author (Phan et al 2013) 149 For the period 1981–2007, 90 dates matched these criteria in November, and 19 for the 150 climatologically drier month December Dates with tropical cyclone activity were excluded 151 using 152 http://weather.unisys.com/hurricane/w_pacific/index.php Cases were subjectively selected 153 for an in-depth investigation based on two criteria: (a) one of the known dynamic features 154 discussed in the Introduction was assumed to be the major forcing of rainfall, i.e., cold fronts, 155 cold surges, TD-type disturbance, or active MJO and CCEW phases; and (b) subjective 156 synoptic analyses of MSLP, geopotential, wind, stream function, velocity potential, and OLR 157 confirmed the suitability of the identified cases This resulted in four cases named “tail end of 158 a cold front” (case I), “TD-type disturbance” (case II), “multiple tropical wave interaction” Joint Typhoon Warning Center Best Track Data, accessed via 159 (case III), and “cold surge with Borneo Vortex” (case IV) The reasons for the naming are 160 described in section 161 To study the identified cases, some derived quantities have been calculated from ERA- 162 Interim These are the vertically integrated humidity fluxes, surface Convective Available 163 Potential Energy (SF-CAPE), the 100–400 hPa vertically averaged potential vorticity as in 164 Fröhlich and Knippertz (2008), and the Froude number The Froude number, defined here for 165 elevation heights higher than 400 m as the ratio of wind speed at 850 hPa and the product of 166 Brunt-Väisälä frequency between 925 and 700 hPa and elevation height, is an approximation 167 if an air parcel will overpass an obstacle or not In case of high wind speeds, low stability, 168 and/or small obstacle the Froude number is large, and the air parcel will likely overpass the 169 obstacle Contrary, in case of a weak wind, high stability, and/or an tall obstacle the Froude 170 number is smaller than one, and the air parcel will not easily overpass the obstacle or will 171 even be forced to pass along the obstacle 172 173 174 Rainfall events a Case I: Tail End of a Cold Front (09–15 November 1982) 175 In the period between 09 and 15 November 1982, highest precipitation amounts 176 occurred in southern parts of the Central Highlands (Figure 2a) Rainfall anomalies with 177 respect to the 1981–2007 period were also positive in the north, but the central part was drier 178 than normal Figure 2b shows the time evolution of rainfall during the event for the entire 179 study region, as well as for the northern, central, and southern parts Two rainfall maxima 180 occurred during this period: the first on 09 and 10 November, and the second from 12 181 November onward; 11 November was rather dry throughout all parts of the Central Highlands 182 (Figure 2b) Therefore, this event can be divided in two periods that will be discussed below 183 The first period of this event was clearly influenced by a subtropical cold front 184 extending deep into the Tropics The cold front belongs to a low-pressure system with its 185 center located over the Yellow Sea on 09 November 1982 at 1800 UTC (Figure 3a) At this 186 time, the cold front was extending equatorward to about 13°N, and the location of the cold 187 front was well reflected in MSLP, wind speed, and horizontal wind shear (Figure 3a) The 188 passage of the cold front can also be seen in the radiosoundings at Hoang Sa (16°50’N; 189 112°20’E) between 09 November 1200 UTC and 10 November 0000 UTC and at Da Nang 190 (16°04’N; 108°21’E, see Figure 1) between 09 November 1200 UTC and 10 November 1200 191 UTC (not shown) Schultz et al (1997) noted that mid-latitude cold fronts frequently lose 192 their frontal character when they reach into the Tropics and can better be described as shear 193 lines, because there is no longer a pronounced temperature gradient but strong winds Yet, 194 both upper-air stations show a considerable drop in low-level temperature on top of a wind 195 shift during the passage of the low-level cold front Thus, though cold fronts are rarely 196 analyzed in surface charts in the southern SCS, it seems justified in this case 197 The three-hourly surface analyses of the NHMS also showed the cold front from 198 Indochina to the Yellow Sea until 09 November 1982 2100 UTC, whereas on 10 November 199 1982 at 0000 UTC the cold front was no longer drawn (not shown) Figure 3a shows strong 200 24-h pressure rise over mainland Asia peaking at hPa per 24 hours over southern China 201 whereas pressure fall ahead of the cold front was on the order of 1–2 hPa suggesting 202 frontolysis The Yellow Sea low was associated with a longwave trough, which is reflected 203 both in 500-hPa geopotential height, and vertically averaged potential vorticity (Figure 3b) 204 The trough originated from a wave disturbance over central Russia on 06 November 1982 and 205 moved eastward with the basic flow (not shown) On 09 November 1982 at 1800 UTC, it 206 reached the southernmost position and the trough axis extended southward to about 21°N 207 (Figure 3b) The trough axis was identified using the zonal 500-hPa geopotential gradient as 208 described in Knippertz (2004) The Chinese name of this station is Xisha Dao (WMO station ID 59981) 682 Figures 683 684 Figure Topographic map of the study area Map of Vietnam and adjacent countries and 685 zoom on Central Highlands region Numbers in the zoomed map correspond with the 686 locations of stations that are used for the analysis of rainfall events (Figures 2, 6, 9, and 12) 687 The stations are listed in Table The dashed lines separate the regions of Vietnam 31 688 689 Figure Tail End of a Cold Front: Rainfall characteristics for 09–15 November 1982 (a) 690 APHRODITE MA V1101 precipitation sums (colors; in mm) and anomalies (contour lines; in 691 mm) and rain gauge precipitation sums and anomalies (numbers; sums in mm, and anomalies 692 in percent of the long-term mean sums) The reference period for anomalies is 1981–2007 (b) 693 Time series of area-averaged (11.125–15.125°N, 107.125–108.625°E) precipitation (black 694 line), long term (1981–2007) mean area-averaged (11.125–15.125°N, 107.125–108.625°E) 695 precipitation (dashed line) and area-averaged precipitation for northern (13.875–15.125°N, 696 107.125–108.625°E; purple line), central (12.625–13.625°N, 107.125–108.625°E; red line), 697 and southern (11.125–12.375°N, 107.125–108.625°E; blue line) parts of the area (source: 698 APHRODITE MA V1101) The grey line in (b) is the mean daily rainfall averaged over all 699 nine stations located in the Vietnamese Central Highlands (cf Figure 1) 32 700 701 Figure Cold front on 09 November 1982, 1800 UTC (a) ERA-Interim mean sea level 702 pressure (colors), 24h change of mean sea level pressure (contour lines), 10 m wind (vectors), 703 and cold front from NHMS surface analysis (b) ERA-Interim 500-hPa geopotential height 704 (contour lines), geopotential height trough axis (bold black line), and 400-100 hPa vertically 705 averaged potential vorticity (colors) (c) Surface to 300-hPa vertically integrated humidity 33 706 flux (vectors), humidity flux convergence (colors) and GridSat brightness temperatures 707 (stippling) 34 708 709 Figure Influence of the cold front on Vietnam on 10 November 1982, 0600 UTC (a) ERA- 710 Interim MSLP (contour lines), geostrophic wind (black vectors), isallobaric wind (red 711 vectors), and GridSat brightness temperatures (stippling) (b) ERA-Interim 925-hPa vertical 712 wind (colors), 10 m wind (vectors), and SF-CAPE (stippling) (c) Topography (colors), ERA- 713 Interim 850-hPa streamlines, and Froude number (stippling/hatching) 35 714 715 Figure Cause of high precipitation amounts after passage of the cold front (a) SF-CAPE 716 (colors), equivalent potential temperature (contour lines), and GridSat brightness temperatures 717 (stippling) on 13 November 1982 1200 UTC (b) GridSat brightness temperatures on 13 718 November 1982 0900 UTC 36 719 720 Figure As in Figure 2, but for TD-type disturbance case: Rainfall characteristics for 01–04 721 December 1986 37 722 723 Figure TD-type disturbance approaching Vietnam (a) Hovmöller diagram of wave-filtered 724 NOAA Interpolated OLR between 0° and 15°N for Kelvin waves (brown) and TD-type 725 disturbances (blue) Contour intervals are -1, -5, and -15 W m-2 Hatching indicates the MJO- 726 filtered OLR lower than -1 W m-2 01–04 December 1986 and Vietnam longitudes are marked 727 with thick black lines (b) GridSat brightness temperatures (colors) and ERA-Interim 2–10- 728 day band-pass filtered 850-hPa wind (streamlines) at 0600 UTC for 26 November to 05 729 December 1986 The bold dashed line in (a) and (b) highlights the Kelvin wave and the bold 730 dotted line highlights the TD-type disturbance in (a) and (b) The longitude ranges of the Bay 731 of Bengal (BoB), the Gulf of Thailand (GoT), Vietnam (VNM), the South China Sea (SCS), 732 the Philippines (PHL), and the Western Pacific (WPac) are marked above panel (b) 38 733 734 Figure TD-type disturbance reaching Vietnam on 02 December 1986, 0600 UTC (a) 735 Surface to 300-hPa vertically integrated humidity flux (vectors), humidity flux convergence 736 (colors) and GridSat brightness temperatures (stippling) (b) ERA-Interim 925-hPa vertical 737 wind (colors), 10 m wind (vectors), and SF-CAPE (stippling) (c) Topography (colors), ERA- 738 Interim 850-hPa streamlines, and Froude number (stippling/hatching) 39 739 740 Figure As in Figure 2, but for Multiple Tropical Wave Interaction case: Rainfall 741 characteristics for 02–05 November 2007 40 742 743 Figure 10 Multiple tropical wave interaction between 02 and 05 November 2007 (a) 744 Hovmöller diagram of wave-filtered NOAA Interpolated OLR between 0° and 15°N for 745 Kelvin waves (brown), TD-type disturbances (blue), and ER waves (grey) Contour intervals 746 are -1, -5, and -15 W m-2 Hatching indicates the MJO-filtered OLR lower than -1 W m-2 02– 747 05 November 2007 and Vietnam longitudes are marked with thick black lines (b) GridSat 748 brightness temperatures (colors) and ERA-Interim 2–10-day band-pass filtered 850-hPa wind 749 (streamlines) at 0600 UTC for 27 October to 05 November 2007 The bold dashed line in (a) 750 and (b) highlights the Kelvin wave; the bold dotted line highlights the ER wave and TD-type 751 disturbance in (a) In (b), the bold dotted line highlights the combined ER wave and TD-type 752 disturbance influence on deep convection The longitude ranges of the Bay of Bengal (BoB), 753 the Gulf of Thailand (GoT), Vietnam (VNM), the South China Sea (SCS), the Philippines 754 (PHL), and the Western Pacific (WPac) are marked above panel (b) 41 755 756 Figure 11 Influence of tropical waves on Vietnam on 03 November 2007, 0600 UTC (a) 757 Surface to 300-hPa vertically integrated humidity flux (vectors), humidity flux convergence 758 (colors) and GridSat brightness temperatures (stippling) (b) ERA-Interim 925-hPa vertical 759 wind (colors), 10 m wind (vectors), and SF-CAPE (stippling) (c) Topography (colors), ERA- 760 Interim 850-hPa streamlines, and Froude number (stippling/hatching) 42 761 762 Figure 12 As in Figure 2, but for Cold Surge with Borneo Vortex case: Rainfall 763 characteristics for 11–15 December 2005 43 764 765 Figure 13 Cold surge and Borneo Vortex (a) ERA-Interim mean sea level pressure (colors) 766 and 925-hPa wind (vectors) on 14 December 2005, 0600 UTC The black box indicates the 767 region to identify a vortex as a Borneo Vortex following Chang et al (2005) (b) Siberian 768 High Intensity (SibH Intensity; thin black line) and its five year running mean (thick black 769 line) and North Pacific Index (NP Index; blue line) for December The period is 1949/50 to 770 2013/14, and the DJF SibH Index is assigned to the year of the December The SibH Intensity 771 for DJF 2005/2006 and the NP Index for December 2005 are marked with the red circles 44 772 773 Figure 14 Post-Borneo Vortex reaching Vietnam on 14 December 2005, 0600 UTC (a) 774 Surface to 300-hPa vertically integrated humidity flux (vectors), humidity flux convergence 775 (colors) and GridSat brightness temperatures (stippling) (b) ERA-Interim 925-hPa vertical 776 wind (colors), 10 m wind (vectors), and SF-CAPE (stippling) (c) Topography (colors), ERA- 777 Interim 850-hPa streamlines, and Froude number (stippling/hatching) 45 ... evolution of early dry-season rainfall events in the 106 Vietnamese Central Highlands The study aims at selecting an, in terms of dynamic forcings, 107 as diverse as possible sample of anomalous rainfall. .. of early dry-season (November-December) rainfall events in 353 the Vietnamese Central Highlands, the major coffee-growing region in Vietnam, were 354 analyzed in this study The 109 rainfall events. .. Using daily rain gauge measurements at nine stations for the 19 period 1981–2007 in the Central Highlands, four dynamically different early dry-season 20 rainfall events were investigated in