Journal of the Meteorological Society of Japan, Vol. 89A, pp. 61--81, 2011.

DOI:10.2151/jmsj.2011-A04

Convective Systems Developed along the Coastline of Sumatera Island, Indonesia,
Observed with an X-band Doppler Radar during the HARIMAU2006 Campaign

Shuichi MORI, HAMADA Jun-Ichi
Research Institute for Global Change (RIGC),
Japan Agency for Marine-earth Science and Technology (JAMSTEC), Yokosuka, Japan

Namiko SAKURAI
National Research Institute for Earth Science and Disaster Prevention (NIED), Tsukuba, Japan

Hironori FUDEYASU
Yokohama National University, Yokohama, Japan

Masayuki KAWASHIMA
Institute for Low Temperature Science, Hokkaido University, Sapporo, Japan

Hiroyuki HASHIGUCHI
Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto, Japan

Fadli SYAMSUDIN, Ardhi A. ARBAIN, Reni SULISTYOWATIAgency for the Assessment and Application of Technology (BPPT), Jakarta, Indonesia

Jun MATSUMOTO
Research Institute for Global Change (RIGC),
Japan Agency for Marine-earth Science and Technology (JAMSTEC), Yokosuka, Japan
Department of Geography, Tokyo Metropolitan University, Hachioji, JapanandManabu D. YAMANAKA
Research Institute for Global Change (RIGC),
Japan Agency for Marine-earth Science and Technology (JAMSTEC), Yokosuka, Japan
Graduate School of Science, Kobe University, Kobe, Japan(Manuscript received 30 April 2010, in ﬁnal form 28 September 2010)
Corresponding author: Shuichi Mori, Research Institute for Global Change (RIGC), Japan Agency for
Marine-Earth Science and Technology (JAMSTEC),
2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061,
Japan.
E-mail: morishu@jamstec.go.jp
6 2011, Meteorological Society of Japan

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Abstract
An overview of convective activity during the HARIMAU2006 campaign conducted from 26 October to 27
November 2006 was presented, focusing on the di¤erences between coastal land/sea and inactive/active phases of
intraseasonal variation (ISV) based on observations using an X-band Doppler radar (XDR) and intensive soundings at Sumatera Island. Diurnal variation (DV) in coastal convections and formation of the coastal heavy rainband (CHeR) along Sumatera Island were also examined in terms of diurnal land-sea migration of coastal convective systems.
Convection in the ISV inactive period (PP1) contained convective rain fractions nearly twice as much as stratiform rain fractions, whereas that in the ISV active period (PP2) comprised convective and stratiform elements almost equally. Vertical proﬁles of radar echo coverage for stratiform rain during PP2 were greater than those during PP1, especially in the lower troposphere over the sea. The radar echo coverage for convective rain over the sea
during both periods was nearly double that over land from the near surface up to 6 km high.
Convection was generated in the southwestern foothills of the mountain range in the early afternoon (12–15
Local Time, LT). Part of the convective system remained over the coastal land and exhibited weak reﬂectivity
until the next morning. The other part migrated o¤shore at a speed of approximately 4 m s1 and intensiﬁed until
around 21 LT while still o¤shore. Additional convective cells also developed o¤shore in the early morning hours,
independent of those that formed over land. Results suggested that the CHeR along Sumatera Island is dictatedby diurnal variations in coastal convective development and consists of the following phases: 1) migration of convection away from the coastal land and its redevelopment in the late evening, and 2) additional generation of convection just o¤shore during the early morning hours.

1. Introduction
Coastal regions over the tropics exhibit uniquecharacteristics in terms of their convective activity.
Heavy rainbands tend to form along coastlines,
and the rainfall variation has a strong diurnal component.
Xie et al. (2006) showed several heavy rainbands
that developed in the boreal summer season along
coastlines over the Asian monsoon region. These
rainbands were identiﬁed using long-term observations from the Tropical Rainfall Measuring Mission (TRMM) satellite. They identiﬁed rainbands
along the Indian coast west of the Western Ghats,
the western coast of Myanmar over the Bay of Bengal and the Andaman Sea, northeastern coast of the
Gulf of Thailand, the eastern coast of Vietnam, andthe northwestern Philippines. They showed thatrainfall distribution in the tropics is not uniform,
but instead is anchored by mountains along thesecoastlines that cause a wind-terrain interaction during the boreal summer monsoon season. Though it
was not mentioned by Xie et al. (2006), a heavy
rainband was also identiﬁed along the southwestern
coastline of Sumatera Island in the Indonesian maritime continent (IMC). This rainband was found in
the boreal summer season, but its e¤ect was also
noted in the long-term annual mean rainfall (Moriet al. 2004).

Diurnal variation (DV) is widely known to be
one of the most dominant modes of convective
activity in the tropics (e.g., Nitta and Sekine 1994;
Ohsawa et al. 2001). Intraseasonal variation (ISV),
including the Madden-Julian oscillation (MJO), is
also an essential mode of convective activity in thetropics (Madden and Julian 1971, 1972). In general,
rainfall DV is characterized by evening (morning)rain over land (sea) due to the di¤erent heat capacities of the two surfaces and related local land-sea
circulations (Johnson et al. 2001; Tao et al. 1996;
Sui et al. 1998; Zhou and Wang 2006). As a result,coastlines exhibit contrasts between evening andmorning rainfall over the coastal land and sea, respectively, and produce unique characteristics ofconvection in those regions. In fact, Mori et al.
(2004) showed clear contrast between the evening(12–23 Local Time, LT) and morning (00–11 LT)
rain along the coastlines of major islands of the
IMC, such as Sumatera, Kalimantan, Jawa, Sulawesi, and Papua Islands, using data from the precipitation radar (PR) on the TRMM satellite. In
addition, they showed that the peak time of rainfall
DV is delayed according to the distance both for inland and o¤shore sides from the coastline. Similarphase delays of rainfall DV are widely seen along
coastlines in the tropics such as the southeastern
coast of the Bay of Bengal (Zuidema 2003), thenorthwestern coast of South America (Mapes et al.

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2003), and the northern coast of Papua (Moteki
et al. 2008). This suggests that coastlines play animportant role in the control of convective DV in
the tropics.
Most coastal rainfall patches (or coastal heavy
rainbands, CHeRs) are concentrated just o¤shore,and not on land (Xie et al. 2006). This cannot be
explained solely in terms of the wind-terrain interaction (Ogura and Yoshizaki 1988). Recently, Wu
et al. (2009) and Hara et al. (2009) also noted thatthe coastal heavy rainband along Sumatera Island
was not well simulated. Even in high-resolution
general circulation models (GCMs), the rainband
was reproduced over the coastal land but not in
the nearshore region. They suggested that problems
with GCMs that cannot simulate convective DV
well (e.g., Yang and Slingo 2001; Neal and Slingo2003) may have caused these errors.
Rainfall levels over the IMC are among the highest globally. Spatial and temporal variations inrainfall a¤ect the global circulation through latent
heat release (Ramage 1968). Because the IMC is
composed of a large number of islands and surrounding sea, both regional and temporal variations in rainfall may be quite complicated due tostrong interactions between land and adjacent sea
through their heterogeneous radiation properties
and local circulations. Convective activity over the
IMC has been studied using satellite, objective
analysis, and daily and/or monthly rainfall data interms of diurnal (Hamada et al. 2008; Qian 2008;
Sakurai et al. 2005; Yang and Slingo 2001), intraseasonal (Hamada et al. 2008; Nitta et al. 1992;
Ichikawa and Yasunari 2008), seasonal (Hamada
et al. 2002; Matsumoto 1992; Tanaka 1994; Murakami and Matsumoto 1994), and interannual variations (Hamada et al. 2002; Yasunari 1981). However, due to the limited frequency of operationalrawinsonde observations in Indonesia (Okamoto etal. 2003), sounding data and their related products
such as the objective analysis data over the IMC
cannot resolve the DV in atmospheric structures. It
is widely known that data from infrared (IR) sensors on geostationary satellites show several hoursof phase delay in the peak time of convective DV
in comparison with in situ surface rainfall observations because IR sensors detect cloud top temperature (Houze et al. 1981; Ohsawa et al. 2001; Kubota and Nitta 2001). Though TRMM PR canresolve the rainfall DV correctly with enough spatial and temporal resolutions over the globe (Hirose
and Nakamura 2005; Hirose et al. 2008), it cannot

63

capture the evolution and decay processes of convection embedded in the DV owing to its low sampling frequency over the equatorial tropics. Moreover, the hierarchical structure of mesoscaleconvective systems (MCSs) especially over coastal
regions and their interaction with synoptic disturbances, including the ISV and/or monsoon, havenot been closely analyzed in previous studies because of the coarse spatial and temporal resolutionsof the data in those studies; nevertheless, such studies are crucial to the discussion of the formation of
CHeRs over the IMC.
Recently, campaign observations of Coupled
Processes in the Equatorial Atmosphere (CPEA)
have been conducted over the mountainous regionof Sumatera Island (Fukao 2006). The structureand dynamics of convective cells have been studied
in detail using an X-band Doppler radar (XDR),
Equatorial Atmosphere Radar (EAR) and intensive
soundings (Kawashima et al. 2006; Mori et al.
2006; Sakurai et al. 2009a; Shibagaki et al. 2006).
However, convection over the southwestern coastal
region of the island, which is an essential region forour study, was not covered by the XDR because of
the complicated high topography around the XDR
and EAR.
Coastal regions in the tropics, especially in Asianmonsoon countries including the IMC, have highpopulations and signiﬁcant industrial activity. A
small di¤erence in the location of CHeRs overland or the sea can greatly a¤ect local communities(e.g., through natural disasters and/or changes inagriculture/water resource management). In addition, this convection can sometimes generate hazardous windstorms in the foothills of mountain
ranges along the coast (Kawashima et al. 2011).
Therefore, it is important to increase our understanding of the physical processes related to the
DV in convective activity in these regions for scientiﬁc and practical reasons.
For this reason, we performed the ﬁrst campaign
observation of Hydrometeorological ARray for
ISV-Monsoon AUtomonitoring (HARIMAU; Yamanaka et al. 2008), called ‘‘HARIMAU2006’’, toinvestigate the characteristics of convective activity
along the southwestern coastline of Sumatera
Island and their interaction with the ISV, mainly
using two XDRs and intensive atmospheric soundings from 26 October to 27 November 2006. This
paper presents an overview of convective activity
during the HARIMAU2006, focusing on the di¤erences between coastal land/sea and inactive/active

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Fig. 1. (a) Averaged annual rainfall over Sumatera Island, Indonesia, observed with TRMM PR during
1998–2006. A small square corresponds to an area of panel (b).
(b) Locations of two X-band Doppler radars (XDRs; MIA and Tiku), rawinsonde stations (Tabing andSiberut), and Equatorial Atmosphere Radar (EAR) in west Sumatera Island with topography. The twocircles show maximum ranges (83 km) of observation from the two XDRs, and the rectangle depicted in
the range of MIA-XDR is an area of analysis in Section 3. Elevation counters are depicted every 500 m.

ISV phases based on observations with the XDR
and intensive soundings at Sumatera Island. In
addition, the DV in coastal convection and formation of CHeR are also examined in terms of diurnal
land-sea migration of coastal convection.
The HARIMAU2006 was performed in collaboration with the Mirai Indian Ocean cruise for the
Study of the MJO-convection Onset (MISMO)
project (Yoneyama et al. 2008). The MISMO deployed the R/V Mirai over the Indian Ocean(80.5 E, EQ) and observatories at Gan Island(73.2 E, 0.7 S) and other islands of Maldives to investigate zonal structure and evolution of the ﬁrst
MJO onset using Doppler radars and intensive
soundings at multiple locations. We aimed to obtain simultaneous observational data to examine
them at the eastern end of Indian Ocean thoughthe HARIMAU2006, however, it was out of scopein this study.

2. Observation and data
2.1 Framework
We conducted the HARIMAU20006 campaign
between 26 October and 27 November 2006 (33days) in the major rainy season over the west
Sumatera Island. Figure 1a shows a 10-year map(1998–2007) of average annual rainfall patternover Sumatera Island obtained from the TRMM
PR. Rainfall greater than 3,000 mm y1 was observed along the southwestern coast of the island.
The CHeR was located over the sea, not overland. The small rectangle inside the ﬁgure corresponds to Fig. 1b, which shows the allocation of
XDRs and sounding stations along the coast with
topographical elevation.
Two XDRs were installed at Minangkabau International Airport (MIA; 0.79 S, 100.30 E, 5 m
above mean sea level, AMSL) and Tiku (0.40 S,

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99.92 E, 3 m AMSL) to observe the behavior of
convection over both the coastal land and sea. In
addition, intensive soundings in 3–6 hour intervals
were conducted at Tabing (0.88 S, 100.35 E, 3 m
AMSL) and at Siberut (1.58 S, 99.20 E, 7 m
AMSL) on both sides of the Mentawai Strait to obtain the atmospheric structure over sub-diurnal
temporal scales. Surface meteorological data wereobtained at those observatories by automaticweather stations (AWSs). Moreover, rainwater was
sampled at high temporal resolution at both Tabing
and Siberut for stable isotope analysis (Fudeyasu
et al. 2011).
2.2 Doppler radars
We deployed two XDRs at the MIA and Tiku
sites, which operated continuously from 26 October
to 27 November 2006. The major speciﬁcations of
the XDRs are summarized in Table 1. The XDRs
obtained three-dimensional reﬂectivity and Doppler
velocities every 6 min through a series of conicalscans with antenna elevation from 0.5 to 50 at
both stations. The observation radii of both XDRs
were 83 km, as shown in Fig. 1b. In addition, the
XDR at MIA made surveillance observations at
one elevation angle (0.5 ) with a radius of 166 km
every 6 min. The reﬂectivity and radial Doppler velocity were interpolated over Cartesian coordinateswith grid intervals of 0.5 km (333  333 grids in a
166  166 km square) in the horizontal and 0.5 km
in the vertical (40 levels from 0.5 to 20 km AMSL)
plane.
Reﬂectivity ﬁelds were partitioned into convective and stratiform regions by using a techniqueproposed by Steiner et al. (1995) because the two
rainfall types have crucial di¤erences in their latent
heat proﬁles (Houze 1982) and a¤ect the surrounding atmosphere di¤erently. We calculated thehourly rainfall rates from the reﬂectivity ﬁeld using
Z-R relations for each rainfall type, where Z and R
are radar reﬂectivity and rainfall rate, respectively.
In this study, we employed the Z-R relations asﬁrst-guess values for rainfall rate estimation in the
TRMM 2A25 version 5 algorithm, i.e., Z ¼ 148
R 1:55 for convective rainfall, Z ¼ 276 R 1:49 forstratiform rainfall (Schumacher and Houze 2003),
to facilitate comparison of the rainfall characteristics among various regions. Then, the rainfall rates
were averaged over the maximum volume-scan coverage area.
We used the radar echo coverage (%) observedby the XDR only at MIA in the following analyses.

65

Table 1. Major speciﬁcations of MIA and Tiku XDRs
and their operating condition during the HARIMAU2006 campaign.
Parameter

Value

Radar site

MIA

Tiku







0.40 S, 99.92 E
3 m AMSL

Location

0.79 S, 100.30 E
5 m AMSL

Manufacture

Japan Radio Company Ltd. (JRC)

Frequency

9770 MHz

9445 MHz

Peak power

70 kW

40 kW

Pulse width

0.5 msPRF

1800 Hz
Dual PRF [600/900 Hz] for PPI
surveillance
mode—

Beam width

0.98 deg

1.1 deg

Signal processor

RVP8

RVP7

Sigmet Product Line
Maximum range

83 km
160 km for PPI
surveillancemode—

Sampling space

200 m

Antennarotation speed

30 deg sec1

Elevation angles

0.5 to 50.0 deg

Nyquist velocity
Time interval

18 elevations

19 elevations

13.8 m s1

16 m s1

Every 6 min

Because the XDR at Tiku frequently experienced
stoppages due to electrical power supply problems,
it could not obtain enough data during the whole
period; however it was utilized for dual Doppler radar analyses and other case studies (Kawashima
et al. 2011). Radar echo coverage was deﬁned as
the area of radar echo (km 2 ) with a radar reﬂectivity of more than 10 dBZ at a speciﬁc height
divided by an area of radar detection (km 2 ) at thesame height. Because of the complicated topography (Fig. 1b), the radar detection area over the

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Fig. 2. Hovmöller diagram of OLR with NCEP wind vectors at 850 hPa averaged over the latitude of 5 S–
5 N from 02 October to 31 December 2006. The vertical solid line near 100 E shows the location of the
MIA-XDR site. The date between two horizontal thick lines (26 October–27 November 2006) indicatesthe HARIMAU2006 campaign period.

land varied greatly, particularly below 2 km AMSL
(ﬁgure not shown).
2.3 Intensive soundings
Rawinsondes (Vaisala RS-92SGP) were launchedat Tabing over the whole observation period of
26 October to 27 November 2006 and at Siberut
during 3–15 November 2006. We took soundings
four times a day (00, 06, 12, and 18 UTC) at bothsites and took additional soundings at four othertimes (03, 09, 15, and 21 UTC) during 4–14, 25,
and 26 November 2006 at Tabing, and 9–13 November 2006 at Siberut. All soundings provided
vertical proﬁles of pressure, temperature, relative

humidity, and horizontal wind every 2 seconds,
which corresponded to a height resolution of approximately 10 m. Sounding data from Tabing
averaged vertically every 100 m were used in thefollowing analyses.
2.4 Surface observations
AWSs (Vaisala MAWS201) were deployed at
MIA, Tiku, Tabing, and Siberut. They provided
surface pressure, temperature, relative humidity, solar radiation, wind direction and speed, and rainfall. The minimum sensitivity for the rainfall
amount and the temporal resolution of the recorded data were 0.5 mm and 10 min at TabingFebruary 2011

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67(Ogasawara RS-102 rain gauge), and 0.2 mm and1 min at the other stations (Vaisala QMR101 rain
gauges). AWS data observed only at MIA wereused in the following analyses.
2.5 Other data
TRMM PR 10-year (1998–2007) precipitation
data based on the monthly 3A25 dataset provided
by the Japan Aerospace Exploration Agency(JAXA) were used to examine the climatology of
the CHeR along the southwestern coastline of Sumatera Island. Both outgoing longwave radiation(OLR) and objective reanalysis data provided by
the National Oceanic and Atmospheric Administration (NOAA) and National Centers for Environmental Prediction (NCEP), respectively, were used
to analyze large-scale convective activity and related circulation during this period. The spatial
and temporal resolutions of both datasets were
2:5  2:5 degrees and 1 day, respectively. An allseason real-time multivariate MJO index (RMM)
provided by the Bureau of the Meteorology Research Centre (BMRC) was also used to conﬁrm
the phases of the MJO activity during this period.
In addition, the GTOPO30 global digital elevation
model (DEM) dataset provided by the U.S. Geological Survey (USGS) with a spatial resolution of
30-min arcseconds was used to conﬁrm the area ofradar detection and depict the topography aroundthe study region.
3. Results
3.1 Large-scale conditions during the study period
The HARIMAU2006 was originally designed toobserve coastal convective activity during the ﬁrst
MJO onset in collaboration with the MISMO.
Hence, it was set in the boreal autumn season (onemonth from late October to late November). A
Hovmöller diagram of OLR and wind vectors at
850 hPa from October to December 2006 is shownin Fig. 2 to conﬁrm synoptic convective activity
and circulations from the western Indian Ocean to
Sumatera Island (5 S–5 N). Low OLR regions correspond to large-scale disturbances that appeared in
the middle of the Indian Ocean (70 –80 E) around
10 November and approached Sumatera Island(100 E) after 15 November. More active convective
regions with much smaller OLR arrived at the island after the middle of December. However, theOLR was generally high, and large-scale convection was inactive around the island before the middle of November. Wind in the lower troposphere

Fig. 3. All-season real-time multivariate
MJO index (RMM) during 01 October to
31 December 2006. The index during the
HARIMAU2006 campaign period was
divided in 5 terms (each 7 days, except thelast term) denoted by each mark.

during the campaign period was easterly in general,
but it was weaker after the large-scale disturbanceapproached the island.
The large-scale disturbance after the middle ofNovember shown in Fig. 2 was diagnosed as a
MJO based on the RMM chart (Fig. 3). The MJO
signal started at the maritime continent sector(RMM 5) from 01 October and then propagatedeastward. Although it became stagnant once justbefore the Indian Ocean by 26 October and 17 November, it started to propagate eastward again overthe Indian Ocean sector (RMM 2–3) smoothly. Because it approached the western tip of the maritime
continent sector (RMM 4) at the end of 23–28 November, we assume that Sumatera Island was
located at the leading edge of the MJO active convective region in the last quarter of the campaign
period. Indeed, because the average speciﬁc humidity at 850 hPa averaged from October to November
2006 (Fig. 4) was concentrated over the eastern Indian Ocean and the west of Sumatera Island, the atmosphere in the lower troposphere was wet enough

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Fig. 4. Horizontal ﬁelds of speciﬁc humidity and wind vectors at 850 hPa obtained by NCEP reanalysis overthe eastern Indian Ocean and western part of Indonesian maritime continent averaged for the HARIMAU2006 campaign period (26 October–27 November 2006). White crosses indicate sounding stations at
Tabing (0.88 S, 100.35 E) in Sumatera Island, R/V Mirai (EQ, 80.5 E), and Gan Island in the Maldives(0.7 S, 73.2 E).

to generate convection, even though it was mainlyin the MJO inactive phase.
Yoneyama et al. (2008) examined the same largescale disturbances during the same period and identiﬁed that this disturbance, which passed over
Sumatera Island during the HARIMAU2006, wasnot exactly caused by the MJO by their analysis
based on Wheeler and Weickmann (2001), although it showed intraseasonal variability. Therefore, in the following analyses, we refer to thelarge-scale disturbance shown during the campaign
period as the ISV.
3.2 Local weather conditions
We examined the day-to-day variation in sounding and area-averaged rainfall data to identify the
local weather conditions during the HARIMAU2006. Figure 5 shows time-height cross sections of (a) equivalent potential temperature, (b)speciﬁc humidity, (c) zonal and (d) meridional
wind speeds observed by intensive sounding from26 October to 27 November 2006 at Tabing (Fig.
1). Potential equivalent temperature and speciﬁc
humidity are shown as anomalies from the periodmean at each height.
Easterly wind appeared in the troposphere during the whole period, as can be seen from the synoptic conditions (Figs. 2 and 4), although the weak
westerly wind was generally the dominant climatol-ogy below 5 km in this period (Okamoto et al.
2003). The easterly wind speed below 10 km decreased, and westerly wind appeared in the beginning (26–31 October), middle (10–15 November),and end (23–27 November) of the period. An apparent westerly wind was observed below 1 km almost every day, which corresponded to the sea
breeze circulation across the northwest-southeast
oriented coastline (Fig. 1b). The meridional wind
showed clear 4–5 day variation below 10 km, withupward propagation that may have been related tothe mixed Rossby-gravity wave because disturbances with similar periods propagated westward
with small zonal wind components (Kawashima etal. 2010). Similar to the zonal wind variation, there
was southerly wind ﬂow almost continuously below
1 km, which had a component of sea breeze circulation, but did not show clear diurnal alternation.
Both the equivalent potential temperature andspeciﬁc humidity clearly showed diurnal variations
from the near surface to the middle or even approaching the tropopause. Both quantities werelarge when the zonal wind was weak easterly or
westerly in the beginning, middle, and last parts ofthe period mentioned above. They were largest in
the lower troposphere during the last 6 days (22–27 November 2006), when the large-scale disturbance approached Sumatera Island as starting the
ISV active phase (Fig. 2).

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Fig. 5. Time-height cross-sectional views of equivalent potential temperature (a), speciﬁc humidity (b), andzonal (c) and meridional (d) wind velocity obtained by intensive soundings at Tabing from 26 October to
27 November 2006. Equivalent potential temperature and speciﬁc humidity are shown as anomalies fromthe period mean at each height.

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Journal of the Meteorological Society of Japan

Figure 6 shows time series of (a) surface pressure,(b) solar radiation, (c) speciﬁc humidity, (d) convective available potential energy (CAPE) and convective inhibition (CIN), and (e) area-averaged daily
rainfall at 2 km AMSL during the campaign period. Data in (a), (b), and (c) were observed with
the AWS installed at the MIA-XDR site. The CAPE
and CIN were obtained from each sounding at
Tabing. The area-averaged daily rainfall was calculated from MIA-XDR observations of every 6 min,and summed to reach the daily rainfall amount.
The area-averaged rainfall (e) could be divided
into three periods: 7–14 November (8 days), 22–27
November (6 days), and others, selecting series ofrainy days which had daily rainfall amounts greateror less than the value of period averaged daily rainfall (2.9 mm day1 ). We hereafter refer to the former two periods as precipitation period (PP) 1 and 2.
As shown in Fig. 6, the daily average surfacepressure (a) exhibited a decreasing trend during theentire period which corresponded to the approachof the large-scale disturbance; however it increased
when the PP1 and PP2 events occurred. Speciﬁc humidity (c) also exhibited long-term variation, whichshowed a general increase before the PP1 and PP2events started and then decreased gradually during
the PPs. On the other hand, solar radiation reacheda higher value during the PP1 and PP2 periods thanon other non-PP days, even though its daily average did not show any speciﬁc variability.
The convective rain fraction (e) was almost double to the stratiform rain fraction in PP1. However,convective and stratiform rain fractions were almost the same in PP2. Although daily rainfall was
slightly less than the period averaged rainfall
amount, three days (28–30 October) at the beginning of the period could also be identiﬁed as a PP
with considerable rainfall. Hence, all PPs in the beginning, middle, and end of entire period occurred
when the zonal wind was weak easterly or westerly.
In addition, the daily average CAPE (d) was at a
maximum just before PP1 and decreased during
PP1. It again increased slightly after PP1, but decreased signiﬁcantly when PP2 started. Although
the CAPE showed variation with large values between the PPs, the CIN did not vary much during
the whole period.
3.3 Coastal convective activitiesa) Vertical proﬁles of echo area
Figures 7 and 8 show time-height cross sectionsof radar echo coverage obtained by the MIA-XDR

Vol. 89A

Fig. 6. Sequential variations in (a) surfacepressure, (b) solar radiation, (c) speciﬁc humidity, (d) convective available potential
energy (CAPE) and convective inhibition(CIN), and (e) area-averaged daily rainfall.
The CAPE and CIN were calculated byeach sounding at Tabing, and the daily
area-averaged rainfall amounts at 2 km
AMSL were observed with MIA-XDR.
Other data were observed with the AWS
at the MIA-XDR site during the HARIMAU2006 campaign period. A black line
in panel (d) indicates daily average valueof CAPE. A horizontal line in panel (e) isthe period mean of area-averaged rainfallof 2.97 mm day1 . The thin solid and thickbroken lines in panel (e) are convective andstratiform rain fractions, respectively.

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71

Fig. 7. Time-height cross sections of radar echo coverage observed with the MIA-XDR during the HARIMAU2006 campaign period for convective rain type over land (a) and the sea (b) regions. The right-handpanels show the average proﬁles for PP1, PP2, and the whole period.

for each surface and rain type, along with the average proﬁles for PP1, PP2, and all periods on theright-hand side. The echo area of the convective
rain was larger in both PP1 and PP2 over the sea(Fig. 7b) than over land (Fig. 7a), especially below
6 km AMSL. In addition, proﬁles of convective
rain over land decreased signiﬁcantly with heightfrom the near surface to 10 km, but proﬁles overthe sea exhibited nearly twice as much coverage as
those over land from the near surface to 6 km
AMSL. There were peaks of echo coverage around
2 km AMSL over the sea. Though values over land
and the sea were larger in PP1 than in PP2, the difference between them was quite small over the entire height range.
On the other hand, areal coverage of stratiform
rain echoes over the sea region (Fig. 8b) was muchlarger in PP2 than that in PP1. The di¤erence became larger as the height decreased. Values overland (Fig. 8a) did not show much di¤erence between PP1 and PP2, although they were a little
larger in PP2 throughout the heights. Proﬁles of
echo area peaked at higher altitudes (around 3 km

AMSL) over the sea than over land and decreased
signiﬁcantly near the surface.
These results indicate that the di¤erent characteristics of convective activities between PP1 and PP2,
as illustrated in Fig. 6e were due to the changes instratiform rather than convective rainfall, especiallyover the sea. In addition, the echo coverage of
convective rainfall over the sea maintained larger
area than that over land throughout both PP1 andPP2.
b)

Diurnal variations
Figure 9 presents the diurnal variations in echo
coverage at 2 km AMSL observed by the MIAXDR for (a) convective rain over land, (b) stratiform rain over land, (c) convective rain over sea,and (d) stratiform rain over sea. The top and sidepanels in Fig. 9 (a)–(d) show variation in daily average and average diurnal variation, respectively,
for PP1, PP2, and the entire period.
The echo coverage of convective rain over land(a) was at a maximum at 15 LT. It did not shift inlocal time much during the whole period. The max-

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Fig. 8.

Vol. 89A

Same as Fig. 7 but for stratiform rain type.

imum diurnal variation was larger in PP1 (4.3%)
than in PP2 (3.7%) (side panel), although the daily
average echo coverage did not di¤er by more thanabout 1.0–1.5% between PP1 and PP2 (top panel).
However, the values over the sea (c) were completely di¤erent. The mean diurnal variation overthe study period (side panel) had a gentle peak between late evening (21 LT) and early morning (04LT) and a minimum at 14 LT. However, deﬁnite
double-maximum peaks were seen at 20–21 LT in
PP2 (4.8%) and 04 LT in PP1 (4.0%). The daily average echo coverage was larger in PP1 than in PP2,
with a maximum of around 2.7% compared with
2.1% (top panel).
The echo coverage of the stratiform rain typeover land (b) show a nocturnal ﬂat maximum between 15 and 01 LT (approximately 20%) following
the convective rain peak (a) and a minimum at 09
LT as its period mean (side panel). The nocturnal
peak in PP2 (23–01 LT) was sharp, while the daily
average echo coverage (top panel) did not di¤ergreatly between PP1 and PP2 (around 10–20%).
Values over the sea (d) showed similar DVs, withnocturnal maxima at 21–22 LT both in PP1 andPP2 and secondary maxima in early morning (01–05 LT). However, the maximum peak in PP2 waslarger (50%) than that in PP1 (24%). As a result,the daily average echo coverage (top panel) wasmuch larger in PP2 (more than 20%) than in PP1(less than 20%).
c)

Diurnal migration of coastal convection
Figure 10 shows a Hovmöller diagram of radar
reﬂectivity in the rectangle depicted in Fig. 1b, as
observed by the MIA-XDR throughout the HARIMAU2006. The reﬂectivity at 2 km AMSL wasaveraged over the short axis of the rectangle. A vertical solid line at 0 km in each panel, which marks
the location of the MIA-XDR, roughly corresponds to the coastline of Sumatera Island. The
rectangle was 160 km long, 20 km wide, and nearly
perpendicular to the coastline. Convection was generated approximately 20–40 km inland from thecoastline almost every day. Some convection cells
migrated toward the o¤shore side (e.g., 29 October,07 and 27 November). However, some other cells(e.g., those on 9, 11, and 15 November) developedo¤shore about 40–80 km from the coastline, independent of those over land. The frequency ofthe convective migration increased, especially in theFebruary 2011
S. MORI et al.

Fig. 9. Diurnal and periodic variations in echo coverage at 2 km AMSL observed with the MIA-XDR for (a) convective rain over land, (b) stratiform
rain over land, (c) convective rain over the sea, and (d) stratiform rain over the sea. Top and side panels in (a)–(d) are the daily averaged variation
and averaged diurnal variation for PP1, PP2, and the whole period, respectively.

73

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Journal of the Meteorological Society of Japan

Vol. 89A

Fig. 10. Hovmöller diagram of radar reﬂectivity at 2 km AMSL in the rectangle depicted in Fig. 1b, observed with the MIA-XDR throughout the HARIMAU2006 campaign period. The reﬂectivity was averaged along the short axis of the rectangle. A vertical solid line at 0 km in each panel, which is the locationof the MIA-XDR, roughly corresponds to the southwestern coastline of Sumatera Island.

last quarter of the campaign period, when the largescale disturbance approached Sumatera Island.
Average diurnal variations in (a) radar reﬂectivity at 2 km AMSL, (b) rainfall intensity, (c) convective rain fraction, and (d) daily accumulated rainfall and topography along the rectangle are shownin Fig. 11. Convection was generated in the southwestern foothills of the mountain range in the early
afternoon (12–15 LT). It developed until 18 LT ata similar position (Fig. 11a). Part of the convective
system remained over the coastal land region andmaintained weak reﬂectivity until the next morning.
The other part migrated o¤shore at a speed ofapproximately 4 m s1 (thick broken lines), before
intensifying signiﬁcantly around 21 LT about 40–

80 km from the coastline. The redeveloped convection maintained its position and strength throughout the night. Other convective cells developed
40–80 km o¤shore between 06 and 09 LT in themorning, independent of those over land. Consequently, the rainfall intensity (Fig. 11b) was highonly in the afternoon to evening over the coastal
land region. However, rainfall intensity over thecoastal sea, on the other hand, showed two peaks:in the late evening and the following morning. As
a result, there were double peaks of the daily rainfall maximum both over the costal sea and land regions of approximately 3.5 mm, but the peak overthe sea (land) was broad (narrow) and produced
larger (smaller) amounts of rainfall (Fig. 11d). Al-

February 2011

S. MORI et al.

75

Fig. 11. Average diurnal variations in (a) radar reﬂectivity at 2 km AMSL, (b) rainfall intensity, (c) convective rain fraction, and (d) accumulated daily rainfall amount and topography along the rectangle in Fig. 1bduring the HARIMAU2006 campaign period. Thick broken lines in (a), (b), and (c) indicate migration
speeds of approximately 4 m s1 . A vertical solid line at 0 km in each panel, which is the location of the
MIA-XDR, roughly corresponds to the southwestern coastline of Sumatera Island, as in Fig. 10.

though more than 50% of the rainfall came fromconvective clouds in only 6 hours (12–18 LT) overthe coastal land region followed by rainfall with asmaller fraction rate (<50%), rainfall over the costal sea region consisted of a convective fraction(>50%) that was maintained more than 12 hours(21–09 LT) through the night to the next morning(Fig. 11c).
Figure 12 shows averaged vertical proﬁles ofwind speed along the rectangle (i.e., perpendicular
to the coastline; positive value for the southwesterlywind) every 3 hours as observed by soundings at
Tabing and a hodograph at 100 m AMSL. Thesewind proﬁles can be partitioned into three heightregions: 1) northeasterly trade winds above
1.5 km, 2) southwesterly monsoonal wind at 0.5–
1.5 km, and 3) land-sea breeze circulation below

0.5 km. The land-sea breeze circulation was dominant around 100 m AMSL, as shown in the hodograph. The sea breeze in the daytime helped to generate new convection over the coastal land region
by local convergence at the foothills of the mountain range. The land breeze during the night createdfavorable conditions for new convection over thecoastal sea region by converging with the synoptic
southwesterly wind, though it was weak and shallow below 1 km AMSL (also see Fig. 5d and 5e).
Although these convergences were not strong, atmospheric instability during the period was enoughfor convective genesis (Fig. 6). Furthermore, large
vertical wind shears (roughly 5 m s1 km1 at 1–
2 km) were fundamental factors for maintaining
the long-lasting nocturnal convection over thecoastal sea (Ogura and Yoshizaki 1988). Because

Journal of the Meteorological Society of Japan76

Vol. 89A

that PP1 (PP2) was in an ISV inactive (active)
phase. Major di¤erences in convective activity between PP1 and PP2, and between land and sea, aresummarized below:

Fig. 12. Average vertical proﬁles of windspeed along the rectangle in Fig. 1b (i.e.,
perpendicular to the coastline; positive
value for southwesterly wind) observed atTabing every 3 or 6 hours during the
HARIMAU2006 campaign period, and ahodograph at 100 m AMSL.

the wind speed of convective migration in the evening corresponded to that of northeasterly tradewind above 2 km, convection seemed to be advected by the synoptic ambient wind on average.
4. Discussion
4.1

Di¤erent characteristics of convection in PP1and PP2, and over land and sea
In this subsection, we examine the di¤erent characteristics of convection observed in our study between PP1 and PP2, and over the coastal land andsea regions. We then compare the results with thosefor convection over the inland region of Sumatera
Island and the coastal sea region of Borneo (Kalimantan) Island of the IMC.
Because the large-scale disturbance was identiﬁed
as an ISV that passed over the MIA-XDR site after
17 November as shown in Fig. 2, we can presume1) The convective rain fraction in PP1 was nearly
twice the stratiform rain fraction. On the other
hand, the rain fractions in PP2 were almost
equal, as shown in Fig. 6e.
2) The vertical proﬁles of echo coverage for convective rain were similar in PP1 and PP2 (although slightly larger in PP1), both over landand sea (Fig. 7), whereas the vertical proﬁle forstratiform rain in PP2 was larger than that in
PP1, especially in the lower troposphere overthe sea (Fig. 8).
3) Although the echo coverage of convective rainover land decreased rapidly with height fromthe near surface to 10 km AMSL, that over thesea maintained almost twice the echo coverageof that over land from the near surface to 6 km
AMSL both in PP1 and PP2 (Fig. 7).
4) The nocturnal rainfall peaks were larger in PP2than those in PP1 in all cases except for convective rain over land. Rainfall peaks in PP1 werelarger only for convective rain in the afternoon
over land and early morning over the sea (Fig.
9).
As shown in Fig. 6, the CAPE during the campaign period was largest just before PP1 and thendecreased until PP2 started. It seems that the convection associated with the large convective rain
fraction in PP1 required high instability to be generated, which it then consumed by creating muchrainfall. The southwesterly wind in the lower troposphere observed during PP1 (Fig. 5c and 5d) also
formed favorable conditions for the generation of
local convection along the coastline, by convergence with the northeasterly land breeze at night(Fig. 12). However, convection composed equallyof both convective and stratiform rain in PP2 did
not require such local instability because the largescale disturbance of ISV supplied heat energy andwater vapor (Fig. 5a and 5b) from the Indian
Ocean, although easterly wind was dominant in
the middle troposphere over Sumatera Island during this period.
These di¤erences in convection between in PP2(ISV active) and PP1 (ISV inactive) are consistentin general with the results of Kawashima et al.
(2006) who studied convective activity during the
MJO active and inactive phases using XDR dataFebruary 2011

S. MORI et al.

from west Sumatera Island. However, their data
covered only the inland region and did not include
the major rainy season. Our results show that thegreatest di¤erences between PP1 and PP2 camefrom convective activity, especially its stratiform
fraction, over the sea rather than over the land, as
shown in Figs. 7, 8, and 9. In addition, the convective rain over the coastal sea region ﬁrst peaked ataround 21 LT (Fig. 9c), almost at the same time as
the stratiform peak (Fig. 9d). Over land, the peakin echo coverage of convective rain at around 15
LT (Fig. 9a) was followed by a gentle increase instratiform rain during the late evening to nighttime(Fig. 9b), as described by Kawashima et al. (2006).
The characteristics of diurnal variation over the sea(Fig. 9c and 9d) were also di¤erent from those for atypical lifecycle of MCS evolution in the tropicalopen oceans (Houze 1982). It is possible that thenocturnal maximum of stratiform fraction overthe coastal sea region comprised a combination ofo¤shore-generated convection and convection advected from over land, which reached its secondary
maximum in the evening. However, we need morecase studies to examine this process in greater detail.
Houze et al. (1981) analyzed convective activity
over the northwestern coastal sea region of Borneo(Kalimantan) Island based on observations with aC-band radar during the boreal winter monsoon
season. Their results showed clear peaks of total
radar echo coverage at 23 LT and 08 LT overcoastal land and sea regions, respectively. This result di¤ered from our ﬁndings, which showed adouble peak time of radar echo coverage for bothconvective and stratiform rain over the coastal sea
region especially in PP1 and PP2. The peak ataround 21 LT, in particular, was not seen in Houzeet al.’s (1981) study. The diurnal variation over thecoastal sea region in our study with a double peaktime may have been caused by diurnal land-sea migration of convection over the southwestern coastline of Sumatera Island (Mori et al. 2004; Sakurai
et al. 2005). This is discussed more in the next subsection. Ichikawa and Yasunari (2006) found that
although there was similar migration of convection
over the northwestern side of Borneo (Kalimantan),
the rainfall over the coastal sea region that, Houzeet al. (1981) studied, peaked in the early morning
because it was not disturbed by diurnal convective
migration from the coastal land because of the Borneo vortex. Therefore, it is a unique mechanism tomaintain the DV in convection over the northwest-77

ern coastal sea of Borneo (Kalimantan) Island andcannot explain the convective characteristics overthe southwestern coastline of Sumatera Island.
4.2 Coastal heavy rainbands
Previous studies revealed the CHeR along thesouthwestern coastline of Sumatera Island after theera of the TRMM satellite (Mori et al. 2004; Sakurai et al. 2005; Sakurai et al. 2009; Wu et al.
2009). Xie et al. (2006) found similar CHeRs arecommonly observed over the Asian monsoon region, such as the Indian coast west of the Western
Ghats, the western coast of Myanmar over the Bayof Bengal and the Andaman Sea, northeastern
coast of the Gulf of Thailand, the eastern coast ofVietnam, and the northwestern Philippines. On thebasis of their analysis of the boreal summer monsoon season (June, July, August), they suggested
that most of these CHeRs could be generated by interaction between the southwesterly monsoon ﬂow
and orographic convergence along the coastlines(wind-terrain interaction). On the other hand, the
CheR along Sumatera Island was seen throughout
the year, even though easterly wind was dominant
in the lower troposphere. Indeed, the o¤shore peakof daily rainfall was clearly observed in our study(Fig. 11d) as well as a comparable peak in the foothills of the mountain range. Rainfall intensity during this period (Fig. 6e) was not enough to produce
the climatological CHeR (more than 3,000 mm y1 )shown in Fig. 1a because our observational period
was mainly in the convectively ISV inactive phase.
The CHeR along the Indian coast near the Western Ghats, which has a rainfall peak over thecoastal ocean rather than in the foothills, was studied by Ogura and Yoshizaki (1988) by using an
idealized numerical simulation. They found thattwo factors were essential to generate the CHeR:
1) heat and moisture ﬂuxes supplied from thecoastal ocean, and 2) vertical wind shear between
the southwesterly monsoon ﬂow in the lower levels
and the easterly wind layer above. The environmental wind proﬁles in our case (Fig. 12) satisﬁed atleast the second requirement. However, the southwesterly wind ﬂow was not stronger during the
campaign period than its climatology (Okamoto
et al. 2003) because it occurred in the convectively
inactive ISV phase.
Although Ogura and Yoshizaki (1988) did not
examine the diurnal variation in coastal convection,
our study (Fig. 11) clearly showed that it must also
be essential to generate the CHeR along Sumatera

78

Journal of the Meteorological Society of Japan

Island. As shown in Fig. 11, convection was generated in the foothills of the mountain range in theearly afternoon. One part of the convective system
was maintained there with weak reﬂectivity and asmaller convective rain fraction throughout thenight. Another part of the convective system migrated into the o¤shore region at a speed of approximately 4 m s1 and then redeveloped ataround 21 LT about 40–80 km o¤ the coastline.
Furthermore, more o¤shore convection developed
independently at around 06–09 LT about 20–
80 km o¤ the coastline, corresponding to the morning rainfall peak also observed in the northwestern
coastal sea region of Borneo (Kalimantan) Island(Houze et al. 1981). Sakurai et al. (2009b) examined
the interaction between convection from the coastal
land and that generated over coastal sea, and thenfound that this interaction enhanced the coastal
rainfall though they examined only one day case.
All these results suggest that the CHeR along theSumatera Island is formed by diurnally developed
coastal convection composed of two major parts:
1) migration of convection from the coastal land
and its redevelopment in the late evening, and 2)
independent generation of convection over thecoastal sea region in the early morning. In addition,
we speculate that speciﬁc interaction mechanisms,
such as the seeder-feeder process between 1) and 2)(Mori et al. 2004), may enhance the CHeR. Moreover, because the Mentawai Strait is shallow andmostly separated from the Indian Ocean by anarchipelago along Sumatera Island as shown in
Fig. 1b, a high sea surface temperature (SST) mustbe formed easily in the daytime if the winds are notstrong. Hence, it is possible that a large amount ofheat and moisture ﬂuxes from the sea surface of the
Mentawai Strait enhance the CHeR as Ogura andYoshizaki (1988) concluded.
5. Concluding remarks
This paper presented an overview of convective
activity during the HARIMAU2006 campaign, focusing on the di¤erences between coastal land/sea
and inactive/active ISV phases based on observations from the XDR and intensive soundings at
Sumatera Island. In addition, the DV in coastal
convection and formation of CHeR were also examined in terms of the diurnal land-sea migrationof coastal convective systems.
Although the campaign period corresponded tothe major rainy season over Sumatera Island, theISV phase was inactive during the beginning andVol. 89A

middle of the observation period. However, thephase became active near the end of period. We deﬁned the two major precipitation periods of PP1
and PP2 as the ISV inactive and active phases, respectively. PP1 contained convective rain fractions
nearly twice that of the stratiform rain fractions,but PP2 was composed of both rain fractions almost equally.
The vertical proﬁles of echo coverage for convective rain were similar in PP1 and PP2 over both thecoastal land and the adjacent o¤shore region. The
vertical proﬁle for stratiform rain in PP2 hadgreater coverage than that in PP1, especially in thelower troposphere over the sea. Although the echo
coverage of convective rain over land decreased
rapidly with height from the near surface to 10 km
AMSL, the convective echo proﬁles over the sea exhibited nearly twice as much coverage as those overland from the near surface to 6 km AMSL, both in
PP1 and PP2. The nocturnal rainfall maxima werelarger in PP2 than in PP1 in all cases except for
convective rain over land. Rainfall maxima during
PP1 were larger only for convective rain in the afternoon over land and early morning over the adjacent sea.
Highly unstable environmental conditions were
responsible for the convection in PP1, which instability was then consumed by creating signiﬁcantamounts of rainfall. The southwesterly wind ﬂow
in the lower troposphere observed during PP1 also
led to favorable conditions for the generation of
local convection along the coastline due to convergence with the northeasterly land breeze at night.
However, that in PP2 was formed due to the largescale disturbance of ISV, which supplied heat energy and water vapor from the Indian Ocean.
Convection was generated in the southwestern
foothills of the mountain range in the early afternoon (12–15 LT). It continued growing until 18
LT at a similar position. Part of the convective system remained over the coastal land region andmaintained weak reﬂectivity until the next morning.
The other part migrated o¤shore at a speed of approximately 4 m s1 , before intensifying signiﬁcantly around 21 LT in the o¤shore region. However, other convective cells developed o¤shore in
the early morning independent of those over theland. As a result, double maxima of the daily rainfall were observed both over the costal sea andland, but the maximum over the sea (land) was
broad (narrow) and produced a larger (smaller)
amount of rainfall.

February 2011

S. MORI et al.

All these results suggest that the CHeR along
Sumatera Island is formed by diurnally developed
coastal convection composed of two major parts:
1) migration of convection from the coastal land
and its redevelopment in the late evening, and 2)
independent generation of convection over thecoastal sea region in the early morning. In addition,
we speculate that speciﬁc interaction mechanisms,
such as the seeder-feeder process between 1) and 2)
and/or the abundant heat and moisture ﬂuxes supplied from the Mentawai Strait between the islandsof Sumatera and Siberut may enhance the CHeR.
However, open questions remain. It is importantto proceed to highly detailed studies of individual
cases to determine how the CHeRs are formed as anext step in employing dual XDR analysis, numerical model simulation, coastal ocean ﬂux monitoring, and newly introduced dual-polarimetric radarobservations.
Acknowledgements
The HARIMAU project, led by Japan Earth
Observing System (EOS) Promotion Program(JEPP) of the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan,
has been operated based on collaboration between
the BPPT (Agency for the Assessment and Application of Technology), BMKG (Meteorology, Climatology, and Geophysical Agency), and LAPAN(National Institute of Aeronautics and Space) on
the Indonesian side, and JAMSTEC, Hokkaido
University, and Kyoto University on the Japaneseside. The authors express their thanks to Professor
Yasushi Fujiyoshi and Mr. Masayuki Ohi of Hokkaido University for coordinating and technicallysupporting the X-band Doppler radar observations
at Tiku and MIA. The authors express their thanks
to Drs. Masaki Katsumata and Hisayuki Kubota
of JAMSTEC, and Taro Shinoda of Nagoya University for their helpful discussions and positive
comments. Thanks are also extended to two anonymous reviewers, whose constructive comments
greatly improved the content of this manuscript.
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