Journal of the Meteorological Society of Japan, Vol. 89, No. 4, pp. 341–361, 2011

DOI:10.2151/jmsj.2011-404

Migration Process and 3D Wind Field of Precipitation Systems
Associated with a Diurnal Cycle in West Sumatera:
Dual Doppler Radar Analysis during the HARIMAU2006 Campaign
Namiko SAKURAI
National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan

Shuichi MORI
Research Institute for Global Change, JAMSTEC, Yokosuka, Japan

Masayuki KAWASHIMA, Yasushi FUJIYOSHI
Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

HAMADA Jun-Ichi
Research Institute for Global Change, JAMSTEC, Yokosuka, Japan

Shingo SHIMIZU
National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan

Hironori FUDEYASU
Faculty of Education and Human Sciences, Yokohama National University, Yokohama, Japan

Yoshikazu TABATA
Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan

Wendi HARJUPA
National Institute of Aeronautics and Space, Jakarta Pusat, Indonesia

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

Manabu D. YAMANAKASATREPS-MCCOE Promotion Ofﬁce, Jakarta Pusat, Indonesia
Research Institute for Global Change, JAMSTEC, Yokosuka, Japan
DEPS-CPS, Kobe University, Kobe, Japan

Jun MATSUMOTO
Department of Geography, Tokyo Metropolitan University, Hachioji, Japan
Research Institute for Global Change, JAMSTEC, Yokosuka, JapanEMRIZAL
Indonesian Meteorological and Geophysical Agency, Jakarta Pusat, Indonesia

Fadli SYAMSUDIN
Agency for the Assessment and Application of Technology, Jakarta Pusat, Indonesia(Manuscript received 18 May 2010, in ﬁnal form 31 May 2011)341

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Abstract
This study describes the three-dimensional structure and migration process of a westward-migrating precipitation system with a diurnal cycle observed on 10 November 2006 in west Sumatera as based on dual-Doppler radar analysis,rawinsonde data, and surface data. The location of convective cell generation over land in the daytime and the timingof an evening change in the migration direction of precipitation systems from landward to seaward were inﬂuenced by
local circulation: in the morning, an isolated convective cell generated near the west coast propagated inland (toward themountains) via the successive generation of new convective cells over the sea breeze. In the afternoon, convective cells ina precipitation system were generated over the western slopes of the mountains surrounding Lake Maninjau by thermallyinduced local circulation and the slope effect. In the evening, precipitation systems located over the mountains started
to propagate toward the sea (westward) in response to a change in local circulation from landward to seaward winds.
Subsequently, precipitation systems were newly generated over the sea near the west coast of Sumatera Island, merging
with the systems that originated over land. These two sets of systems formed a larger system (long axis > 100 km) thanthat over land (long axis of several tens of kilometers). The expanded precipitation system had a convective region witha long axis oriented parallel to the west coast of Sumatera Island and a short axis (∼7 km) oriented perpendicular to thewest coast at the leading edge of the system. The echo top height of the system was located at approximately 13 km
and an anvil moved faster than the convective region above 6 km in height. The convergence was formed by an easterly
wind component in the system and a southerly wind component over the sea around the leading edge of the system atthe lowest layer. The expanded system migrated farther offshore at a speed of about 5 m s−1 via the self-replication of
convective cells over the convergence at the leading edge of the system and via advection by ambient wind in the lower
troposphere. The convergence regions at the leading edge of the system were continuously strengthened by downward
transportation of horizontal momentum below 4 km, meaning that the system could be maintained for a long time andmigrate offshore for a long distance.

1. Introduction
The Indonesian maritime continent, which is locatedin the equatorial tropics, is characterized by the world’s
largest amounts of rainfall and latent heat release (Ramage 1968). Heavy rainfall occurs over both land andthe surrounding sea. A diurnal cycle of convective activity is one of the dominant phenomena in the tropics(Hendon and Woodberry 1993). There exists a distinct
difference in the timing of the maximum in convective
activity between land (evening) and sea (morning) areas
within the Indonesian maritime continent (Murakami
1983; Nitta and Sekine 1994; Yang and Slingo 2001).
The timing of the maximum in convective activity is delayed from the coastline to the sea off the coast (Yangand Slingo 2001).
The land-sea contrast in the timing of maximum convective activity over Sumatera Island is caused by the
migration of cloud systems (Mori et al. 2004; Sakurai et al. 2005): convective activity becomes intensein the western part of Sumatera Island during the daytime and cloud systems migrate westward and eastward
Corresponding author: Namiko Sakurai, National ResearchInstitute for Earth Science and Disaster Prevention (NIED), 31, Tennodai, Tsukuba, Ibaraki 305-0006, Japan.
E-mail: sakurain@bosai.go.jpc⃝2011,
Meteorological Society of Japan

from this area for several hundred kilometers during thenight, meaning that convection is active over the sea
around Sumatera Island during the morning. Sakurai
et al. (2005) reported that westward migration is more
frequent than eastward migration. In addition, several
studies have shown that annual rainfall over the sea
in the vicinity of west Sumatera is much greater thanthat over the island itself, and that most of the heavy
rainfall over the sea in this region occurs during nighttime (Hara et al. 2009; Mori et al. 2004; Wu et al.
2009). These studies speculated that the abundant rainfall over the sea near west Sumatera is related to thewestward migration of precipitation systems. In thesestudies, two-dimensional and wide-range data collectedby satellite systems such as Geostationary Meteorological Satellite (GMS) infrared-1 (IR1) and precipitation
radar (PR) data from the Tropical Rainfall Measuring
Mission (TRMM) were mainly used for analyses. However, these studies using satellite data did not clarify
the mechanism of the migration of precipitation systems or the distinct difference in rainfall amount between land and sea. Consequently, it is necessary to
perform ground based observations to capture the threedimensional structure of migrating precipitation systems with a diurnal cycle at high temporal and spatial
resolutions.
Several numerical studies using General Circulation

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Model (GCM), regional model, or Global Cloud Resolving Model (GCRM) have sought to simulate thediurnal cycle of convective activity throughout the Indonesian maritime continent (Arakawa and Kitoh 2005;
Fujita et al. 2011; Hara et al. 2009; Neal and Slingo
2003; Sato et al. 2009; Wu et al. 2009). Unfortunately,
the previous studies using GCMs did not adequately
represent the phase of the diurnal cycle or the spatial
distribution of precipitation: the simulated maximum
precipitation is several hours earlier than that observed
around the western coast of Sumatera Island and precipitation is much weaker than that observed over thesea near west Sumatera (Arakawa and Kitoh 2005; Haraet al. 2009; Neal and Slingo 2003). Neal and Slingo(2003) noted that the correct representation in a GCM
of a diurnal cycle derived from migrating precipitation
systems might lead to improvements in terms of the underestimated precipitation in the region. Recently, Wu
et al. (2009), who used a regional model with nonhydrostatic and cloud system-resolving, succeeded in replicating the westward migration of precipitation systems
with a diurnal cycle in the coastal region of west Sumatera. Using the regional model, the authors were able tosimulate a more realistic precipitation distribution andphase of diurnal cycle than those of GCMs. In addition,
Sato et al. (2009) and Fujita et al. (2011) reported thatthe spatial distribution of precipitation and the phaseof diurnal cycle over Sumatera Island and its surrounding sea were realistically simulated by GCRM. Thesenumerical studies suggested that a westward-migrating
precipitation system with a diurnal cycle is a key factor
in determining the spatial distribution of precipitation
around the western coast of Sumatera Island. However,few observational studies have considered the threedimensional structure and migration process of precipitation systems with a diurnal cycle in this region.
An X-band Doppler radar (XDR) was installed in
mountainous terrain on Sumatera Island as part of
the Coupling Process in the Equatorial Atmosphere(CPEA) project, which provided three-dimensional information on the internal structure of migrating precipitation systems with high temporal and spatial resolution (Fukao 2006; Kawashima et al. 2006; Sakurai et al.
2009). However, the XDR only covered the land area
in west Sumatera, meaning that the internal structuresof migratory precipitation systems over the sea remain
unknown.
The Hydrometeorological ARray for ISV-Monsoon
AUtomonitoring (HARIMAU) project, which operated
from 2005 to 2010 (Yamanaka et al. 2008), involved
an intensive observation campaign in west Sumatera
between 26 October and 27 November 2006 (HARI-

343
5˚

Malaysia

[m]
0˚

0˚

3500
Sumatera-5˚

Lake Maninjau

95˚

100˚

Kototabang

105˚

3000
2500

83 km

Tiku
83 km MIA

2000

Tabing

1500

-1˚

1000
Siberut
99˚:XDR and AWS

500
100˚
:Rawinsonde and AWS

101˚

0

:EAR and AWS

Fig. 1. Locations of two XDRs (MIA and Tiku),
rawinsonde stations (Tabing and Siberut), andEAR (Kototabang) in west Sumatera Islandwith topography. AWSs were deployed at
MIA, Tiku, Tabing, Siberut, and Kototabang.
Two circles indicate the observation ranges(83 km) of the two XDRs located at MIA and
Tiku. A box at the upper-right corner shows
topography of Sumatera Island.

MAU2006) using two XDRs that covered the sea area
off the west coast of Sumatera Island, rawinsondes, andEquatorial Atmosphere Radar (EAR) (Fig. 1), yielding
the three-dimensional wind ﬁeld in westward-migrating
precipitation systems with a diurnal cycle. The purposeof the present study is to describe the internal structure and migration process of a westward-migrating
precipitation system with a diurnal cycle and to reveal the mechanism of the westward migration using
HARIMAU2006 data. A westward-migrating precipitation system observed on 10 November 2006 is analyzed
based on HARIMAU2006 data.
The remainder of this manuscript is organized as follows. Section 2 outlines the observations and data, andSection 3 presents a case study on the internal structure and temporal evolution of precipitation systems observed on 10 November 2006. Section 4 provides discussions on internal structure and migration mechanismof a westward-migrating precipitation system over thesea. Finally, the main ﬁndings of the study are summarized in Section 5.

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Table 1. Major parameters of the X-band Doppler Radars (XDRs).
Items
Transmit frequency and polarization
Transmit power
Beam width
Pulse width
Pulse repetition frequency
Antenna rotation speed
Rain detection range
Nyquist Velocity

Description
MIA
Tiku
9.770 GHz, Horizontal
9.445 GHz, Vertical
70 kW
40 kW
0.98 deg
1.1 deg
0.5 µs
0.5 µs1,800 Hz
1,800 Hz30 deg sec−1
30 deg sec−1
83 km
83 km
16 m s−1
13.8 m s−1

2. Observations and data
Two XDRs were installed at Minangkabau International Airport (MIA) (100.30◦ E, 0.79◦ S) and Tiku
(99.92◦ E, 0.40◦ S) in west Sumatera, Indonesia, in October 2006 as part of the HARIMAU project (Fig. 1).
Figure 1 shows the topography and locations of observation stations in west Sumatera. In the observation area of the two XDRs, the coastline trends from
northwest to southeast parallel to steep mountains in
the eastern part of the observation area. Lake Maninjau (LM), a caldera lake, is located in the mountains
within the observation area covered by the two XDRs.
During HARIMAU2006, each XDR obtained volume
scan data on reﬂectivity and Doppler velocity every 6
minutes within an 83 km radius (Table 1). The sampling resolution of XDR data was 200 m in the radial
direction and 1.0◦ in the azimuthal direction. Each volume scan at MIA (Tiku) consisted of 18 (19) elevation
angles from 0.5◦ to 50◦ . The Doppler velocity and reﬂectivity were interpolated into a Cartesian grid systemwith horizontal and vertical spacing of 0.5 km using a
Cressman weighting function (Cressman 1959). DualDoppler analysis was performed using the variational
method proposed by Shimizu et al. (2008). Several precipitation events were observed by the two XDRs during
HARIMAU2006 (Mori et al. 2011). The present study
analyzes a westward-migrating precipitation system observed on 10 November 2006 based on dual-Doppler
data. Other precipitation events during HARIMAU2006
are analyzed with the two XDRs data by Fudeyasu et al.
(2011) and Kawashima et al. (2011).
Intense rawinsonde observations were carried out at
Tabing station of the Indonesian Meteorological andGeophysics Agency (BMG; now the Indonesian Meteorological Climatological and Geophysical Agency,
BMKG) (100.35◦ E, 0.88◦ S) for 33 days (from 26 Oc-

26OCT.
2006

01NOV.

06NOV.

11NOV.

16NOV.

21NOV.

26NOV.
80E

90E

100E

110E

120E

210

220

230

240

250

(K)

Fig. 2. Time-longitude cross-section of TBB on
the equator based on MTSAT IR1 data witha spatial resolution of 0.5◦ ×0.5◦ between 26
October and 27 November 2006. Solid line
indicates 100◦ E, which corresponds to the location of the central mountain range upon Sumatera Island, on the equator.

tober to 27 November 2006) and at Siberut (Siberut National Park Ofﬁce) (99.20◦ E, 1.58◦ S) for 11 days (4–14
November 2006) as part of HARIMAU2006 (Fig. 1).

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Fig. 3. Horizontal distributions of TBB over Sumatera Island on 10 November 2006. The square at 1400 LST
indicates the map area shown in Fig. 1.

Fig. 4. Temporal variations in the horizontal distribution of precipitation systems at a height of 1.0 km from 1012
to 1048 LST on 10 November 2006. Thick and thin circles indicate 30 and 15 dBZ, respectively. The thick line
indicates the coastline and contours show altitude at intervals of 500 m.

Rawinsonde data were obtained every 3 hours on 10
November 2006. We analyzed the rawinsonde data
edited every 100 m in the vertical at Tabing. Surface data were obtained every minute during HARIMAU2006 at MIA, Tiku, Tabing, Siberut, and Kototabang using automatic weather stations (AWS). The
present analysis employed data on wind speed and winddirection at the ground surface as observed at Tiku.

3. Structure and evolution of migratory cloud andprecipitation systems
3.1 Temporal variations in the horizontal distributionof cloud systems
Climatologically, the period of HARIMAU2006 corresponded to one of two rainy seasons in central Sumatera (March-May and September-November; Hamada

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Fig. 5. As for Fig. 4, but from 1200 to 1830 LST on 10 November 2006.

2003) that appear to be related to annual variations(i.e., a north-south shift about the equator) in the intertropical convergence zone (ITCZ) (Murakami andMatsumoto 1994; Okamoto et al. 2003). However,the period of HARIMAU2006 coincided with a pos-

itive phase of an Indian Ocean Dipole Zonal Mode
event and weak to moderate-strength El Nino, meaning that convection was suppressed in the warm pool
regions of the eastern Indian Ocean and western Paciﬁc (McPhaden 2008; Yoneyama et al. 2008). FigureAugust 2011

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Fig. 6. As for Fig. 4, but from 1830 to 2100 LST on 10 November 2006.

2 shows a time-longitude cross-section of black bodytemperature (TBB ) by Multi-functional Transport Satellite (MTSAT) IR1. Despite the fact that observations
were made during a period of suppressed convective activity, a diurnal cycle of convective activity occurred almost every day and the westward migration of cloudsystems was commonly observed in central Sumatera
during HARIMAU2006.
The typical diurnal cycle of westward migration ofcloud systems was observed on 10 November 2006.
Figure 3 shows the horizontal distribution of TBB . Convection became active (TBB ≤ 230 K) from 1300 LST(data not shown) and cloud systems grew to several hundred kilometers in horizontal extent over the west coastof Sumatera Island by 1600 LST. The cloud systems
then migrated westward; the core of the systems had
moved offshore by 2000 LST. At 0000 LST on 11
November, the core of the cloud systems was locatedover offshore islands, approximately 150 km from thewest coast of Sumatera Island.
3.2 Temporal variations in the horizontal distributionof precipitation systems
The migration of precipitation systems observed by
XDR was more complicated than that of cloud systems

observed by MTSAT IR1. Figures 4 and 5 show temporal variations in the horizontal distribution of precipitation systems at a height of 1.0 km from 1012 to 1830
LST on 10 November 2006. An isolated convective cell
was generated near the west coast of Sumatera Island
about 20 km from the south slope of Lake Maninjau(LM) at 1012 LST (Fig. 4a). New convective cells weresequentially initiated toward the south slope of LM, anda convective cell was generated over the south slope of
LM at 1036 LST (Figs. 4b, c). The convective cell gradually intensiﬁed to a precipitation system with a horizontal scale of approximately 20 km, thereafter generating new convective cells in a westward direction andmigrating to the southwest slope of LM by 1200 LST(Figs. 4d, 5a).
Between 1200 and 1800 LST, the precipitation systems changed their migration direction and shape several times over land, but they did not migrate to over thesea. From 1200 LST, a precipitation system migrated
northwestward and left the area observed by the XDRlocated at MIA (Figs. 5a–e). At 1248 LST, a new convective cell developed over the northwest slope of LMand became organized into a precipitation system witha horizontal scale of approximately 10 km at around1330 LST (Figs. 5c, d). The precipitation system mi-Journal of the Meteorological Society of Japan

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16

14

[m/s]

12

0

Height [km]

10

-3

8

-6

6

-9

4

-12

2

-15
10(m/s)00700
10 NOV

1000

1300

1600

1900

Local Time

2200

0100
11 NOV

0400

Fig. 7. Time-height cross-section of horizontal velocity (arrows; upward represents northward) from 0700 LST on
10 November to 0400 LST on 11 November 2006, as observed by rawinsondes launched at Tabing. Shading
indicates zonal wind.

grated southwestward and merged with other precipitation systems, becoming organized into a line-shaped
precipitation system at around 1430 LST (Figs. 5e–g). The line-shaped system remained west of LM forabout 1 hour (Fig. 5h). From 1530 LST, the line-shaped
system showed a change in shape to a wedge-shaped
system west of LM (Figs. 5i–k). At 1630 LST, thewedge-shaped system again became a line-shaped system, aligned north-south (Fig. 5l). From 1630 LST,
convective cells in the northern half of the line-shaped
system migrated northward and decayed (Figs. 5m, n).
In contrast, convective cells in the southern half of thesystem migrated eastward, merging with a precipitation
system that had remained south of LM; the merged precipitation system changed its movement direction andmigrated to over the sea (Figs. 5m–p).
Convective activity was initiated over the sea from1830 LST (Fig. 6). New precipitation systems weregenerated over the sea and developed as they migrated
northwestward (Figs. 6a–c). The precipitation systems
that originated from over land and those generated overthe sea became merged into a larger precipitation system than those over land (Figs. 6d, e). The expanded
system migrated offshore (in a westward direction) from1930 LST at a speed of about 5.0 m s−1 and left the area
observed by XDRs at around 2200 LST (Figs. 6e–h).
Mori et al. (2011) reported the monthly mean of thediurnal cycle in precipitation systems for the present
study area: the precipitation area observed over landin daytime started to migrate toward the sea at around1800 LST at a propagation speed of approximately 4.0
m s−1 . Thus, the present results are largely consistent
with those of Mori et al. (2011), indicating that the caseexamined in the present study is typical of the diurnal
cycle of convective activity in west Sumatera during
HARIMAU2006.
3.3 Time variations in horizontal wind
Figure 7 shows a time-height cross-section of horizontal wind at Tabing obtained by 3-hourly rawinsonde
observations on 10 November 2006. From 2 to 14 km
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8Wind speed (m/s)

6

4

2

0
0000

0600

1800

1200

Wind direction (degree)

360
330
300
270
240
210
180
150
120
90
60
30
0

Wind direction
Wind speed

2359

Local Time
Fig. 8. Temporal variations in surface wind direction (red circles) and speed (blue triangles) at Tiku on 10 November
2006. Red and blue lines indicate monthly mean surface wind direction and speed, respectively.

and wind speed was stronger at higher altitudes. Local circulation was observed below 2 km in height withwesterly (easterly) wind during the daytime (nighttime)
below 1 km. Local circulation was also found in surface observations at Tiku (Fig. 8): southwesterly windblew between 0800 and 1700 LST, subsequently changing to southeasterly. The surface wind speed gradually
increased from 0800 LST, peaking at around 1600 LST(7.7 m s−1 ) and rapidly decreasing thereafter. The surface wind speed during the nighttime was weak, generally less than 2.0 m s−1 . The surface wind speed anddirection on 10 November 2006 at Tiku had a similar
diurnal cycle to that of the monthly mean in November
2006, indicating that typical local circulation was observed on this day. Similar trends in surface wind wereobserved at MIA and Tabing (data not shown).
3.4 Precipitation systems over land
Subsection 3.2 described temporal variations in the
migration of precipitation systems on 10 November
2006, revealing several changes in their migration directions and shapes during the daytime. We classiﬁed themigratory precipitation systems that occurred over landin the daytime into three phases as follows (the internal
structure of the precipitation systems in each phase aredescribed below):
1. Morning phase: isolated convective cells weregenerated near the west coast of Sumatera Island andpropagated to mountainous areas via the self-replicationof convective cells.
2. Afternoon phase: precipitation systems that developed over the slope of LM showed repeated changes intheir migration directions and shapes.
3. Evening phase: precipitation systems migrated
from land to sea.
Precipitation systems with various shapes and migration directions were assigned to the 2nd phase, as theyshared a common origin over the slope of LM and theyremained over land throughout their lifetimes. The internal structures of the various cases are not shown orcompared in this paper, although the internal structureof one of the cases is described below.
During the 1st phase, an isolated convective cell was
generated near the west coast at 1012 LST and new cells
were successively generated toward the south slope of
LM. Figure 9 shows the internal structure of convective
cells at 1024 LST on 10 November 2006. Southeasterly
wind was observed at a height of 2.0 km in a convective
cell (Fig. 9a), and a similar horizontal wind ﬁeld was observed at higher levels (data not shown). Southwesterly
wind was observed at heights of 0.5 km (Fig. 9b) and 1.0
km in the convective cell (data not shown). Similarly, asouthwesterly blew across the surface at Tiku (Fig. 8).
A southwesterly wind turned to an updraft at the leadingedge of the cell, and the updraft turned to a southeasterly above 2.0 km (Figs. 9a, c). During this phase, theecho-top height of the cells was relatively low (≤6 km).
As a case of the 2nd phase, the internal structure andformation process of a line-shaped precipitation systemover the west slope of LM, observed between 1248 and
1530 LST on 10 November 2006, is shown here. As outlined in Subsection 3.2, a convective cell was generated
over the northwest slope of LM at 1248 LST and migrated southwestward to become a line-shaped precipi-

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Fig. 9. Horizontal distributions of reﬂectivity and horizontal wind at heights of 2.0 km (a) and 0.5 km (b) andvertical cross-section of reﬂectivity and wind along line A-A′ (c) at 1024 LST on 10 November 2006. In (a) and(b), the thick line indicates the coastline and thin lines show altitude at intervals of 500 m. Dual-Doppler radar
analysis was conducted within the overlapped area of the two solid circles shown in Fig. 1 and the area indicatedby two dashed circles, except for the area in which the intersection angle is less than 30◦ . Gray shading in (c)indicates topography.

tation system between 1248 and 1430 LST (Figs. 5c–g). The line-shaped system was maintained between
1430 and 1530 LST (Fig. 5h). The echo-top height ofthe precipitation system was located at approximately 9km (Figs. 10c–e). Southerly and southwesterly winds
were dominant below 0.5 km (Figs. 8, 10a), whereas aneasterly wind component was predominant at 2.0 km(Fig. 10b) and above this height (Fig. 7). Southerly
wind in the lower troposphere struck the west slope ofLM and turned to an updraft, and convective cells weregenerated over the west slope of LM. The convective
cells were advected by the easterly wind, which was
dominant above 2 km in height (Figs. 10c–e). The easterly wind turned to a downdraft over the adjacent plain,
where the convective cells decayed there (Figs. 10c–e).

This process continued to the west of LM and the lineshaped precipitation system was maintained for 1 hour.
During the 3rd phase, precipitation systems began
to migrate from land to sea in the evening. Figure
11 shows the horizontal distribution of horizontal windand reﬂectivity at a height of 0.5 km during 1600–1830
LST. Horizontal wind at 1600, 1630, and 1700 LST ata height of 0.5 km was directed toward LM (southerly,
southwesterly, and westerly) and the precipitation systems remained over land. Some of the precipitation systems migrated from the plain located west of LM to thesouth slope of LM, as described in Subsection 3.2. From
1730 LST, horizontal wind started to blow from LM to
the west coast, and the precipitation systems around LM
started to move from LM to the west coast. At 1800

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Fig. 10. Horizontal distributions of reﬂectivity and horizontal wind at heights of 0.5 km (a) and 2.0 km (b), andvertical cross-sections of reﬂectivity and wind along line B-B′ at 1348 LST (c), 1400 LST (d), and 1412 LST (e)on 10 November 2006. In (a) and (b), thick and thin lines indicate the coastline and altitude at intervals of 500
m, respectively. Dual-Doppler radar analysis was conducted within the overlapped area of the two solid circles(see Fig. 1) and the area indicated by two dashed circles, except for the area in which the intersection angle isless than 30◦ . Gray shading in (c)–(e) indicates topography.

and 1830 LST, northeasterly wind was dominant on thesouthwestern slope of LM. The surface wind at Tiku
was southwesterly between 1600 and 1700 LST, changing to southeasterly between 1700 and 1800 LST, showing that the timing of the change in wind direction below 0.5 km corresponded to the timing of change in the
migration direction of precipitation systems.
Here, we discuss the relationship between local circulation and the generation and migration of precipitation
systems or convective cells over the land in daytime.

During the 1st phase, direction along which new convective cell formed corresponded to the leeward sideof horizontal wind in the lower layer. During the 2ndphase, the development of convective cells over thewestern slope of LM was inﬂuenced by the horizontal
wind direction in the lower layer: the convective cells
were generated by an updraft induced by local thermal
circulation and the slope effect. On the other hand, themotion of the convective cells was affected by easterly
wind in the upper layer. During the 3rd phase, the mi-352

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Fig. 11. Temporal variations in horizontal distribution of reﬂectivity and horizontal wind at a height of 0.5 km from1600 to 1830 LST on 10 November 2006. Thick and thin lines indicate the coastline and altitude at intervalsof 500 m, respectively. Dual-Doppler radar analysis was conducted within the overlapped area of the two solid
circles (see Fig. 1) and the area indicated by two dashed circles, except for the area in which the intersection
angle is less than 30◦ .

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Fig. 12. Horizontal distribution of reﬂectivity and horizontal wind at a height of 0.5 km at 1930 LST on 10 November
2006. Thick and thin lines indicate the coastline and altitude at intervals of 500 m, respectively. Dual-Doppler
radar analysis was conducted within the overlapped area of the two solid circles (see Fig. 1) and the area indicatedby two dashed circles, except for the area in which the intersection angle is less than 30◦ .

gration direction of precipitation systems corresponded
to the horizontal wind direction in the lower layer. The
surface wind direction between 1024 and 1830 LST on
10 November 2006 at Tiku roughly corresponded to thehorizontal wind direction at 0.5 km height observed by
XDRs. Surface wind at Tiku on 10 November 2006
showed local circulation which was typical of that observed in November 2006, as mentioned in Subsection
3.3. Accordingly, it is proposed that local circulation
affected not only the occurrence direction of new convective cells in the daytime, but also the timing of theevening change in the migration direction of precipitation systems from land to sea.
3.5 Precipitation systems over the sea
From 1830 LST, precipitation systems were generated over the sea near the west coastline and developed

northwestward in association with southerly or southeasterly wind in the lower troposphere (Fig. 12). The
precipitation systems from land migrated to the sea,accompanied by easterly or northeasterly wind in thelower troposphere (Fig. 12). These two sets of precipitation systems became merged into a much larger
precipitation system than those over land, being greaterthan 100 km along the long axis and oriented parallelto the west coast of Sumatera Island (Fig. 13a). Figure 13 shows the horizontal distribution of reﬂectivity
and horizontal wind at 2.0 km height, a vertical crosssection of reﬂectivity and wind, and mean proﬁles ofzonal, meridional, and horizontal winds along line C-C′
in Fig. 13a at 1954 LST. The narrow convective region
with a core of reﬂectivity exceeding 45 dBZ and oriented perpendicular to the west coast was observed at
the leading edge of the precipitation system (Fig. 13b).

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Vol. 89, No. 4

Fig. 13. The horizontal distribution of reﬂectivity and horizontal wind at a height of 2.0 km (a), a vertical cross–section of reﬂectivity and wind along line C-C′ (b), and mean proﬁles of zonal wind (red), meridional wind
(blue), and horizontal wind (dashed) (c) at 1954 LST on 10 November 2006. Dual-Doppler radar analysis was
conducted within the overlapped area of the two solid circles (see Fig. 1) and the area indicated by two dashedcircles, except for the area in which the intersection angle is less than 30◦ . Blue contours in (b) indicate a verticalvelocity of 0.0 m s−1 .

The width of the convective region with a reﬂectivity
exceeding 35 dBZ was approximately 7 km at a heightof 2.0 km (Figs. 13a, b). A downdraft was observed
below 1.0 km at the leading edge of the precipitation

system (Fig. 13b). A southerly inﬂow at a height of 2.0
km turned to an updraft above the downdraft (Figs. 13a,b). The updraft merged with ascending easterly windfrom the weak echo region in the rear of the convective

August 2011

N. SAKURAI et al.
2 m/s

DISTANCE NORTH OF MIA (km)DISTANCE NORTH OF MIA (km)0.5 km

20

10

10

0

2000 LT2012 LT

-70

2 m/s

20

0.5 km

0

355

-60

2036 LT2048 LT

-50

-10
-80

-40

-70

DISTANCE EAST OF MIA (km)2 m/s

DISTANCE NORTH OF MIA (km)DISTANCE NORTH OF MIA (km)0.5 km

20

10

10

0

2012 LT2024 LT

-70

-50

2 m/s

20

0.5 km

0-60

DISTANCE EAST OF MIA (km)-602048 LT2100 LT-50-40

DISTANCE EAST OF MIA (km)

-10
-80

-70

-60

-50

DISTANCE EAST OF MIA (km)2 m/s

DISTANCE NORTH OF MIA (km)0.5 km2024 LT2036 LT

20

10

0

-70

-60

-50

-40

DISTANCE EAST OF MIA (km)Fig. 14. Temporal variations in the horizontal distribution of a precipitation system, divergence, and horizontal wind
at a height of 0.5 km from 2000 and 2100 LST on 10 November 2006. The thick black and gray contours are
reﬂectivity greater than 30 dBZ. Thin dashed contour and gray shading indicate divergence less than −0.1×10−3
s−1 . Contour interval of the convergence is 0.5×10−3 s−1 .

356Journal of the Meteorological Society of Japan

Vol. 89, No. 4

Fig. 15. As for Fig. 13, but for line D-D′ at 2024 LST.

region and had a peak of about 8 m s−1 at a height of 9
km (Fig. 13b). The echo-top height of the precipitation
system was located at approximately 13 km (Fig. 13b).
An anvil located above 6 km in height moved faster thanthe core of the convective region because of a strong
easterly wind in the upper troposphere (Figs. 13b, c), as
observed by rawinsondes (Fig. 7).
The migration process of the precipitation systemover sea was investigated based on data from the two

XDRs (Figs. 14, 15). Figure 14 shows the horizontal
distribution of horizontal wind, divergence, and reﬂectivity at a height of 0.5 km from 2000 to 2100 LST on
10 November 2006. In the precipitation system, two
migration processes were observed. In the ﬁrst migration process, the system migrated westward maintaining
its structure as shown in Fig. 13: horizontal wind converged at the leading edge of the system in the lower
layer and a convective region was accompanied with anAugust 2011

N. SAKURAI et al.

15

DISTANCE NORTH OF MIA (km)0.5 km

10

5
(km)

4
3

0

2
1
-5
-60

-55

-50

-45

-40

DISTANCE EAST OF MIA (km)Fig. 16.
Backward trajectory results for air
parcels placed at a height of 0.5 km aroundthe leading edge of the precipitation system at2024 LST on 10 November 2006 (cross mark).
Positions of the air parcels are plotted every 6
minutes by circle mark. Colors of the circle
mark are heights of air parcels. Contours indicate reﬂectivity every 5 dBZ starting at 20
dBZ. Thick contour indicates reﬂectivity at
30 dBZ.

updraft above the convergence. Migration speed of thispart was about 5 m s−1 , similar to the ambient windspeed in the lower troposphere (Figs. 7, 13b, c). In thesecond migration process, a new convective cell was
generated at about 5 km ahead of the leading edge ofthe precipitation system (Fig. 14). Internal structure of
the system in the second migration process is shown in
Fig. 15. At 2024 LST, a new cell had developed with acore of reﬂectivity exceeding 40 dBZ at a height of 2.0
km in front of an old cell (Figs. 15a. b). An updraft wasobserved in the new cell and a downdraft was predominant below 5 km in height within the old cell (Fig. 15b).
After the old cell decayed, the new cell became partof the precipitation system (Fig. 14). The precipitation
system migrated via the above two migration processes
at a speed of approximately 5 m s−1 from 1930 LST until around 2200 LST in the area observed by the XDRs
(Fig. 14). Consequently, the migration mechanism of
the precipitation system over the sea is considered tohave been advection by ambient wind in the lower troposphere and self-replication of convective cells at theleading edge of the system.

357

These two migration mechanisms were considered tobe concerned with the convergence in the lower troposphere. As shown in Fig. 14, the convergence region was mainly observed around leading edge of the
precipitation system at the lowest layer. The convergence was formed by an easterly wind component in thesystem and a southerly wind component over the sea.
The wind speed of the easterly wind component at theleading edge of the system was 4–6 m s−1 , which waslarger than land breeze (less than 2.0 m s−1 ). Therefore, the easterly wind component should be composedof not only land breeze but also other sources. Downdraft was also considered as possible formation mechanism of the easterly wind around the leading edge ofthe system at the lowest layer; however, the downdraft
was weak during the dissipating stage of the convective cell, as shown in Fig. 15. Therefore, surface wind(land breeze) and downdraft alone are unable to explain
the formation of the wind speed in the convergence region. We performed a trajectory analysis to investigate
the formation mechanism of the convergence around theleading edge of the precipitation system (Fig. 16). Airparcels were placed at a height of 0.5 km around theleading edge of the system at 2024 LST and traced backward in time for 61 minutes. We assumed that the windﬁeld was maintained for 6 minutes. The trajectory of
air parcels was tracked using wind data renewed every
minute. Air parcels were transported from east or southeast and from 4 km in height to the convergence region
around the leading edge of the precipitation system atthe lowest layer (Fig. 16). The ambient wind speed ataround 4 km in height was 4–6 m s−1 (Figs. 7, 13c,15c), similar to the wind speed in the convergence region observed by the two XDRs (Fig. 14). Therefore,
we consider that downward transportation of horizontal momentum below 4 km also played an importantrole in intensifying the wind speed around leading edgeof the system at the lowest layer. Since ambient easterly wind was predominant below 14 km on 10 November 2006 (Fig. 7), horizontal momentum below 4 km
could be continuously transported downward, meaningthat the system was maintained over the sea for a longtime (more than 90 minutes) and migrated offshore fora long distance.
4. Discussion
As mentioned in the Introduction, little is known
about the internal structure and migration process of awestward-migrating precipitation system with a diurnal
cycle over the sea off Sumatera Island because, in a previous project (CPEA), an XDR was installed in mountainous terrain and did not cover the sea area off the west

358

Journal of the Meteorological Society of Japan

coast of the island. In the present study, we aimed to ﬁll
gaps in our knowledge using data from two XDRs installed near the west coast of Sumatera Island, providing
coverage of the sea off the west coast of the island. Thissection summarizes the internal structure and migration
mechanism of a westward-migrating precipitation system with a diurnal cycle over the sea off the west coastof Sumatera Island. The results are then compared withthe ﬁndings of previous studies in the region.
4.1 Internal structure of a westward-migrating precipitation system over the sea
In the present study, as shown in MTSAT image(Fig. 3), the horizontal scale of westward-migrating
cloud systems with a diurnal cycle was several hundred
kilometers over the sea, which is consistent with thatof westward-migrating cloud systems shown in previous studies based on satellite data (Sakurai et al. 2005;
Sakurai et al. 2009). Based on data from ground based
radar data, we investigated the internal structure of awestward-migrating precipitation system over the sea
and revealed that the precipitation system was muchsmaller than the cloud systems. The precipitation system had a convective region with a long axis (>100km) oriented parallel to the west coast of Sumatera Island and a short axis (∼7 km) oriented perpendicular tothe coast. An anvil of the precipitation system moved
faster than the convective region above 6 km in height(Fig. 13) and the echo top height of the precipitation
system was located at approximately 13 km (Fig. 13).
The precipitation system over the sea was much larger
than that observed over land in the daytime (long axisof several tens of kilometers) because precipitation systems that originated from over land merged with thosegenerated during the evening over the sea near the west
coast of Sumatera Island (Fig. 11). These precipitation systems with different origins were accompanied
by different winds in the lower layer: easterly or northeasterly wind in the precipitation systems from landand southerly or southeasterly wind in those generated
over the sea (Fig. 12). Mori et al. (2011) described theevening-time expansion of the westward-migrating precipitation system over the sea, near the west coast of
Sumatera Island based on monthly mean single radar
data collected as part of HARIMAU2006. In the present
study, the expansion process of the system was revealed
for the ﬁrst time based on dual-Doppler radar analysis.
We discuss the relationship between the expansion
process of the precipitation system and abundant rainfall over the sea near the west coast of Sumatera Island.
Previous studies on the climatological diurnal cycle ofrainfall over Sumatera Island have shown that annual

Vol. 89, No. 4

rainfall over the sea, near the west coast of the island,is much greater than that over the island itself, and thatmost of the rainfall over the sea occurs during nighttime (Hara et al. 2009; Mori et al. 2004; Wu et al.
2009). These earlier studies speculated that the abundant rainfall over the sea near west Sumatera is related
to a westward-migrating precipitation system with a diurnal cycle. The results obtained in the present study
support the speculation in these earlier studies: we revealed the expansion process of a westward-migrating
precipitation system over the sea near the west coast of
Sumatera Island. However, only one case was presented
in the present study; further analyses are required to investigate if the expansion of the precipitation system examined in the present study is common to all systems in
the region and to what degree the rainfall produced by
the expanded system contributes to the abundant rainfall
over the sea.
4.2 Mechanism of westward migration of the precipitation system over the sea
In the case analyzed in the present study, a precipitation system located over the mountains of Sumatera Island started migrating to the sea in association
with a change in local circulation from landward to seaward at around 1730 LST (Fig. 11). After the systemmerged with other system generated over the sea near
the west coast, the system migrated further offshore via
two mechanisms. The ﬁrst migration mechanism is advection by ambient wind in the lower troposphere because the migration speed of the precipitation systemover the sea was about 5 m s−1 , similar to that of ambient wind in the lower troposphere where the easterly
wind component was dominant above 2 km. During theadvection by ambient easterly wind in the lower troposphere, a convergence in the lower layer and an updraft
above the convergence were maintained in convective
region in the system. The second mechanism is successive generation of convective cells at the leading edge ofthe system (Fig. 15) arising from convergence between
an easterly wind component from the dissipating stageof the convective cell and the ambient southerly windover the sea in the lower layer (Figs. 14, 15). The convergence at the leading edge of the system played an
important role in these two migration mechanisms. We
suggested that the main factor behind convergence formation at the leading edge of the system was the downward transportation of horizontal momentum below 4
km because air parcels at the lowest layer at the leading edge of the system were transported from 4 km inheight (Fig. 16) and the ambient wind speed and direction below 4 km were similar to the wind speed and di-August 2011

N. SAKURAI et al.

rection in the lower layer at the leading edge of the system (Figs. 7, 14, 15). This view is also supported by theobservation that the land breeze (∼2 m s−1 ) and downdraft from the dissipating stage of the convective cellwere too weak to explain the wind speed in the lower
layer at the leading edge of the system (Figs. 8, 15).
The above mechanisms of the westward-migration of
the precipitation system are now compared with the results of previous studies. The ﬁrst migration mechanism (i.e., advection by ambient wind in the lower troposphere) is consistent with the mechanism proposed
in previous studies that considered ground-based radar
data (Mori et al. 2011; Sakurai et al. 2009). The second migration mechanism (i.e., the successive generation of convective cells at the leading edge of the precipitation system) is also consistent with the mechanism
proposed in previous studies (Mori et al. 2011; Sakurai
et al. 2009; Wu et al. 2009). However, there is a difference in the type of the wind that converges with ambient wind in the lower layer: in the present study, notonly land breeze and downdrafts from old convective
cell, but also downward transportation of horizontal momentum below 4 km strengthen the convergence in thelower layer at the leading edge of the system. Whereas
in Mori et al. (2011), a land breeze is the main component of the wind that converges with ambient wind in
the lower layer, and Wu et al. (2009) emphasized therole of a gust derived from an old convective cell. In
summary, the two migration mechanisms proposed in
the present study to explain the westward migration of
precipitation system are consistent with the mechanisms
proposed in previous studies, which suggests these are
the main mechanisms of migration in west Sumatera.
However, there are conﬂicting ideas regarding the typeof wind that converges with ambient wind in the lower
layer. Consequently, some questions remain in termsof the westward migration mechanism. Further studies
are required to answer these questions, including casestudies based on observational data and sensitivity experiments using numerical models.
5. Conclusion
This study examined the internal structure and migration process of a westward-migrating precipitation system with a diurnal cycle observed on 10 November 2006
in west Sumatera based on dual-Doppler radar analysis,rawinsonde data, and surface data obtained as part of
HARIMAU2006, along with MTSAT IR1 data. The location of convective cell generation over land in the daytime and the timing of evening changes in the migration
direction of precipitation systems from landward to seaward were inﬂuenced by local circulation: in the morn-

359

ing, an isolated convective cell was generated near thewest coast and new convective cells were successively
generated over the sea breeze (Figs. 4, 9). In the afternoon, precipitation systems showed repeated changes inmigration direction and shape over land (Fig. 5). A casestudy of precipitation systems observed over land in theafternoon revealed that convective cells in a precipitation system were successively generated over the western slopes of the mountains surrounding Lake Maninjau (LM) by thermally induced local circulation and by
the slope effect (Fig. 10); the precipitation system wasmaintained for 1 hour (Figs. 5g, h). From 1730 LST,a mountain breeze started to blow from LM toward thewest coast, resulting in migration of the system toward
the sea (Fig. 11).
From 1830 LST, precipitation systems began generation over the sea near the west coastline of the island(Fig. 6a). The systems generated over the sea and thosemigrating from land merged to produce a larger precipitation system than those over land (Figs. 6b–e, 12). The
expanded precipitation system had a convective region
with a long axis oriented parallel to the west coast of
the Island (>100 km) and a short axis (∼7 km) oriented
perpendicular to the west coast at the leading edge ofthe system (Fig. 13). The echo top height of the system
was located at approximately 13 km and an anvil moved
faster than the convective region above 6 km in height.
After 1930 LST, the system migrated farther offshore ata speed of about 5 m s−1 , similar to the ambient wind direction and speed in the lower troposphere (Figs. 6d–h).
The mechanisms of migration for the system over thesea were advection by ambient wind in the lower troposphere and the successive generation of convective cells
at the leading edge of the system (Fig. 14). We consider
that continuous downward transportation of horizontal
momentum below 4 km strengthened the convergenceat the leading edge of the system, resulting in the maintenance of the system over the sea for a long time andthe migration over the sea for a long distance (Fig. 16).
Migrating precipitation systems associated with a diurnal cycle in west Sumatera have been investigated previously in terms of their climatological characteristics
based mainly on long-term satellite data (Mori et al.
2004; Murakami 1983; Nitta and Sekine 1994; Sakurai et al. 2005; Yang and Slingo 2001). The CPEA andHARIMAU projects made use of ground-based observations by XDRs, yielding three-dimensional data withhigh spatial and temporal resolution, which provided anadvance in our understanding of the diurnal cycle of migrating precipitation systems in the west Sumatera region. The present study is the ﬁrst to document thethree-dimensional structure of the wind ﬁeld and the mi-Journal of the Meteorological Society of Japan

360

gration process of precipitation systems associated witha diurnal cycle in the region. Because we presented
only a single case study, it is necessary to perform additional case studies under different atmospheric conditions (e.g., ambient wind speed and direction, season,and phase of intraseasonal variation) in order to obtain acomprehensive understanding of the three-dimensional
structure and migration process of migrating precipitation systems with a diurnal cycle in this region. Another intensive observation campaign, employing dual
Doppler radar, was conducted during April and May of2007 in the same area as that considered in the present
study, also the XDR at MIA is now operating as part of
the HARIMAU project, producing data for future analyses.
Acknowledgements
The HARIMAU project was supported by the Japan
Earth Observing System (EOS) Promotion Program(JEPP) of the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan. Data usedin this study such as the two XDRs at MIA and Tiku,
rawinsondes at Tabing, and the surface wind data at
Tiku were obtained by the HARIMAU project. We
heartily thank colleagues at the Agency for the Assessment and Application of Technology (BPPT), BMG,
and the National Institute of Aeronautics and Space(LAPAN) on the Indonesian-side, and the Japan Agency
for Marine-Earth Science and Technology (JAMSTEC),
Hokkaido University, and Kyoto University on theJapanese-side for their strong collaborations in performing observations. We would also like to thank Dr.
Takeshi Maesaka of the National Research Institute for
Earth Science and Disaster Prevention (NIED) for many
useful suggestions regarding the analysis of radar data.
Thanks are also extended to the constructive commentsof the Editor (Prof. Takehiko Satomura) and two anonymous reviewers who greatly improved the content of
this manuscript. Dr. Tokio Kikuchi of Kochi University is thanked for providing MTSAT IR1 data. Figures
were drawn using Grid Analysis and Display System(GrADS) and Generic Mapping Tools (GMT).
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