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Time–Space Characteristics of Diurnal Rainfall over Borneo and Surrounding Oceans
as Observed by TRMM-PR
HIROKI ICHIKAWA
Graduate School of Environmental Studies, Nagoya University, Nagoya, JapanTETSUZO YASUNARI
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan(Manuscript received 25 April 2005, in final form 1 October 2005)
ABSTRACT
Five years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data were used toinvestigate the time and space characteristics of the diurnal cycle of rainfall over and around Borneo, anisland in the Maritime Continent. The diurnal cycle shows a systematic modulation that is associated withintraseasonal variability in the large-scale circulation pattern, with regimes associated with low-level easterlies or westerlies over the island. The lower-tropospheric westerly (easterly) components correspond toperiods of active (inactive) convection over the island that are associated with the passage of intraseasonal
atmospheric disturbances related to the Madden–Julian oscillation. A striking feature is that rainfall activity
propagates to the leeward side of the island between midnight and morning. The inferred phase speed ofthe propagation is about 3 m s⫺1 in the easterly regime and 7 m s⫺1 in the westerly regime. Propagation
occurs over the entire island, causing a leeward enhancement of rainfall. The vertical structure of thedeveloped convection/rainfall system differs remarkably between the two regimes. In the easterly regime,
stratiform rains are widespread over the island at midnight, whereas in the westerly regime, local convective
rainfall dominates. Over offshore regions, convective rainfall initially dominates then gradually decreases inboth regimes, while the storms develop into deeper convective systems in the easterly regime. Aside fromleeward rainfall propagation, shallow storms develop over the South China Sea region during the westerly
regime, resulting in heavy precipitation from midnight through morning.

1. Introduction
The weather over the Maritime Continent (Ramage1968) is characterized by heavy rainfall throughout theyear. The region is a major atmospheric heat sourcethat plays an essential role in the earth’s climate system.
The unique environment over the Maritime Continent
includes complex land/ocean geography, surrounded bywarm ocean waters. This environment favors the development of frequent deep convection on multiple temporal and spatial scales. Land–sea contrasts associatedwith major islands in warm oceans generate complex
local circulations that play an important role in the energy and water cycle processes of the region (Neale andCorresponding author address: Hiroki Ichikawa, Graduate
School of Environmental Studies, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8601, Japan.
E-mail: s040101d@mbox.nagoya-u.ac.jp© 2006 American Meteorological Society

Slingo 2003). Much of the active deep convection occurs in response to the interactions of the several time–
space-scale circulation systems over and around
Borneo, which is located at the center of the Maritime
Continent (Chang et al. 2005a). The linkage between
island-induced convection and the large-scale circulation is crucial for convective organization over this region.
The Maritime Continent has a pronounced diurnal
cycle in convection and precipitation that is influenced
by surface heating. Previous studies using satellite
cloud imagery have shown a distinct land–sea contrast
that is characterized by late afternoon/evening convection over land, and nighttime and morning convection
over the surrounding ocean (Murakami 1983; Nitta andSekine 1994; Yang and Slingo 2001). There is also large
regional variability in the diurnal cycle because of thecomplex forcing mechanisms related to surface inhomogeneities (Ohsawa et al. 2001). Diurnally forced

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land–sea differential heating and terrain-slope heating
all initiate local circulations that can support organized
cloud clusters that are characterized by a pronounced
diurnal cycle (Williams and Houze 1987). Especiallyover coastal regions, the development of convection isdominated by the sea–land breeze (Saito et al. 2001).
One of the characteristics observed in the diurnal
variability of convection over the Maritime Continent is
the phase propagation of the diurnal signal. Yang andSlingo (2001), using high-resolution Cloud Archive
User Service (CLAUS) brightness temperature data,
showed that a widespread coherent diurnal variation in
the convection peak propagates offshore around theisland. This suggests that the strong diurnal signal overland moves out over the surrounding ocean as a gravity
wave. Mori et al. (2004) used Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) data
to examine more regional characteristics of precipitation over Sumatra and found that diurnal variation of
rainfall over that island is characterized by a migrationfrom the southwestern coastline to both inland and offshore regions. Liberti et al. (2001) examined geostationary meteorological satellite (GMS) brightness temperature data over New Guinea and showed an offshore propagation of cloud systems that moved
northward toward open ocean. Numerical simulations
have shown that an eastward-propagating precipitation
system that develops over the lee of mountain ranges is
responsible for the nocturnal rainfall maximum overinland areas of Indochina (Satomura 2000). Cold gravity currents have been shown to influence the propagation. Propagating diurnal cycles also occur over the Bayof Bengal (Webster et al. 2002; Zuidema 2003), east
Asia (Asai et al. 1998), North America (Wallace 1975;
Carbone et al. 2002), and the northwestern coast ofSouth America (Mapes et al. 2003a,b; Warner et al.
2003). However, details of propagation have not yet
been investigated over Borneo, which is a major islandin the Maritime Continent.
Large-scale low-level winds are an important modulating factor in the diurnal cycle. Houze et al. (1981)
examined nocturnal rainfall activity over the seasnorthwest of Borneo during the Winter Monsoon Experiment (WMONEX). Land breezes converge withnortheasterly monsoons to generate offshore convection, which develops into an organized precipitation
system that produces heavy rainfall between midnight
and morning. Oki and Musiake (1994) found similar
morning rainfall associated with offshore breezes over
the Malay peninsula. They used rain gauge data to analyze the seasonal and diurnal cycles of precipitation andshowed a windward shift in the coastal morning
maxima in precipitation counter to the predominant1239

monsoon wind direction. The above studies underline
the importance of interactions between diurnally forced
land–sea circulation and prevailing monsoon winds ingenerating strong diurnal variations at some locations.
However, a more systematic description of the largescale impact on convection by lower winds in the diurnal cycle has not been reported.
Since late 1997, TRMM satellite PR data have clarified some spatial and temporal variabilities in precipitation structure. Precipitation radar data yield threedimensional rainfall distributions over the whole tropical area (e.g., Short and Nakamura 2000; Petersen and
Rutledge 2001; Hirose and Nakamura 2002; Petersen etal. 2002). Nesbitt and Zipser (2003) described the complex diurnal cycle over the Maritime Continent andshowed diurnal variations in individual precipitation
systems identified by PR and TRMM Microwave Imager (TMI) data. Precipitation systems appeared
mostly between afternoon and late evening over land,
but precipitation associated with mesoscale convective
systems (MCSs) lingered past midnight. The number of
MCSs increased over adjacent sea regions during morning hours. In addition to these large-scale features, amore regional investigation of rainfall activity provided
a more advanced understanding of the precipitation
characteristics of this region of complex terrain.
This study examined temporal and spatial variations
in diurnal rainfall over Borneo and adjacent oceans. A
particular question addressed was how the low-level
prevailing wind modulates the diurnal cycle of rainfall
over the island. High-resolution PR data for 5 yr
yielded precipitation characteristics and their variability. Section 2 outlines the data used in this study and thecomposite method. In section 3, general features of thediurnal rainfall cycle, and the time–space structure of
rainfall activity during different wind regimes, are presented using the results of a composite analysis. This
extracted the influence of low-level winds on the diurnal cycle. Section 4 presents a summary and discussion.

2. Data and methods
The data used in this study include TRMM PR for 5
yr (1998–2002). PR data provide the three-dimensional
structure of a swath of rainfall from the surface to 20
km above the earth ellipsoid. The swath is 215 km wide,with a horizontal resolution of 4.3 km ⫻ 4.3 km at thenadir. Vertical resolution is 250 m. Rainfall estimate
sensitivity is about 0.5 mm h⫺1 (Kummerow et al. 1998,
2000). Although the data have high resolution, temporal sampling is 0.5 times day⫺1 at the equator because of
the satellite’s orbit. It takes about 46 days for TRMM torepeat a pass over a given location at a given local time.

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Because of this sampling constraint, this study examined diurnal cycles from a 5-yr composite of PR datawith no less than 3-h bin temporal resolution. This resolution allows a presentation of the diurnal cycle of rainfall. The present study used near-surface rain products
from the TRMM Science Data and Information System(TSDIS) 2A25 version 5 algorithm (Iguchi et al. 2000).
Algorithm 2A23 (Awaka et al. 1997) classifies rainfall
as convective or stratiform. Such rainfall classification
is important when determining the vertical profiles oflatent heating and their influence on large-scale circulation fields (Houze 1989, 1997).
Gridded TRMM PR rainfall data were produced bycomputing 5-yr total rainfall in 0.05° ⫻ 0.05° and 0.5° ⫻
0.5° grid boxes for each local time at each grid point.
This computation allowed an examination of the spatial
variations in diurnal rainfall over Indonesia. Histograms of storm height, defined by the top of the highest
range bins with significant reflectivity, were also constructed between the surface and 15 km, at an intervalof 1 km, with the same spatial and temporal resolution
as the rainfall composite. The combination of surface
rainfall data from PR with information on deep andshallow storms described by storm height yielded threedimensional convective characteristics. Hourly gridded
data were smoothed with a 3-h running mean to minimize sampling errors.
The effect of the low-level prevailing wind on thediurnal cycle is discussed in this study. Changes in convection under different wind regimes were revealed
through composite analyses centered on the island of
Borneo. Zonal winds at 850 hPa from 5 yr (1998–2002)
from the National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Atmospheric Model Intercomparison Project II (AMIP-II)
reanalysis dataset (hereafter referred to as NCEP RII;
Kanamitsu et al. 2002) were used to classify the windregime. Daily, area mean zonal wind time series for abox covering Borneo shown in Fig. 1 (1.5°S⬃4.0°N,
111.0°⬃116.5°E) were smoothed using a 5-day runningmean; the wind data were then partitioned into easterly
and westerly regimes. A detailed classification definition is given in section 3b. TRMM PR surface rainfall
data were composited in 0.05° grid boxes for each classified wind regime.

3. Resultsa. General features of diurnal rainfall activity
Figure 1 shows the spatial distribution of rainfall inferred from TRMM PR surface rainfall data over the
Maritime Continent in the morning and during late evening. There is a pronounced diurnal cycle of rainfall

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FIG. 1. Rainfall over the Maritime Continent in the morning(0600–1000 LT) and late evening (1800–2200 LT). The rectangular box is the area within which the area mean zonal wind timeseries are calculated. The horizontal dotted line over Borneo is adomain of calculation for the vertical distribution of storm heightin Fig. 2.

between land and ocean. Rainfall is suppressed overthe islands during the morning, but strong convection
develops offshore, dropping much rain. The situation
dramatically reverses in the evening, when rainfall activity is concentrated over Sumatra, Borneo, and New
Guinea.
Borneo is located near the center of the Maritime
Continent. The terrain of Borneo and the sea–landbreeze creates a peculiar diurnal cycle in convection.
Figure 2 shows time sequences of the vertical distribution of storm height frequency and surface rainfall(all—black solid, convective—red solid, and stratiform—black dotted) along 1°N averaged with ⫾0.5° inlatitude (dotted line in Fig. 1) to highlight the twodimensional structures in precipitation and convective
development. Shallow storms appear over the island,
over the coast, and in the central mountains, at 1100–
1300 LT. By 1400–1600 LT, coastal storms have developed, enhancing surface rainfall. Between 1700 and1900 LT, deeper storms overspread much of the island
resulting in increased surface rainfall, while coastal
rainfall dissipates. Most of the rain from 1100–1300 LT
to 1700–1900 LT is convective. At night, convective activity is concentrated over the central part of the island.
From 2000 LT through 0100 LT, deeper storms converge over inland areas, and rainfall persists until midnight over the central part of the island. The dominant1 APRIL 2006

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FIG. 2. Time sequence of the vertical distribution of storm height frequency from 3.5 to 9 km over Borneo along
1°N averaged with ⫾0.5° in latitude (horizontal line in Fig. 1). Surface rainfall amount is also noted in each timebin (black solid: all rain, red solid: convective rain, black dotted: stratiform rain). Land terrain is described along
the bottom (sea is shown by blue line).

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FIG. 3. Zonal wind at 850 hPa from NCEP R2 data for an area over Borneo (1.5°S–4.0°N, 111.0°–116.5°E;
rectangular box in Fig. 1) and smoothed by a 5-day running mean.

rainfall type changes at night. About half of the rainfall
is stratiform from 2300 LT to 0100 LT. From 0200–0400
LT until morning, offshore convection develops on
both sides of the island as convection over land gradually dissipates.

b. The low-level wind variation over Borneo andthe large-scale circulation field
Oscillations in the low-level zonal wind associatedwith variations in the large-scale circulation field are aprominent feature over and around Borneo. Figure 3
shows a time series of zonal wind at 850 hPa from
NCEP RII data averaged over Borneo (1.5°S– 4.0°N,
111.0°–116.5°E: rectangular box in Fig. 1) andsmoothed by a 5-day running mean. Fluctuations in
low-level flow over the island occur mainly in the in-traseasonal and interannual time scales. Seasonal variations do not cause distinct variability over the island.
Between January and April 1998, easterly winds persisted during an El Niño event, when submonthly scaledisturbances are unlikely to dominate. A prolonged period of easterly winds, presumably due to anomalous
Walker circulations, corresponded to a significant decrease in rainfall over the Maritime Continent (Lau andWu 1999; Walsh and Newbery 1999). Wind directionchanged intermittently from easterly to westerly andvice versa in association with convective variability influenced by the passage of an intraseasonal-scale disturbance such as the Madden–Julian oscillation (MJO;
Madden and Julian 1971, 1972, 1994). Intraseasonalvariability for the five years is most strongly dominatedby the westerly phase rather than the easterly phase.

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FIG. 4. Power spectra for 850-hPa zonal winds over Borneo.

This is especially true for the stronger westerly windsthat prevail during boreal winter, when the MJO has apeak amplitude (Madden and Julian 1972; Madden
1986; Gutzler and Madden 1989).
A power spectrum of the zonal wind time series inFig. 3 is shown in Fig. 4 to clarify prominent frequencies
in the wind field variability. It is obvious that the intraseasonal variation is the most prominent variability
in the time series of Fig. 3. Pronounced variations inzonal wind that occur with periods of about 30 and 60
days likely reflect the influence of the MJO. The spectrum shows relatively higher power at a period of 120
days, but actual changes in wind direction occur at intraseasonal frequencies, as seen in Fig. 3. As the MJO
propagates eastward from the equatorial Indian Ocean
toward the Pacific Ocean, low-level winds change fromeasterly to westerly over Indonesia as the MJO enhances or suppresses convection. Therefore, low-level
equatorial westerly (easterly) winds basically correspond to periods of active (inactive) convection over
the Maritime Continent because of the MJO (Chang etal. 2005a).
Three wind regimes were identified that highlight the
influences of the low-level wind pattern variability on
diurnal rainfall activity over the island. The smoothed
time series of zonal winds (Fig. 3) were first partitioned

into easterly (EE) and westerly regimes. The westerly
regime was further partitioned into weak westerly(WW) and strong westerly (SW) phases based on a
threshold average wind speed of 3 m s⫺1. Figure 5
shows composite midtropospheric relative humidity(averaged from 700 to 400 hPa) and upper-level divergence (200 hPa) for each wind regime to depict convective activity. Despite the simple partitioning methodology, clear differences in atmospheric features for eachwind regime are obvious. Westerly regimes correspondto periods of active convection over the Maritime Continent, as reflected in the large-scale upper-level divergence and wetter conditions around the Sumatra,
Borneo, and Sulawesi islands. Convection is inactive
when easterly winds prevail, and the regions onlyaround Sumatra and western Borneo are rather humid.
These three composites also characterize the MJO.
There is enhanced convection in the Indian Ocean during the EE regime. Convection approaches the Maritime Continent in regimes WW and SW.
Although most of the variability of the wind fieldoccurs in the intraseasonal time scale over the island,
there are rather large differences of circulation fieldfrom northern winter and summer because of the seasonal reversal of the prevailing wind. To clarify theseasonal attribution of each composite, the seasonal

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FIG. 5. Wind field at 700 hPa (vectors), relative humidity averaged from 700 to 400 hPa (shaded), anddivergence at 200 hPa [contour (⫻1 000 000 s⫺1)] for each wind regime.

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ICHIKAWA AND YASUNARITABLE 1. The seasonal distribution of days in each wind regime
over Borneo.

SW daysWW daysEE days

Boreal winter(October–March)

Boreal summer(April–September)

Total

346
257
308

133
434
348

479
691
656

statistics of occurrence of each wind regime are examined as shown in Table 1. A strong westerly phase prevails mostly in boreal winter. On the other hand, WW
tends to occur in boreal summer, while it also contains
a transition period of intraseasonal variability between
EE and SW in boreal winter. During the boreal summer
season, the intraseasonal variation occurs between EE
and WW, and the occurrence of SW is rare. Associatedwith this seasonality, each composite also characterizes
a primary circulation pattern related to the seasonal
circulation system as well as the intraseasonal variability.
Figure 6 shows composite 925-hPa wind and atmospheric instability defined by the difference of equivalent potential temperature between 850 and 500 hPa. In
the SW composite, there is a strong cyclonic circulation
over the South China Sea region called the Borneo
vortex, which appears as a persistent feature of the boreal winter climatology (Johnson and Houze 1987).
This synoptic-scale disturbance contributes to the variability of circulation field and deep convection over andaround Borneo in the interaction with the MJO (Changet al. 2005a). The cyclonic circulation of the vortex is
influenced by the increased equatorial westerly associated with the MJO extended from the Indian Ocean to
Borneo and the northeasterly from the South China
Sea. The latter flow is partly related to the cold surges(Chang et al. 1983) associated with the Asian wintermonsoon, which has an important impact in the lowlevel circulation patterns over the South China Sea inthe synoptic time scales. In our composite, the cold
surge event that is defined as the averaged 925-hPa
meridional wind between 110° and 117.5°E along 15°N
(horizontal line in Fig. 6) exceeds 8 m s⫺1, occurs
mainly in the SW regime (SW—72 days, WW—6 days,and EE—8 days) for 5 yr. The strength of the Borneo
vortex is amplified by the presence of the cold surge
because of the increased shear vorticity due to thestronger northeast wind (Chang et al. 2005a). Thus, thecold surge acts to enhance the offshore activity and give
much precipitation over the South China Sea. In thecase of WW, the clockwise turning of the winds thatcross the equator can be observed around Sumatra andBorneo. The southeasterly winds from the southern

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latitude turn northeastward, which extends to the Asianmonsoon region. The circulation pattern that appearedin WW is similar to the wind field observed in the boreal summer (e.g., Chang et al. 2005b, their Fig. 3).
Thus, all in all, the composites used in this study basically represent the intraseasonal variability, but also
contain a significant feature of the seasonal circulation
pattern. The atmospheric condition is more stabilized
in the westerly regime compared to the easterly regime
associated with the circulation and convection patterndifferences over the Maritime Continent. The stablecondition extends from the Indian Ocean to the west of
Borneo, which changes to the rather unstable condition
over the east of Borneo and around Sulawesi in SW. In
contrast, the thermodaynamical instability is enhanced
over and around the major islands in EE.

c. The influence of low-level winds on diurnal
rainfallFigure 7 shows the composite mean wind field andrainfall distribution for each wind regime. Large-scale
intraseasonal atmospheric variations primarily change
the low-level flow over Borneo, as shown in Fig. 7.
Prevailing westerlies over the island are, in most cases,
associated with northwesterly flow from the South
China Sea during the SW regime as discussed in theprevious section. Weak westerly regime composites include an anticyclonic feature with the weak wind fieldover the island. During EE, convergence in moderate
easterlies occurs over the southwestern part of the island. The daily rainfall distribution also changes associated with the three wind field patterns. The rainfall
center is located over the western part of the island
during the EE regime, but over the middle part in the
WW regime. In the SW regime, the rainfall enhancement over the island occurs rather locally over the eastern lee foot area as compared to the other two regimes.
Furthermore, during the SW regime, widespread rainfall occurs over the South China Sea northwest of
Borneo. This is related to the strong cyclonic circulation of the Borneo vortex.
In addition, diurnal rainfall has different patterns in
the three wind regimes. Diurnal variations in rainfall
during EE and SW are shown in Figs. 8 and 9, respectively. Rainfall patterns during WW exhibit featuresintermediate between SW and EE (i.e., daytime rainfall
persists over the central part of the island until pastmidnight, as shown in Fig. 2). The rainfall patterns between the EE and SW regimes are similar during theday; afternoon rainfall occurs in coastal regions andsubsequently migrates inland in the evening. However,significant differences appear from midnight to morning as follows.

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FIG. 6. Wind field at 925 hPa (vectors) and equivalent potential temperature (␪e) difference between
850 and 500 hPa [shaded; ␪e(850 hPa)⬃␪e(500 hPa)] for each wind regime. The black horizontal line in
SW is the area for the surge index (see text for details).

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FIG. 7. Composite rainfall and 850-hPa winds for each wind regime. The black line describes island orography.

During the EE regime (Fig. 8), rainfall is suppressedon the east coast from 2000–2200 to 0200–0400 LT andis concentrated over the western half of the island. Enhanced rainfall occurs along the lee of the central
mountain range and over the coastal seas, as is clearlyshown at 2300–0100 and 0200–0400 LT. A relative reduction in rainfall over the eastern part of Borneo is
obscured at 0500–0700 LT because convection originating over Sulawesi Island reaches coastal regions ofBorneo in the morning. At 0800–1000 LT, convection
over land weakens as convection off the coast strengthens.
During the SW regime (Fig. 9), the rainfall that covers the entire island at 2000–2200 LT decreases along
the western coast at 2300–0100 LT. An area of suppressed rainfall expands and propagates eastward overthe island from 0200–0400 to 0800–1000 LT. As in the
EE regime, areas of enhanced rainfall are influenced by
local terrain and occur over the eastern lee of themountains at 2000–2200 and 2300–0100 LT, and overcoastal regions at 0200–0400 LT. In addition, a convective system develops along the northwestern coast inlate evening (2300–0100 LT) and overspreads the adjacent seas from late night through morning. The increased rainfall over the water northwest of Borneo(Fig. 7) is the result of this offshore convection. Thisnocturnal convection may be related to an organizedmesoscale system that prevails during the winter monsoon over the South China Sea, as studied by Houze etat. (1981) using WMONEX data. Leeward propagation
is likely to occur at larger scales over the entire island.
Figure 10 shows a time–distance cross section for adomain perpendicular to the central mountain range ofBorneo to highlight the differences in the different
wind regimes of the east–west propagation of rainfall
activity. Interestingly, the direction of rainfall propagation changes systematically from the west during EE tothe east during SW. In both regimes, rainfall occurs in
the afternoon over a large area, but areas of suppressed
rainfall propagate westward (eastward) during EE(SW). Thus, convection persists until midnight only
over the western (eastern) side of the mountain during
EE (SW), so more rainfall appears over the western(eastern) part of the island during EE (SW). During
SW, a maximum in daily rainfall also appears offshoreof the west coast because of a local westwardpropagating rainfall system that develops at about 0000
LT (Fig. 9).
The inferred phase speed of leeward propagation for
significant rainfall in Fig. 10 is ⬃ ⫺3 m s⫺1 during EEand ⬃7 m s⫺1 during SW. To reveal the relationshipbetween the phase speeds and the surrounding windvelocity, Fig. 11 shows the vertical profile of the zonal
wind velocities averaged over Borneo (1.5°S–4.0°N,
111.0°–116.5°E: rectangular box in Fig. 1) in each regime (closed circle: EE, and open circle: SW). In EE,
wind condition is dominated by the easterly in all over
the troposphere. The velocity increases linearly fromthe surface (⬃0 m s⫺1) to the upper troposphere (⬃10m s⫺1 at 200 hPa). In contrast, in SW, the westerlies
prevail in the lower level (⬎500 hPa) with its maximum(6 m s⫺1) at 700 hPa, and strong easterly wind shear
exists in 700 to 200 hPa. Comparing the phase speedwith the wind field profile (Fig. 11), we can observe that

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FIG. 8. Time series of rainfall distribution over Borneo in the EE regime.

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FIG. 9. Same as Fig. 8, but for the SW regime.

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FIG. 10. Time–distance cross sections of regime rainfall along rectangle A averaged in the domain perpendicular
to the central mountain range. Land terrain (solid line) and daily accumulated rainfall (crossed line) are describedat the bottom of each diagram.

the leeward propagation for rainfall is closely linked to
the lower-tropospheric wind in both regimes. Specifically, the inferred phase speed is roughly close to the
700-hPa wind speed in both EE and SW.

d. Vertical structure of diurnal rainfall activity
The storm height characteristics of rainfall systems
differ in the different wind regimes. Figure 12 shows thespatial distribution of mean storm height in each windregime. Deeper convection is associated with rainfall
propagation and occurs over the leeward side of theisland. However, comparisons of Figs. 12 and 7 suggestthat deeper storms do not necessarily coincide withheavy rainfall. Average storm heights are higher in EE,
exceeding 7000 m, and high storms are more frequent in
EE than in the other regimes associated with the conditionally unstable condition over the Maritime Conti-

nent (Fig. 6). Deeper storms tend to develop over thewestern coastal seas and over the southern plains in EE.
Rainfall amounts differ little between coastal seas andthe western lee of the mountains, but the mean storm
height is lower over the lee. Higher storms, predominantly over the western part of Borneo in EE, move toeastern Borneo as the regime switches to WW and SW.
Higher storm tops appear over the eastern part of theisland in the SW composite, especially over the northern lee of the mountains. Offshore activity shows lessvertical development in SW, with moderate storm
heights from 5000–6000 m over the adjacent ocean. The
rainfall distribution (Fig. 7) suggests that these relatively shallow storms nevertheless produce a lot of precipitation over the South China Sea.
Figures 13 (EE) and 14 (SW) show averaged storm
height distributions in each 3 h of the day to highlight
how the storm top changes diurnally. Shallow storms

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FIG. 11. Vertical profile of zonal wind velocities for an area overBorneo (1.5°S–4.0°N, 111.0°–116.5°E: rectangular box in Fig. 1) ineach regime (closed circle: EE, open circle: SW).

are predominant over Borneo at 1100–1300 and 1400–
1600 LT in both regimes, then rain starts over land (Fig.
8 and Fig. 9). After 1700–1900 LT, deeper storms develop and differences between EE and SW develop thatare associated with the leeward propagation of rainfall.
During EE (Fig. 13), deeper storms develop over western Borneo. Over land, deeper storms occur over the1251

southern plains than over the western lee of the mountains at 2300–0100 and 0200–0400 LT. However, rainfall
is heavy near the mountain range in Fig. 8. The rainfall
off the west coast from midnight to morning in Fig. 8
corresponds to highly developed storms. During SW(Fig. 14), deeper storms develop over the eastern sideof the island, especially over the lee of the mountains,from 2000 to 2200 LT. These deeper storms gradually
weaken from 2300–0100 to 0200–0400 LT as convection
propagates off the east coast. The regime mean distributions (Fig. 7; Fig. 12) suggest that the rainfall systems
that develop over the South China Sea coast from
2300–0100 to 0800–1000 LT in Fig. 9 are not necessarily
deeper storms.
To elucidate storm/rainfall evolution in the east–west
propagation, time sequences of the vertical distributionof storm height frequency and surface rainfall (allrain—black solid, convective rain—red solid, and stratiform rain—black dotted) averaged along the rectangle
A (shown in Fig. 10) are shown in Figs. 15 for EE, andFig. 16 for SW, respectively. The data are smoothed by
a 3-h running mean in each hour. The convective characteristics differ between EE and SW not only in thetiming and duration of storms but also in the verticaldevelopment and rainfall type. During EE (Fig. 15),
shallow convection develops, producing rainfall overthe coast and central mountain range in the afternoon.
This coastal convection gradually migrates toward central Borneo from 1300 to 1700 LT. Convection over themountains develops vertically in the evening (1700–
1900 LT), and deeper storms begin to appear over western Borneo from 2000 to 2200 LT. Convection overland continues until midnight with higher storm devel-

FIG. 12. Mean storm height distribution in each wind regime.

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FIG. 13. Time series of mean storm height distribution over Borneo in the EE regime.

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FIG. 14. Same as in Fig. 13, but for the SW regime.

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FIG. 15. Time series of vertical distribution of storm height frequency from 3.5 to 11 km over Borneo, averaged along rectangle A
in the EE regime. The surface rainfall amount is indicated in each time bin (black solid: all rain, red solid: convective rain, black dotted:
stratiform rain). Land terrain is described along the bottom (sea is shown by the blue line).

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FIG. 16. Same as Fig. 15, but for the SW regime.

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opment and a growing area of rainfall as the storms
propagate westward. Large amounts of stratiform rainfall occur over land at night. In fact, stratiform rainfall
amounts exceed convective rainfall amounts after 0100
LT. Before convection over land ceases, offshore activity starts, between 2300 and 0100 LT over the west
coast. That convection rises to nearly 10 km during themorning hours. Most of the rainfall is convective, which
dominates until 0800–1000 LT and gradually decreases.
In the SW regime (Fig. 16), the onset of convection islater than in EE; shallow convective clouds appear at
1400–1600 LT over the west coast and the mountain
range. The coastal storms gradually evolve into widespread shallow storms at 1900–2200 LT over the western part of the island. Over the eastern part of theisland, deeper storms develop at 1800–2000 LT over themountain range. These storms evolve and propagate
eastward from 2100 to 0200 LT. In contrast to the EE
regime, in which stratiform rainfall overspreads muchof the land (Fig. 15), rainfall in SW is predominantly
convective with a sharp maximum. After 0200–0400 LT,
convection over land gradually ceases, and rainfall
propagates eastward, offshore of the east coast. However, these coastal storms do not develop vertically tothe extent of storms over land. The dominant rain typegradually changes from convective to stratiform. Relatively shallow storms appear over regions offshore of
the west coast at 0000–0200 LT. Those storms thenspread to the South China Sea between midnight andmorning.

4. Summary and discussion
The space–time characteristics of diurnal rainfall activity were investigated over the Maritime Continent
using TRMM PR data for 5 yr and NCEP RII data. A
prominent diurnal cycle in rainfall with a distinct land–sea contrast was obvious. Over Borneo Island, convection that starts in the afternoon over the coast and thecentral mountains develops vertically and subsequently
covers much of the island in the evening, with enhanced
rainfall. At night, convective activity converges over thecentral part of the island, and rainfall persists until midnight over inland areas. Convective rainfall is predominant during the daytime. Stratiform rains increase atnight, producing half of the rainfall at midnight.
The annual course of daily rainfall cycle exhibits asystematic intraseasonal variability as a persistent feature of the convection over Borneo and adjacent waters. This variability can be decomposed into three regimes denoted as SW, WW, and EE, based on low-level
zonal wind (850 hPa) speed values over the island.
Composites of the three wind regimes have shown that

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the changes in zonal wind vectors over Borneo are associated with changes in large-scale circulation patterns
related to intraseasonal variations. Strong westerlies
tend to occur seasonally, during boreal winter, while
the wind direction is most likely to change from easterly
to westerly with the passage of intraseasonal atmospheric disturbances such as the MJO. Atmosphericconditions over the Maritime Continent are wet (dry)owing to the large-scale enhanced (suppressed) convection in the SW (EE) regime.
The diurnal cycle of rainfall and convective structurechanges, depending on these zonal wind regimes in thelower troposphere. Figure 17 shows schematics of thediurnal cycle of convection/rainfall activity overBorneo, synthesizing the overall results of this study.
After the shallow convective rainfall over the island in
the afternoon, the differences in diurnal rainfall cycle
between EE and SW become apparent. The most striking feature is that rainfall activity propagates to theleeward side of the island from midnight to morning ineach wind regime. The inferred phase speed of the
propagation is about 3 m s⫺1 in EE and 7 m s⫺1 in SW.
The dynamics of leeward propagation of convection/
rainfall seems to differ between EE and SW. Deeper
storms tend to occur in EE than SW associated with themore conditionally unstable condition. However, thestructures of the developed storms are rather complicated. Over land, stratiform rainfall covers much of theisland at midnight in EE. In contrast, convection in SW
causes heavy local convective rainfall. A significant feature in SW is the eastward propagation of the verticallydeveloped convective rainfall starting from the lee sideof the mountain range, which suggests the developmentof the squall line-type convective system. The deepstorms in SW appear under the strong vertical easterly
wind shear condition in the upper troposphere. The
propagating squall line activated at the lee foot of themountain was simulated by Satomura (2000) over the
Indochina Peninsula. In his study, the convection system is triggered by the intrusion of cold air from thewindward side into the leeward side and also by themountain wave, and propagates at the speed of thegravity current under the interaction process between
the cold pool from the convection and an ambient windnear the surface. A similar mechanism may be responsible for the eastward propagation of the convection in
SW. The occurrence of convection in EE is also likely
to be related to the orographic forcing of the central
mountain range. However, in EE, longer-lived MCSs
which have wider area of stratiform rainfall tend todevelop under the weaker background wind speed. The
local circulation associated with complex land terrain
and the land surface condition of vegetation may pre-

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FIG. 17. Schematics of diurnal rainfall/convective activity over Borneo: (left) EE regime and (right) SW regime.
Phase speed is indicated in each regime (EE : thin solid: EE, : 3 m s⫺1, SW : thick solid: 7 m s⫺1). Dashed line in
SW denotes the evolution of shallow storm systems over the west coast. Borneo vortex (BV) in the lower level isschematically shown with a vortex-like arrow.

sumably be more important for the convection and nocturnal stratiform precipitation in EE.
Over offshore regions, initiation of convection may
be associated with the land breeze for the leewardpropagating storms. Convective rainfall is predominant
initially, and gradually decreases in both regimes. How-ever, storms in EE show deeper development of convection. In addition to the conditional instability, coastline direction over the west coast, which is parallel tothe prevailing wind at 925 to 850 hPa, may be an important factor for the vertical development. This maylead to produce more convergence between large-scale

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lower wind and land breeze blowing perpendicular tothe coast, and yield deeper convection in EE.
Furthermore, over the offshore region of the northwest of Borneo, significant rainfall activity can be observed in the SW regime during nighttime to morning.
The offshore activity is more shallow and stratiform ascompared to the deeper convective cells in EE. The
different characteristic of the nocturnal rainfall in SW islikely to be related to the presence of the Borneo vortex, which has an important impact in convection over
and around Borneo (Chang et al. 2005a). As mentioned
above, conditional instability is reduced during SW
compared to EE around Borneo, especially over thewest coast where the cyclonic circulation is dominated.
In the condition, nocturnal convection is enhanced in
response to large-scale convergence associated with thecyclonic circulation. Thus, shallow convection triggeredby land breeze spreads its area and propagates into alarge area of South China Sea, which causes widespread
stratiform rainfall. On the other hand, the deeper
storms developed in EE tend to stay more close to thewest coast associated with the lower-level convergenceenhanced by the prevailing wind and local circulations
around there, which result in localized rainfall over theoffshore region of the west coast.
As observed over Borneo, the leeward propagationof convection occurs also over Sulawesi in EE. The
relatively shallow storms triggered by the land breeze
propagate westward from the coast of Sulawesi to
Borneo during nighttime to morning. The convection
has an important role to enhance the rainfall amountover the eastern coastline of Borneo. When the storms
reach the east coast of Borneo, the propagation of theconvection is blocked by the land terrain of the island,
and is ceased. However, the storms’ activity is reenhanced as the convection comes close to Borneo because the land breeze intrusions from Borneo to the
propagating storms make new convergence ahead of
the previous convective systems from Sulawesi. Thus
the rainfall persists over the east coast of Borneo until
the afternoon convection develops over the coastal landarea associated with the sea breeze. The shallower
storms, in spite of the large atmospheric instability, may
be related to the complex coastline shape of the relatively small island that may prevent convection there
from organized mesoscale system.
The difference of the nocturnal convection/rainfall
activity may also influence the timing and intensity of
the convection following afternoon over the island. In
both regimes, daytime rainfall over the island is dominated by shallow convection over the coast and mountains. However, as compared to EE, the occurrence ofafternoon convection during SW is later and weaker,

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especially over the east coast. During the previous
nighttime, in SW, shallow clouds spread over a wide
range of the island and prevail until the morning hours.
Increased blocking of solar insolation in the several
hours after sunrise by the extensive morning clouds
cover likely contributes to the delay of afternoon convection (Rickenbach 2004). Furthermore, the development of the deeper storms in the evening to nighttime
would lead to stabilize the atmosphere by the convective overturning, which causes weaker afternoon convection in SW, especially over the eastern part of theisland. The suppression of convection over the east in
SW may also be a response to the lower-level divergence associated with the rather anticyclonic component extended from the Borneo to Sulawesi at 925-hPa
winds as compared to the cyclonic circulation over thewest.
A prominent feature through these results is that leeward propagation occurs over the whole of the island.
A possible mechanism of the propagation may be theadvection by the background low-level wind. The inferred propagation phase speed is roughly close to the
700-hPa wind speed in each regime. The actual situation is far more complicated, however, because thecharacteristics of the propagating rainfall activity differ
between EE and SW, not only in the timing and duration of rainfall but also in dominant rain types andstorm heights. The complex land terrain, which is notzonally symmetric, and differences in moist static instability of the atmosphere between SW and EE complicate the situation and its analysis. In addition to thedevelopment of the propagating storm associated withthe mountain range as simulated by Satomura (2000),
the occurrence of the gravity wave due to the strong
diurnal signal over coastal land would be important forthe propagation over the offshore region as suggested
by Yang and Slingo (2001) and Mapes et al. (2003a).
Several mechanisms may be responsible for the behavior of the diurnal cycle, and a detailed investigationis needed to reveal the processes in organizing convection, including the numerical simulation of a highresolution nonhydrostatic model. The propagating diurnal signal over the island suggests that the diurnal
cycles of convection over the island are characterized
not by a simple response to diurnal land–sea thermal
contrast but also by the presence of propagating diurnal
disturbances that are presumably embedded within theintraseasonal disturbances.
Acknowledgments. The TRMM 2A25 dataset used inthis paper was provided by the Japan Aerospace Exploration Agency. The authors thank Prof. Kenji Nakamura for numerous helpful comments. This study1 APRIL 2006

ICHIKAWA AND YASUNARI

was partly supported by the twenty-first century COE
program Dynamics of the Sun–Earth–Life Interactive
System of Nagoya University, Nagoya, Japan. This
study is also supported by a Grant-in-Aid for Scientific
Research (A) (1), No. 14204044 from the Japan Society
for the Promotion of Science (JSPS).
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