Time-space characteristics of diurnal rainfall over Borneo and surrounding oceans asobserved by TRMM-PR
Hiroki Ichikawa

ANDTetsuzo Yasunari

Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan
Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
E-mail: s040101d@mbox.nagoya-u.ac.jp, Phone: +81-52-789-5470, Fax: +81-52-789-3436

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how the low-level prevailing wind modulates the diurnal

Introduction

The Indonesian Maritime Continent is characterized as

cycle of rainfall over the island. The detail analysis wasa huge amount of rainfall throughout year, and plays themade by using high resolution TRMM PR data.

essential role as a center of atmospheric heat source ofearth climate system. Much of the active deep

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convection occur associated with the several time-space

This study uses near-surface rainfall rate products

circulation system and convective organization over the(2A25ver.5) from the TRMM PR for 5 years(1998-2002).

Borneo and surrounding oceans, which is located overRainfall data are girdded by computing monthly total

the central part of the Maritime Continent (Chang et al.

rainfall for 0.05 * 0.05 and 0.5 * 0.5 grid boxes for each2005),.

local time at each grid point in order to examine spatial

Satellite

observed

convective

cloudsshow

Data and Study method

theand temporal variability of diurnal rainfall activity.

pronounced diurnal cycle variations over the Maritime

Furthermore, composite analysis is performed toContinent (Nitta and Sekine 1994, Yang and Slingo

investigate diurnal rainfall variations in different wind2001). One of the major characteristics observed in theregimes centered on Borneo Island. 5years day were

diurnal variability of convection over the Maritimepartitioned into easterly and westerly regime based on

Continent is the phase propagation of the diurnal signal

the 850hpa zonal wind speed over the island. PR(Yang and Slingo 2001). The low-level prevailing flow issurface rainfall are composed for each wind regime.

the important factors in modulating diurnal cycles.
Houze (1981) pointed out that land breeze convergencewith northeasterly monsoon generate the offshore
convection

northwestern sea of Borneo island. Oki and Mushiakedays associated with the passage of the intra-seasonal(1994) showed a morning peak of rainfall on windward

scale disturbances called as Madden-Julian Oscillation

coast for monsoon wind over Malay Peninsula. However,(MJO). The low-level westerly (easterly) over the island

more systematic description of large scale impact ofcorresponds to the period of active (inactive) convective

environmental flow has not been made. Furthermore,

phase over the Maritime Continent in the intraseasonal

details

variability. Associated with the large-scale frequent

propagation

tomorninghave

notoverThe pronounced variation in the zonal wind over theBorneo Island occurs with a periodicity of about 30-60themidnightResults

theofafter3

yetbeeninvestigated over Borneo.

(inhibited)

The purpose of this study is to present temporal andmoistened (dry) condition in the westerly (easterly)

spatial variations in diurnal rainfall over Borneo andregime.

adjacent oceans. A particular question addressed wasconvection,

theregion

experiencesaIn the different wind regimes, rainfall amountdistribution exhibits a systematic change (Fig 1a).

rainfall (all rain - black solid, convective rain - red solid,

Interestingly, the rainfall amount tends to increase overstratiform rain - black dot) averaged along the rectangle

the leeward side of the island. Compared to the westerly

in Fig. 2 to elucidate storm/rainfall evolution in theregime, much more rainfall can be observed over the

east-west propagation. The convective characteristics

island in the easterly regime. On the other hand, in the

differ between easterly and westerly not only in thewesterly regime, there is a wide spread rainfall area

timing and duration of storms but also in the verticalover the South China Sea off the northwest coast region,

development and rainfall type. During easterly regime

which correspond to strong cyclonic circulation. In order(Fig. 3 right), shallow convection develops, producing

to compare the vertical structure of convection to therainfall over the coast and central mountain range in therainfall activity, the spatial distribution of mean storm

afternoon. This coastal convection gradually migrates

height (top of precipitation) is shown in Fig 1 (b).

towards

Associated with leeward enhancement of rainfall, the1700-1900LT. Convection over the mountains develops

deeper storms tend to occur over the leeward side of thevertically in the evening (1700-1900 LT), and deeperisland.

notstorms begin to appear over western Borneo fromnecessarily appear where much rainfall amount is given,

2000-2200 LT. Convection over land continues until

and it is apparent that deeper storms develop in themidnight with higher storm development and a growing

easterly regime than the westerly regime. In addition, it

area of rainfall as the storms propagate westward. Large

is interesting to note that a lot of precipitation over theamounts of stratiform rain fall over land at night. In fact,

South China Sea region is brought by relatively lower

stratiform rainfall amounts exceed convective rainfall

storm.

amounts after 0200-0400 LT. Before convection overHowever,higherstorm

heightdoes

In order to reveal how the leeward increase of rainfall
isassociatedwithrainfall

from1400-1600

LTto

land ceases, offshore activity starts, between 2300 and0100 LT over the west coast. That convection rises totime-longitude cross diagrams of rainfall are shown innearly 10 km during the morning hours. Most of theFig 2. A notable feature shown in this figure is arainfall is convective, which dominates until 0800-1000

systematic change of the direction in rainfall propagation.

LT and gradually decreases.

In both regimes, afternoon rainfall tends to spread overIn the SW regime (Fig. 3 left), the onset of convection

wide-ranging

area

is later than in EE; shallow convective clouds appear at

propagates to westward (eastward) during easterly

1400-1600 LT over the west coast and the mountain(westerly). Thus convection persists until midnight overrange. The coastal storms gradually evolve into

the leeward side of the island, which results in morewidespread shallow storms at 2000-2200 LT over thedaily rainfall. The inferred phase speed of thewestern part of the island. Over the eastern part of thepropagation is about 10m/s, which is roughly close toisland, deeper storms develop at 1700-1900 LT over thethe 700hpa wind speed. In the westerly regime, themountain range. These storms evolve and propagate

west-ward propagating local rainfall system developed

eastward from 2000-0100 LT.

in midnight, and gives much precipitation over theregime, in which stratiform rainfall overspreads much ofwestern sea region.

the land (Fig. 3 right), rainfall in SW is predominantly

Figure 3 shows the time sequences of the verticalconvective with a sharp maximum. After 0200-0400 LT,

distribution of storm height frequency and surface

convection over land ceases, and rainfall propagatesbut

diurnalBorneoactivity,

area,

thecentral

suppressed

rainfall

In contrast to the EEeastward, offshore of the east coast. However, theseregime, stratiform rains are widespread over the island

coastal storms do not develop vertically to the extent ofat midnight, whereas in the westerly regime, local

storms over land. The dominant rain type graduallyconvective rainfall dominates. Over offshore regions,changes from convective to stratiform. Relatively

convective rainfall initially dominates then gradually

shallow storms appear over regions offshore of the west

decreases in both regimes, while the storms develop

coast at 2300-0100 LT. Those storms then spread tointo deeper convective systems in the easterly regime.

the South China Sea between midnight and morning.

In addition to leeward rainfall propagation, shallow
storms develop over the South China Sea region during

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the westerly regime, resulting in heavy precipitation fromSummary and Conclusions

Five years of Tropical Rainfall Measuring Mission

midnight through morning.

(TRMM) Precipitation Radar (PR) data were used toThe propagating diurnal signal over the island

investigate the time and space characteristics of thesuggests that the diurnal cycle of rainfall over the island

diurnal cycle of rainfall over and around Borneo. The

is characterized not by a simple response to diurnal

diurnal cycle shows a systematic modulation that island-sea thermal heat contrast but by the interaction

associatedwith the large-scale intraseasonal disturbances.

withintraseasonalvariability

inthe

large-scale circulation pattern, with regimes associatedwith low-level easterlies or westerlies over the island.
The lower-tropospheric westerly (easterly) components
correspond to periods of active (inactive) convection

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over the island that are associated with the passage of

Chang, C. -P., P. A. Harr, and H. -J. Chen, 2005:

intraseasonal atmospheric disturbances related to theSynoptic disturbances over the equatorial South

Madden-Julian Oscillation.

China Sea and Western Maritime Continent during

A striking feature is that rainfall activity propagates atabout 10 m

Referencesboreal winter. Mon. Wea. Rev, 133, 489-503.

s¯¹ to the leeward side of the island

Houze, R.A., S.G. Geotis, F.D. Marks, and A.K. West,between midnight and morning. Propagation occurs

1981: Winter monsoon convection in the vicinity ofover the entire island, causing a leeward enhancement

north Borneo. Part1: Structure and time variation ofof rainfall. However, the spatial characteristics of theclouds and precipitation. Mon. Wea. Rev, 108,propagation and of storm development

1595-1614.

are not

symmetric for the easterly and westerly regimes. In theNitta, T. and S. Sekine, 1994: Diurnal variation ofeasterly regime, rainfall persists over the island untilconvective activity over the tropical western Pacific.

midnight, and offshore convection off the west coast

J. Meteor. Soc. Japan, 72, 672-641.

develops independent of convection over the land,Oki, T. and K. Musiake, 1994: Seasonal change of thestarting before the activity over land ceases. In contrast,

diurnal cycle of precipitation over Japan andthe westerly regime is characterized by rainfall activity

Malaysia. J. Appl. Meteor., 33, 1445-1463.

that propagates directly from the island to offshore of theeast coast of the island. Furthermore, the verticalstructure of the convection/rainfall systems differs
remarkably between the two regimes. In the easterly

Yang, G. Y., and J. M. Slingo, 2001: The diurnal cycle inthe Tropics. Mon. Wea. Rev., 129, 784-801.

6Figures(a)Daily accumulated rainfall
Easterly

Westerly

[m/s]

[m/s]
[mm/day](b)Daily average Storm Height
Easterly

Westerly[m]Fig.1 (a) Daily rainfall amount distribution and (b) mean Storm Height (left; easterly regime, right; westerly regime).
[M]

Easterly

WesterlyA

[M]

LST

LST

Topography[M]

Daily Rainfall
Topography[mm/day]

Daily Rainfall
Topography

Fig.2 Time-longitude cross diagram of rainfall along the rectangle A averaged in domain perpendicular to the central
mountain range (left; easterly regime, right; westerly regime).

Fig. 3 Time sequence of vertical distribution of storm height frequency from 3.5km to 11km over Borneo averaged
along rectangle A (in Fig. 2) in easterly (right) and westerly (left) regime. Surface rainfall amount are also described ineach time bins (black solid: all, red solid: convective, black dot: stratiform). Land terrain is described at bottom area
(Sea region are shown by blue line).