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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D07109, doi:10.1029/2009JD012155, 2010

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ArticleDiurnal rainfall pattern observed by Tropical Rainfall
Measuring Mission Precipitation Radar (TRMM‐PR)
around the Indochina peninsula
H. G. Takahashi,1,2 H. Fujinami,3 T. Yasunari,1,3 and J. Matsumoto1,2Received 30 March 2009; revised 5 November 2009; accepted 16 November 2009; published 15 April 2010.

[1] This study addressed the diurnal cycle of rainfall during the summer monsoon season(May to September) around the Indochina peninsula, with a focus on the diurnal cycle’s
relationship to terrain. The investigation used 10 year (1998–2007) Tropical Rainfall
Measuring Mission Precipitation Radar (TRMM‐PR) observations. Results revealed thatthe diurnal variations in rainfall over the Indochina region had three distinct peaks. An
early afternoon maximum of rainfall occurred along the mountain ranges and on coastal
land. Evening rainfall was observed near the foot of mountain ranges, in a valley, and in abasin‐shaped plain; this rainfall weakened before the middle of the night. Heavy rainfall
in the early morning was found around the coasts over the eastern Gulf of Thailand and the
Bay of Bengal, as well as over the eastern Khorat Plateau. We found that nearly half ofthe total rainfall occurred in the early morning over these regions, which indicated that earlymorning rainfall significantly contributes to the climatological rainfall pattern. Note thatthe regions with early morning heavy rain did not correspond to windward faces ofmountains but to the windward plain or to an offshore area apart from the mountain ranges
in the windward direction. Additional examination of rainfall frequency and rainfall
intensity showed that this early morning heavy rainfall was composed of frequent orlong‐lasting rainfall events with a strong intensity.
Citation: Takahashi, H. G., H. Fujinami, T. Yasunari, and J. Matsumoto (2010), Diurnal rainfall pattern observed by TropicalRainfall Measuring Mission Precipitation Radar (TRMM‐PR) around the Indochina peninsula, J. Geophys. Res., 115, D07109,
doi:10.1029/2009JD012155.

1. Introduction[2] The diurnal cycle of rainfall and convective activity isa basic characteristic of the climate over the tropics [e.g.,
Wallace, 1975; Murakami, 1983; Nitta and Sekine, 1994; Yang
and Slingo, 2001; Nesbitt and Zipser, 2003]. The timing of
the diurnal cycle of the convective system is generally associated with the energy budget through the radiation budget;
this relationship may be an essential component of the Earth’s
climate. The diurnal rainfall cycle is closely associated withthe land‐sea distribution, the terrain, and the associated local
circulation.
[3] Over the past three decades, infrared radiation (IR)and gauge‐observed rainfall data have been used to examine
the diurnal variations in convective activities over the southeast
Asian monsoon region [e.g., Murakami, 1983; Nitta andSekine, 1994]. These pronounced diurnal variations during
1
Research Institute for Global Change, Japan Agency for Marine‐Earth
Science and Technology, Kanagawa, Japan.
2
Graduate School of Urban Environmental Sciences, Tokyo
Metropolitan University, Tokyo, Japan.
3
Hydrospheric Atmospheric Research Center, Nagoya University,
Nagoya, Japan.

Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2009JD012155

the Asian summer monsoon season have been found overthe Indochina peninsula, the Tibetan Plateau, the Bay of
Bengal, and the South China Sea [Nitta and Sekine, 1994].
Ohsawa et al. [2001] investigated diurnal rainfall overSoutheast Asia using gauge‐observed rainfall data andsatellite‐derived IR data. The rainfall observations included
a clear diurnal cycle over the Indochina peninsula, which
varied somewhat from the results derived from IR. Interestingly, Ohsawa et al. [2001] also found that late night andearly morning rainfalls were stronger than daytime rainfalls;
these results were obtained from rain gauge observations
from a few stations located inland of the Indochina peninsula.
Satomura [2000] simulated an eastward (leeward) moving
rainfall system using a two‐dimensional cloud‐resolving
model. This appears to partly explain the diurnal cycle ofrainfall inland of the Indochina peninsula. Recently, Hirose
and Nakamura [2005] used 6 year PR (Precipitation Radar)data from the TRMM (Tropical Rainfall Measuring Mission)
to show distinct diurnal cycles of rainfall over the entire
tropical Asian monsoon region. They found diurnal patternsof rainfall that are typical for the tropics, with an evening
maximum over land and an early morning maximum aroundthe coast. The evening rain over land consisted of small
rainfall systems, while the morning rain along the foothills of
the Himalayan mountains were large‐scale rainfall systems.

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TAKAHASHI ET AL.: DIURNAL RAINFALL CYCLE OVER INDOCHINA[4] Diurnal variations in cloud activity and rainfall over the
Asian monsoon region, including the Indochina peninsula,were investigated associated with the land‐sea distribution;
however, variations associated with regional scale orography
over the Indochina peninsula have not been investigated,even though these variations are strongly affected by regional
scale terrain (spatial scale ranging from several tens of kilometers to more than a hundred kilometers).
[5] Several earlier studies have used TRMM‐PR data toinvestigate the diurnal cycle of rainfall associated with
regional orography over the Asian monsoon region. For
example, Fujinami et al. [2005] investigated the diurnal
variations in rainfall over the mountain‐valley terrain of thesouthern Tibetan plateau. They found that rainfall activity
usually occurs over the mountains in early afternoon and overthe valley in late evening. Bhatt and Nakamura [2005]
showed that there was a late evening to early morning rainfall over the southward slope of the Himalayas. Singh andNakamura [2009] investigated diurnal rainfall variations
over the central Tibetan Plateau and found late afternoon peak
rainfall over hilly regions, and late evening to early morning
peak rainfall over valleys and lakes. These studies investigated the diurnal rainfall cycle associated with regional scale
orography, and speculated the existence of local circulations
over regions with sparse observation network. In other words,
the diurnal variations of cloud and rainfall activities provide
a means to visualize a portion of the local circulation, which
is useful for an understanding of local circulations associatedwith these variations over regions with a sparse observation
network.
[6] The use of long‐term TRMM‐PR observations fromspace allows the total amount of rainfall and the diurnal
rainfall cycle in the tropics to be investigated. The 10 year
average of these data can resolve the diurnal cycle of rainfall
and can capture the climatology of total rainfall [e.g., Hirose
et al., 2008]. The advantage of long‐term TRMM‐PR observations is that they have almost uniform quality over sea,land, and mountainous areas. Conversely, gauge‐observed,
IR‐estimated, and microwave‐estimated rainfalls are lessadvantageous for analyzing the total amount of rainfall anddiurnal rainfall cycles related to the regional scale terrain.
Rain gauge observations have been inadequate for describing
the diurnal cycle of rainfall over Indochina terrain and overthe sea, since their locations were limited to populated andlow‐altitude areas. The advection of upper level clouds [e.g.,
Inoue et al., 2006] interferes with the investigation of diurnal
cloud activity related to regional scale terrain. The advectionof the upper tropospheric clouds also adversely affects rainfall products from IR data. Recently, microwave imagers
such as TRMM Microwave Imager have been available forrainfall estimation; they are useful for observations overoceans, but their accuracy over land is poor. Although
TRMM‐PR observations of rainfall over the mountain ranges
may include uncertainties, possibly due to ground clutter, theability to detect clear diurnal variations over the mountain‐
valley regions shows that TRMM‐PR is sufficiently able toinvestigate diurnal variations in rainfall over the Indochina
region.
[7] Prior to the present study, the pronounced diurnal variations in convective activities over the Indochina peninsula

D07109have only been analyzed using gauge‐observed rainfall andIR‐based convective indices. Also, the diurnal variationsof rainfall related to regional scale terrain have not beenpreviously investigated. We used 10 year TRMM‐PR data
to investigate diurnal rainfall cycles over and around the
Indochina peninsula as they relate to the regional orography.
In addition, rainfall intensity, rainfall frequency, and theirdiurnal cycles were investigated to understand characteristicsof rainfall in this region. Previous studies have showed thatthe rainy season over the Indochina peninsula occurs during
5 months, from May through September [e.g., Matsumoto,
1997; Takahashi and Yasunari, 2006]; thus we focusedon that period. Section 2 describes the data used in thisstudy. Section 3 presents results of the TRMM‐PR observations. Possible mechanisms for the observed phenomena
are discussed in section 4, and conclusions are presented in
section 5.

2. Data[8] The primary rainfall data set used here was the TRMM‐
PR rainfall 3G68 product version 6, which is a gridded versionof the TRMM‐PR rainfall 2A25 product [Iguchi et al., 2000].
Hereafter, we refer to these rainfall data as TRMM‐PR. The
3G68 product includes the grid‐averaged hourly rainfall
amount, the grid‐accumulated numbers of rainfall pixels, andthe grid‐accumulated total number of observation pixels
every hour. These data had a 0.5 × 0.5 degree grid resolution within the tropics and subtropics (approximately
36.5°S–36.5°N). The 10 years of TRMM‐PR observations
spanned from 1998 to 2007. We defined a rainfall frequency
and a rainfall intensity to describe the characteristics of thediurnal rainfall cycle and the spatial distribution of rainfall
totals. The three parameters were calculated from the 3G68
product at each hourly time slot. The rainfall amount, RA,
was the total amount for 10 years divided by the total
observation number. The rainfall frequency, RF, was thenumber of rainfall pixels divided by the total number of pixels
for the TRMM‐PR observation, expressed as a percent. A
rainfall pixel was identified when the 2A25 algorithm judged
the existence of near‐surface rainfall. The rainfall intensity,
RI, was calculated as the mean rainfall amount divided by therainfall frequency. RI is the rain rate when a near‐surface
rainfall exists (near‐surface rain rate >0 mm h−1). Because thestudy area was limited to the Indochina peninsula, we usedunified local time along 105°E.
[9] As a second data set, we compared the amount of
rainfall from TRMM‐PR with gauge‐observed rainfall data.
We used a 30 year period of daily rainfall data (1971–2000)
that was collected as part of the Global Energy and Water
Cycle Experiment (GEWEX) Asian Monsoon Experiment(GAME), and the Monsoon Asian Hydro‐Atmosphere Scientific Research and Prediction Initiative (MAHASRI). Less
than 10% of the rain gauge data were missing at each station.
[10] To discuss the local circulation associated with diurnal
variations in rainfall over the Indochina peninsula, we used
3 years of surface wind speed and direction data (1998–2000)
observed at meteorological stations by the Thai Meteorological Department. Only 3 years of data were available toresolve the diurnal cycle. The wind observations were taken

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D07109similarities, a spatial correlation rs was calculated using thefollowing equation:
rs ¼

rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃrﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
XN
XNðx  xÞðyi  yÞ=ðx  xÞ2ðy  yÞ2 ;
i¼1 i
i¼1 i
i¼1 i

XN

where N is number of stations, xi is the 30 year average of
total rainfall measured from May through September at theith station, yi is the 10 year average of total rainfall observedby TRMM‐PR from May through September at the closest
grid to the ith station, x is the average for all xi, and y is theaverage for all yi. The spatial correlation coefficient rs was
0.78, which was significant at the 99.9% confidence limit.
This suggested that the TRMM‐PR observations substantially captured the climatology of the spatial distribution of
rainfall over the Indochina peninsula. The TRMM‐PR values
were slightly underestimated relative to the rain gauge measurements at some stations on the Indochina peninsula(Figure 3). The TRMM‐PR rainfall provides a grid average
Figure 1. Terrain elevation and geography of the study
area. Shading indicates terrain elevation.
at 3 h intervals. The data were also collected by the GAME
and MAHASRI activities.
[11] In addition, we used monthly mean horizontal winds
at 850 and 200 hPa, from the Japanese 25 year Reanalysis
Project (JRA25) [Onogi et al., 2007] data. This provided
data availability to study the relationship between the rainfall distribution and large‐scale atmospheric circulation.
JRA25 data for the Indochina peninsula were taken from the27 year period spanning from 1979 to 2005, for 5 months ofthe summer rainy season (May to September).

3. Results
3.1. Background Monsoon Circulations Overthe Indochina Region[12] The terrain features are shown in Figure 1, and thelong‐term (1979–2005) May–September mean winds are
depicted in Figure 2. At 200 hPa (Figure 2a), the easterlies
prevailed, associated with the circulation related to the
Tibetan High. However, at 850 hPa (Figure 2b), low‐levelmonsoon westerlies prevailed from the Bay of Bengal to theSouth China Sea. A large amount of rainfall was observedon the windward side of the north‐south mountain ranges
within the Asian monsoon region; this was suggested to becaused by the orographic lifting of the low‐level monsoon
westerlies [e.g., Xie et al., 2006].
3.2. Spatial Distribution of TRMM‐PR Rainfall[13] Prior to analyzing the diurnal rainfall cycle, we
examined the spatial distribution of the total rainfall during
the rainy season over the Indochina peninsula, comparing
data from TRMM‐PR (Figure 3a) with 30 year (1971–2000)
rain gauge climatology data (Figure 3b). The TRMM‐PR data
showed large amounts of rainfall occurring around the coastsof the eastern Bay of Bengal and the Gulf of Thailand, andalso west of the Annam mountain ranges (Figure 3a); thisagreed quite well with the peaks shown by the rain gauge
observations (Figure 3b). To further examine these spatial

Figure 2. Long‐term (1979–2005) summer rainy season(May–September) mean wind fields at (a) 200 hPa and(b) 850 hPa.

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Figure 3. (a) Summer (May–September) mean amount of
rainfall observed by TRMM‐PR. The amount of rainfall
was averaged for 10 years (1998–2007). (b) Summer meanamount of rainfall observed at meteorological stations for
30 years (1971–2000). The rain gauge stations were missing
fewer than 10% of the data.
(50 × 50 km2), whereas the rain gauge observations arepoint measurements; this could explain the differences
between the values.
[14] The lowest amounts of rainfall were found along theridges and over the leeward side of high mountains (Arakan,
Dawna, and Annam mountain ranges). Their total rainfall
during the summer monsoon season was less than 5 mm d−1.
The highest amounts of rainfall in the region (over 15 mm d−1)
were observed offshore in the eastern Bay of Bengal and in
the eastern Gulf of Thailand, and also at the windward sideof the Annam mountain range (eastern Khorat Plateau).
These areas of high rainfall corresponded to the windwardsides of the Indochina mountain ranges. Interestingly, thiscorrelation did not hold everywhere; low total rainfall wasobserved along both the windward and leeward faces of
the Dawna mountain ranges. It is notable from the mean
total rainfall distribution that the maximum rainfall did notoccur over the windward faces, but over the windward plain
or off shore.
3.3. Diurnal Pattern of Rainfall[15] As previous studies have found, a diurnal rainfall
cycle is dominant over the Indochina peninsula [Ohsawa etal., 2001; Hirose and Nakamura, 2005]. In this study, we
examined the 3 h mean diurnal cycle of 10 year climatological rainfall occurring from May through September(Figure 4). To facilitate the examination of diurnal rainfall

D07109patterns in low‐rainfall regions, the contribution rate for each3 h period is also shown in Figure 4. The contribution rate isdefined as the proportion of rainfall during a 3 h period to thetotal rainfall amount over 24 h.
[16] Starting at 0700–1000 local time (LT) (Figure 4, topleft), little rainfall was observed over the Indochina peninsula.
Early afternoon rainfall occurred along the mountain crestsand over coastal land at approximately 1300–1600 LT, thenweakened in the evening. At several locations where there is aplain in proximity to a mountain range (around the north of
Bangkok, and over a basin‐shaped plain around Tonle Sap
Lake in Cambodia), evening rainfall began at approximately
1600–1900 LT. The evening rainfall was strongest at 1900–
2200 LT, and weakened before the middle of the night. Heavy
rainfall over the eastern Khorat Plateau in northeastern
Thailand began around 2200–0100 LT. Note that the transition of rainfall activity is discontinuous from 1900–2200 LT
to 2200–0100 LT. The heavy rainfall over the eastern Khorat
Plateau peaked at 0400–0700 LT. In addition, early morning
rainfall (0100–0400 to 0400–0700 LT) over the sea near thecoasts occurred at the eastern Gulf of Thailand and at the Bayof Bengal. Heavy early morning rainfall over both land andsea weakened later in the morning. Note also that heavy
rainfall was observed over the eastern Khorat Plateau during
the early morning hours; this was consistent with rain gauge
observations [Ohsawa et al., 2001].
[17] Interestingly, we found that the spatial distribution of
rainfall during the early morning hours (0100–0400 LT and
0400–0700 LT) was very similar to the distribution of total
rainfall, particularly in the higher‐rainfall regions (Figure 3).
This similarity suggests that early morning rainfall is a significant contributor to the climatological rainfall pattern inthis region. The contribution of early morning rainfall to thetotal amount of rainfall is shown in Figure 5. Early morning
rainfall was defined as the amount of rainfall occurring
over 8 h, between 0100 and 0900 LT. The regions where
the proportion of early morning rainfall was nearly 50% werelocated near the coast of the eastern Gulf of Thailand and the
Bay of Bengal, and over the eastern Khorat Plateau. The
regions of high overall rainfall (Figure 3) and early morning
rainfall basically coincide (Figures 4 and 5).Our results
indicate that an understanding of early morning rainfall
would deepen our understanding of the basic mechanism of
monsoon rainfall.
3.4. Diurnal Cycle in Surface Winds[18] An analysis of the surface winds over Thailand canbe used to discuss the local atmospheric circulations associated with the diurnal rainfall variations. Diurnal components of winds are calculated as the difference between the
3 year averaged daily mean winds and the 3 year averaged
3‐hourly winds (Figure 6). At 1000 LT and 1300 LT, almost
all winds flow toward mountain ranges and can converge
over the mountain crests. This pattern continues until 1600 LT.
At 1900 LT, the diurnal components of the surface winds at
almost all stations have drastically changed direction to flow
toward basin‐shaped plains; it is likely these are downslope
winds. Also, westward winds have developed over the central
Khorat Plateau at 1900 LT, and these westward winds converged with the southward winds north of Bangkok at both1900 and 2200 LT. Along the coast around Bangkok,
northward winds were observed at 1900 LT. By 0100 LT,

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D07109Figure 4. The 3 h mean diurnal cycle of 10 year climatological rainfall (TRMM‐PR) for May–September(1998–2007). The colors indicate the intensity of rainfall (mm h−1). The thick contours denote the contribution rate, which is defined as the proportion of the 3 h rainfall to the total rainfall; the 20% and
30% contours are shown.
the diurnal components in the surface winds have becomesouthwestward over almost the entire Indochina peninsula,and they continued until 0700 LT. The southwestward winds
along the coast were observed from 2200 LT to 0700 LT.
[19] The winds toward the mountain ranges explain theoccurrence of rainfall over the mountain ranges during early
afternoon. Also, the convergence between the westward andsouthward winds is consistent with the evening rainfall
north of Bangkok. These results clearly indicate that there
are different mechanisms for onset of the evening rainfall
compared with that of the early afternoon rainfall. This difference was not discussed in previous studies which described
them as afternoon‐evening rainfall. Also, it is possible thatthe northward winds from the Gulf of Thailand enhanced theconvergence and supported the trigger of evening convective

activities north of Bangkok, from 1900–2200 LT. As we
mentioned above, diurnal variations in rainfall have a discontinuous transition between 1900–2200 LT and 2200–
0100 LT. This timing corresponded with the disappearance of
the wind convergence north of Bangkok. These diurnal windcharacteristics probably affect the diurnal rainfall variations
over the Indochina region. However, the mechanism of earlymorning rainfall is still unclear, which will be discussed in
section 4.
3.5. Rainfall Frequency and Rainfall Intensity[20] Previous studies have investigated only the climatological mean diurnal rainfall pattern, but a given rainfall
amount may be caused by a larger number of weaker rainfall
events or a smaller number of stronger rainfall events. To

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D07109Figure 5. The contribution of the early morning rainfall to
the total rainfall. Thick contours denote the proportion of theamount of rainfall over 8 h, from 0100 to 0900 LT, to thetotal rainfall.
understand the characteristics of the diurnal rainfall cycle
and the spatial distribution of rainfall totals, we investigatedthe daily mean rainfall frequency and the rainfall intensity(Figure 7).
[21] The spatial distributions of both the rainfall frequency (Figure 7a) and rainfall intensity (Figure 7b) werequite similar to that of the total rainfall (Figure 3a). Over

Figure 7. Spatial distribution of the daily mean (a) rainfall
frequency and (b) rainfall intensity.

Figure 6. Diurnal variations of surface winds observed at meteorological stations of TMD. Diurnal components of wind are shown. The number in the top right corner of each plot indicates local time. The bottom
right plot shows daily mean (DM) winds. The wind direction is indicated in the direction of an arrow, andwind speed is shown in gray color. The diurnal component of wind (the daily mean wind) is shown withupper (lower) shading scale.
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D07109Figure 8. Diurnal cycle of total amount of rainfall, rainfall frequency, and rainfall intensity, averaged
over the regions. The gray thick line indicates the diurnal cycle of total amount of rainfall. The black
dashed line denotes the rainfall frequency, and the black solid line shows the rainfall intensity. The rangesof total amount, rainfall frequency, and rainfall intensity are given by RA, RF, and RI, respectively. The
region locations are shown in the top left plot.
the eastern Bay of Bengal (98°E, 14°N), total rainfall wasover 15 mm d−1, which was approximately 4 times greaterthan the rainfall over the Dawna mountain range in western
Indochina (99°E, 15°N). The rainfall frequency over theeastern Bay of Bengal was about 15–20%, indicating thatits rainfall occurred twice as frequently as the rain over the
Dawna Mountain range. The rainfall intensities over theeastern Bay of Bengal and over the Dawna Mountain range
were about 3.5–4.5 mm h−1 and under 2.5 mm h−1, respectively. Thus, both the rainfall frequency and the rainfall
intensity contributed to the climatological rainfall pattern.
[22] The diurnal cycles of the total amount of rainfall, therainfall frequency, and the rainfall intensity are shown in
Figure 8. These three components are presented as averages
over each of the five regions shown in Figure 8 (top left).
Regions 1, 2, 3, 4, and 5 represent the coast of the eastern
Bay of Bengal, the eastern Khorat Plateau, the coast of theeastern Gulf of Thailand, around Ho Chi Minh City in Viet
Nam, and the Dawna mountain range, respectively. The
sizes of Regions 3 and 5 were small since only the windward coast and the Dawna mountain region were examined.
In Region 1, the total rainfall peaked at 0500 LT, corre-

sponding to peaks of rainfall frequency and rainfall intensity,with the frequency providing the more significant contribution. Region 2 shows a nocturnal maximum of rainfall, with
contributions from both the rainfall frequency and the rainfall
intensity. In Region 3, two rainfall peaks were observed, aminor peak in the evening and a major peak in the earlymorning. Region 4 and Region 5 had rainy peaks occurring in
the early afternoon, which could be distinguished from theevening rainfall. The rainfall intensity during the early afternoon in Regions 4 and 5 is weaker or comparable to therainfall intensity during early morning. These results indicated that the heavy early morning rainfall over Regions 1–3(the coast of the Bay of Bengal, the eastern Khorat Plateau,
and the coast of the Gulf of Thailand) consisted of frequentor long‐lasting rainfall events with heavier rainfall intensity.
This is consistent with the results of Hirose and Nakamura[2005], who reported the frequent detection of large rainfall
systems over these regions.
[23] Regions 1–3, which have dominant early morning
maximums of rainfall, also showed some rainfall during thedaytime. Conversely, in regions with daytime maximums,
the rainfall frequency is reduced during the morning hours.

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D07109Figure 9. Time‐longitude cross section of rainfall over the Indochina peninsula along (a) A–A′, (b) B–B′,
and (c) C–C′ as shown in (d) cross section. The bottom of each plot in Figures 9a–9c shows the elevation of
the terrain along each cross section.

Although these trends contributed to the total rainfall, thediurnal cycles of rainfall amount were prominent.

4. Discussion[24] In this section, we discuss the mechanisms of theclimatological diurnal rainfall cycle and how they relate to
the Indochina terrain. Three hour running mean values ofthe diurnal rainfall cycle are plotted in Figure 9. Early afternoon rainfall occurred at approximately 1500 LT over themountain ranges (106°E in Figure 9a, 108°E in Figure 9b,
and 104°E, 108.5°E in Figure 9c). These rainfalls likely
resulted from the convergence of thermally induced upslope
winds. Evening rainfall occurred over the basin‐shaped plainsof Cambodia (104.5°E–107.5°E in Figure 9c) and north of
Bangkok (101°E in Figure 9b), and the valley between the
Pegu and Dawna mountain ranges (97°E in Figure 9a). The
evening rainfall north of Bangkok was caused by the convergence between the southward downslope winds and thewestward winds from the Khorat Plateau (Figure 6). Theseevening rainfalls that occurred over a plain in proximity to amountain range likely come from the mountain range orrelate to a downslope wind. It is possible that the evening
rainfall over the plain of Cambodia is also related to aland‐lake circulation over and around the Tonle Sap Lake.
Rainfall activities along these sections decrease after theevening, corresponding to the discontinuous transition of
rainfall from 1900–2200 LT to 2200–0100 LT (Figure 4).
This suggested that the eastward traveling rainfall systemsuggested by Satomura [2000] was not climatologicallyobserved from the TRMM‐PR results. Around 0500 LT,
heavy rainfall was observed over the eastern Bay of Bengal(94°E in Figure 9a, 97.5°E in Figure 9b), over the coast of the
Gulf of Thailand (103°E in Figure 9c), and on the windwardside of the Annam Mountain range (104.5°E in Figure 9a,
106°E in Figure 9b). Note that the evening rainfall did not
persist through the middle of the night, so that the earlymorning rainfall was likely newly developed.
[25] Note also that the early morning rainfall does notdevelop over the windward faces, but develops over thewindward plain or over the sea close to shore. This appeared
to be related to the convective activities over the Bay of
Bengal, in which the convections developed over the sea
near the shore [Zuidema, 2003]. Observational evidence
suggests that the climatological rainy peak was not causedby orographically forced upslope winds. If it were, highrainfall would occur over the windward slopes, and it would
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rainfall region would occur during the daytime, due to thedaytime enhancement of upslope winds caused by the superposition of the mean monsoon westerlies and the thermallyinduced upslope winds. Since this was not observed, itsuggests that the concept of monsoon rainfall being causedby orographic lifting of the monsoon westerlies should bereconsidered to include the processes of the diurnal cycleof rainfall.
[26] Grossman and Durran [1984] reported that offshore
convective activity peaked west of the Western Ghats
Mountains of India; their results correspond to our findings oflarge rainfall away from windward slopes, over and aroundthe Indochina peninsula. Also, the atmospheric structure overthe Indian monsoon region, namely the low‐level westerlies
and the upper‐level easterlies, corresponded to similar structures over the Indochina monsoon region. Although Grossman
and Durran [1984] did not discuss the diurnal rainfall cycle,
the observational evidence suggests a mechanism underlying
the onset of nonwindward and early morning rainfall.
[27] The mechanism of early morning rainfall around thisregion is still unclear. In a classical paper, Houze et al. [1981]
discussed the offshore rainfall during early morning, andproposed that the onset of offshore rainfall was due to aconvergence of land breeze and low‐level monsoon flow.
This mechanism might explain the early morning windward
rainfall, although the atmospheric structure was different.
Because early morning rainfall was found only over thewindward side of the mountain ranges, the mountain breezes
may contribute to the local circulation in this region. Over the
Indochina peninsula, Satomura [2000] simulated an eastward(leeward) moving rainfall system using a two‐dimensional
cloud‐resolving model. This moving rainfall system might
explain the peak timing in the diurnal cycle of rainfall overthe eastern Khorat Plateau, although the simulated peakoccurred earlier than the observed peak. Kataoka andSatomura [2005] used a three‐dimensional cloud‐resolving
model to suggest a possible mechanism for a nocturnal
rainfall system in Bangladesh, although the onset and peaktimes of their simulated rainfall were somewhat different
from the early morning rainfall times in our study. The
rainfall system in their study developed during the night, due
to greater atmospheric instability at night than during theday. Also, in conjunction with the off‐slope peak of rainfall,they simulated the onset of a rainfall system at the foot of
the windward mountain ranges without katabatic winds,although katabatic winds had been considered a major candidate for the rainfall mechanism. The authors suggested
another possible mechanism, in which a cold pool in frontof a mountain range provides the same effect as the mountain
range.
[28] Although an analysis of the surface winds could
partially discuss the local circulation associated with thediurnal variations in rainfall, the discussions are limited tothe surface winds. To fully understand the mechanism behindclimatological early morning rainfall, additional observationsof rainfall and local circulations are necessary.

5. Conclusion[29] This study used 10 year (1998–2007) TRMM‐PR data
to investigate the diurnal rainfall cycle, rainfall intensity, andD07109rainfall frequency over the Indochina peninsula during thesummer monsoon season (May to September), with a focus
on their relationship to the regional scale terrain. To discuss
the local circulations associated with the diurnal cycle inrainfall, we also used 3‐hourly surface wind observations
over Thailand.
[30] We used rain gauge measurements over land to verify
that the 10 year TRMM‐PR observations can capture theclimatological mean pattern of rainfall. We examined thedetailed diurnal pattern of rainfall and its association with
the Indochina terrain, using 3 h mean TRMM‐PR rainfall
observations. Previous studies showed an evening maximum of rainfall over land and a morning maximum over thesea. Our results revealed that the diurnal variations in rainfall over the Indochina region can be classified into three
distinct peaks. We also observed early afternoon rainfall overmountain ranges and over coastal land. Evening rainfall
occurred around 1900–2200 LT over the foot of mountain
ranges, in a valley (north of Bangkok), and over a basin‐
shaped plain (Cambodia). The results of the surface wind
analysis suggested that the evening rainfall was associatedwith downslope winds from surrounding mountain ranges,
and that this evening rainfall was generated by a separate
mechanism than early afternoon rainfall. Heavy early morning
rainfall was observed near the coasts over the eastern Bay of
Bengal and over the eastern Gulf of Thailand, as well as overthe eastern Khorat Plateau. It was found that nearly half ofthe total rainfall occurred in the early morning over theseregions. Therefore, we concluded that early morning rainfall
is the primary component of the climatological mean rainfall pattern. The examination of rainfall frequency, rainfall
intensity, and their diurnal cycles showed that heavy rainfall
during the early morning over the higher‐rainfall regions
consisted of frequent or long‐lasting rainfall events with astrong intensity. Note that regions with dominant earlymorning rainfall events also had some rain during the daytime, which contributed to the large total amount of rain.
These new findings were not reported in previous studies thatinvestigated only the diurnal phase and amplitude of rainfall.
[31] It was noteworthy that evening rainfall did not persist
through the middle of the night, suggesting that the earlymorning rainfall may have redeveloped, although the mechanism for triggering early morning rainfall has not beenclarified.
[32] Finally, heavy rainfall events were observed in theearly morning, away from windward mountain faces. Thiscannot be explained by the traditional understanding of monsoon rainfall, that it is caused simply by the orographicallyforced uplifting of the monsoon westerlies. Therefore, theconcept of monsoon rainfall should be reconstructed toinclude the processes of the diurnal cycle of rainfall.
[33] Acknowledgments. The rain gauge data were kindly provided
by the Department of Meteorology and Hydrology in Myanmar, the Thai
Meteorological Department, the Department of Meteorology and Hydrology
in Laos, the Department of Meteorology in Cambodia, and the National
Hydrometeorological Service of Vietnam. The surface wind data were also
provided by the Thai Meteorological Department. These data were collected
as part of the GAME‐Tropics (GAME‐T) and Monsoon Asian Hydro‐
Atmosphere Scientific Research and Prediction Initiative (MAHASRI) projects. We thank the agencies and people related to those projects. TRMM‐PR
rainfall data (3G68 product) were obtained from ftp://trmmopen.gsfc.nasa.

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TAKAHASHI ET AL.: DIURNAL RAINFALL CYCLE OVER INDOCHINA

gov/pub/3G68 via FTP. We also thank three anonymous reviewers fortheir helpful suggestions and comments.

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