Pre-monsoon rains and onset of monsoon over the Indochina Peninsula* Jun MATSUMOTO1 and Masashi KIGUCHI2(1: Dept. of Earth & Planetary Science, Univ. of Tokyo, 2: Graduate School of Science, Kyoto Univ.)*Department of Earth & Planetary Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo
113-0033, JAPAN, e-mail: jun@eps.s.u-tokyo.ac.jp
Abstract
The rainfall phenomena in the pre-monsoon season over the Indochina Peninsula are investigated using geopotential height, windand moisture fields by the NCEP/NCAR reanalysis, and OLR, precipitation and GPS data during the GAME-IOP year, 1998. Theintermittent rainfall events during the pre-monsoon period are brought in wide region over the central part of the Indochina Peninsula. Inearly and middle April, and early May, the lower OLR regions, representing the heavy convective activity, extend southward from themid-latitude zone to the Indochina Peninsula. The composite analysis in these three rainfall events show that these rainfall events arebrought by the trough passage in the upper troposphere which migrates eastward in the mid-latitude westerlies along the southern
periphery of the Tibetan Plateau. The moisture inflow in the lower troposphere related with these rainfall events is found to come mainly
from the south and east, which shows sharp contast with the situations of inflow mainly from the west after the monsoon onset in middle
May.
Former climatological studies have shown that the summer rainy season starts earlier in the inland area of Thailand and the onset ofsummer rainy season is prior to the establishment of the summer monsoon circulation over the Indochina Peninsula. Part of such rainfall
is shown to be brought by an intermittent rain phenomenon under the mid-latitude westerlies prior to the monsoon onset.
keywords: Pre-monsoon rain, Indochina Peninsula, mid-latitude westerly, moisture flux, wind field
1.

Introduction

The appearance of strong convection over the Indochina
Peninsula, indicative of the earliest start of the summer
monsoon over the Asian continent, has been documented insome recent literatures. The monsoon onset over theIndochina Peninsula occurrs in early or middle May andmigrates northward from the southern part of the Bay ofBengal [Wang and LinHo, 2002]. Matsumoto [1997] pointed
out that the onset of rainy season is earlier in late April when
Indochina is still under the mid-latitude westerly regime in
the inland region of the Indochina Peninsula than in thecoastal region along the Bay of Bengal. These studies,
however, only discussed the onset of the rainy season on amean field. In general, it is well known that the rainfall
phenomena during summer monsoon over the Asian summer
monsoon region are produced by the southwesterly monsoon.
However, the rainfall phenomena before the establishment ofmonsoon circulation in each year have not been studied,although they may contribute to the establishment ofmonsoon circulation. Thus, this paper elaborates on thecharacteristics of pre-monsoon rains over the Indochina
Peninsula during the GAME-IOP (GEWEX (Global Energyand Water Cycle Experiment) Asian Monsoon Experiment Intensive Observation Period), 1998.

2. Data
The daily rainfall data used in this study are obtained by
the GAME-Tropics dataset for the GAME-IOP year, 1998.
They are observed and archived by the meteorologicaldepartments of Thailand, Laos, and Cambodia. The station
distribution is shown in Figure 1.
Moreover, we use the daily mean precipitable water in
the atmosphere derived from the Global Positioning System(GPS) observations conducted by the GAME-Tropics

Fig. 1: Map of the study area. Light and dark shadings
indicate elevations greater than 200 and 1000 m,respectively. Open circles are the rainfall stations usedin this study. Black triangles are the GPS stations usedin this study. Northern and southern one indicate
Chiang Mai and Bangkok, respectively.
project in the same year. These observation stations are at
Bangkok and at Chiang Mai. Furthermore, we investigate
the atmospheric conditions using daily mean reanalysis dataof various meteorological elements produced by theNational Centers for Environmental Prediction/National
Center for Atmospheric Research (NCEP/NCAR). As anindicator of the convective activity, the daily meaninterpolated OLR data provided by the National Oceanic andAtmospheric Administration (NOAA) are utilized. The
spatial resolution for both data is 2.5degree.

3. Rainfall during the pre-monsoon period
Figure 2 shows the time series of area averaged station
precipitation (44 stations within the box of 12.5-17.5N,
100-105E shown by the rectangular box in Figure 1) for theperiod from March to May 1998. This region is selected as arepresentative region of the central area of the Indochina
Peninsula. The intermittent rainfall events during thepre-monsoon period are seen in early March (marked by A),
the period from late March to early April (marked by B, andC), middle April (marked by D) and early May (marked byE). According to the distribution of precipitation (notshown), these phenomena are rainfall events in which
rainfalls occur not only at some points but also in wide areaof research domain.

4. Synoptic condition
4.1. Trough passage in the upper troposphereAs a factor of convective activity, we can point out thepassage of trough in the upper troposphere that may bringinflow of cold air in the upper troposphere and the upward
flow in the troposphere. Then, in order to investigate therelationship between the precipitation phenomena and thetrough in the upper troposphere, the temporal changes ofgeopotential height field at 300 hPa layer are analyzed. The
troughs appear during the rainy events in the pre-monsoon
period (A, B, C, D and E).
In order to reveal the wind circulation at 300 hPa layer
on rain days, we carry out the composite analysis. Figure 3
shows the composite map of wind field at 300 hPa layer onFig. 2: Area averaged rainfall (black bar) and the ratio ofthe stations where rainfall is observed (dashed line) fromMarch to May in 1998 based on the daily rainfall
amount at 44 stations shown in Figure 1. The dashed linein the non-rain days is neglected. Unit of the rainfall andthe ratio are mm/day and %, respectively. The shading
indicates the period when the ratio of stations whererainfall is observed is exceeding 10%.
the rain events(a), that on the non-rain days during theperiod from March to monsoon onset(b), and the difference
between the two(c). The period during the rain events using
this composite analysis is the periods C, D and E when thestrong convection occurred. It is clear that the trough at 300
hPa stretches from Myanmar to the eastern Bay of Bengal,
that is the west of Thailand, on rain events. In contrast, on
non-rain days, the strong westerly jet appears over northernpart of the Indochina Peninsula and trough is not observed
around the Indochina Peninsula. The difference between therain events and the non-rain days shows that there is a
cyclonic anomaly over the Indochina Peninsula. This
property of the cyclonic anomaly means the presence of thetrough. It is clarified that the existence of the trough in theupper troposphere is important for the convective activity.
Furthermore, in order to show the migration of thetrough in the upper troposphere associated with the rainfall
events, we compose daily wind circulation at 300 hPa. It isclarified that this trough is generated in the eastern India
located in the westerly jet trough in the southern edge of theFig. 3: Composites of the 300 hPa wind vector and the speed ofzonal Plateau
wind for(a)shown).
rain periods C, D and E, and (b) theTibetan(notnon-rain days from March to mid-May, 1998. The bottom panel (c) shows the difference between (a) and (b). The
difference is calculated by subtracting (b) from (a). Contour interval is 10 m/s. Light and dark shadings in (a) and (b) are
more than 20 m/s and more than 40 m/s, respectively. And light and dark shadings of (c) are less than 0 m/s and less than
20 m/s, respectively.

4.2. Moisture convergence in the lower troposphere

Fig. 4: The vertical-time cross section of height anomaly fromthe 30-day running mean value (contoured) and ω (shaded) at
15N, 102.5E for the period from March to May in 1998. The
unit of height anomaly and ω are m and 10-2 Pa/s,respectively. Contour interval of height anomaly is 15 m lessthan 0 m. Shading is exceeding 5x10-2 Pa/s. Shading interval
is 5x10-2 Pa/s. Shadings in the upper part of figure indicate theperiods when the rainfall events occur.

To reveal the change of the vertical circulation associatedwith the trough passage in the upper troposphere, the relation
between the anomalous geopotential height and the upward
flow ω is shown. Figure 4 shows the vertical-time cross sectionof the anomalous geopotential height and ω. The upward flow
is strengthened when the negative value of the anomalous
geopotential height indicating the trough passage during thepre-monsoon period. In contrast, the trough passages andupward motion are not coincident after the monsoon onset(during the period F). Therefore, it can be said that the upwardflow associated with the trough passages in the upper
tropospheric westerlies during the pre-monsoon period isstrengthened over the central part of the Indochina Peninsula.
From the above analysis, it is clarified that the trough in theupper troposphere moves eastward along the southern edge ofTibetan Plateau.

The convergence of moisture is indispensable for theconvective activity that brings precipitation. Therefore, we
examine distribution and transportation of moisture. We
indicate the time series of the precipitable water at Bangkok andChiang Mai derived from GPS observations (not shown). GPSobservation begins from April in 1998. The variations of twokinds of precipitable water data derived from the reanalysis andGPS observations are, in general, similar except for themagnitude of the oscillation, so we utilized the reanalysis data
for further discussion.
The precipitable water increases ahead of the periods B, C,
D and E. The moisture amount over the Indochina Peninsula
increases before the occurrence of active convection. From thetime series of vertically integrated moisture inflow from eachdirection, the relationship between the moisture inflow from theeast and rain events is focused. The moisture inflow from theeast is positive value during the periods A, B, C, and E. The
moisture inflow from the west and east become weak for theperiod D. In contrast, the moisture inflow mainly from the west
and south during the period F. On the other hand, themeridional moisture inflow is almost from the south during theperiod from March to May. There is the strengthening ofmoisture convergence before the appearance of strong
convection for the periods B, C and D. It is thought that thestrengthening of moisture convergence produces the increase of
precipitable water. As shown in the above analysis, it is seenthat the moisture convergence during the rain events is broughtby the moisture inflow from the east and south.
To reveal whether the moisture inflow from which
direction contributes to intermittent rain events during thepre-monsoon period quantitatively, the composite analysis of
the vertically integrated moisture inflow into the study areafrom each direction is carried out. Figure 5 shows the composite
map of the vertically integrated moisture inflow on rain events(during the periods C, D and E) (a) and on non-rain days for theperiod from March to the date of monsoon onset (b),respectively. According to the above composite analysis, it isFig. 5: Composites of the vertical integration of zonal and meridional moisture fluxes passing across boundaries of the box(12.5-17.5N, 100-105E) and the precipitable water in this box on (a) the periods C, D and E, (b) the non-rain days from March toMay, and (c) the period of F. Arrows indicate the direction of flux. The numeral written in the center of the box is the precipitablewater in the box. The unit of the moisture flux and that of the precipitable water are 106 Kg/s and 106 Kg/m2, respectively.

Fig. 6: Same as Figure 3, but for the 925 hPa. Contour interval is 2 m/s. Light and dark shadings is less than -2 m/s and more than
2 m/s, respectively.

clarified that the moisture convergence becomes
strengthened on the rain events. In contrast, Figure 5(b)indicates that there is the moisture divergence on non-raindays. The moisture inflow from the south and north is almost
the same amount in both rain events (Figure 5(a)) andnon-rain days (Figure 5(b)). But the characteristics of theinflow from the zonal direction are drastically different
between them. On non-rain days, the amount of moisture
outflow to the east is higher than that of moisture inflow
from the west. Although amount of moisture inflow isdifferent, such characteristic is similar to that after themonsoon onset (Figure 5(c)). It is suggested that equatorial
westerly and mid-latitude westerly are distinguished over theIndochina Peninsula in summer monsoon season and in
non-rain days, respectively. In contrast, convergence of
zonal direction occurs on rain days. This result indicates thatthe moisture convergence when intermittent rain eventsoccur is produced by the strengthening of inflow from theeast.
To investigate the atmospheric circulation field related
to strengthening of easterly, the composite map of the windfield in the lower troposphere is constructed. The line
indicating the boundary between westerly and easterly is
located at approximately 100E on rain events. It is clear thatsoutheasterly flows into the inland region of the Indochina
Peninsula. In contrast, the westerly is dominant over the
Indochina Peninsula on non-rain days. The features are moreclearly shown by taking the difference between (a) and (b).
Figure 6(c) is a subtraction of (b) from (a). We can see thatanomalous easterly on rain events covers from the western
North Pacific Ocean to the Indochina Peninsula.

5. Conclusions
In this study, the conditions when the intermittent
rainfall during the pre-monsoon period occurred in 1998 arerevealed using daily mean reanalysis data. It is indicated thatthe intermittent rainfall events during the pre-monsoon
season are accompanied by the passage of the westerly

trough in the upper troposphere and the moisture inflow
from the south and east in the lower troposphere. The
westerly trough in the upper troposphere moves eastward
along the southern edge of the Tibetan Plateau. The
boundary between westerly and easterly is located in thecentral region of the Indochina Peninsula just before the rain
events and the easterly from the South China Sea isstrengthened from the start of the rain events.
The analysis of this study is carried out only in 1998. To
reveal the generality of the results in these analyses, theinvestigation of rainfall during the pre-monsoon period for
longer period is needed. The present study clarified that theintermittent rain events during the pre-monsoon period areproduced by the trough passage in the upper troposphere andthe moisture convergence in the lower troposphere. To
understand the mechanism of rainfall phenomena, we should
be clarified using the regional numerical model in future. In
this sutdy, it is not clear whether the rainfall events during
the pre-monsoon period influence the establishment ofmonsoon circulation. The heat budget analysis over theIndochina Peninsula during the pre-monsoon period should
be carried out. Furthermore, the role of the air-landinteraction for the monsoon onset shoud be investigated inthe future.
References
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Wang, B., and LinHo, Rainy season of the Asian--Pacificsummer monsoon, J. Climate, 15, 386-398, 2002.