Isotopic Composition of Rainfall in Thailand Depends on theConvective System∗
Kimpei Ichiyanagi1 , Kei Yoshimura2 , Manabu D. Yamanaka1,3(1: Institute of Observational Research for Global Change, JAMSTEC, 2: Institute ofIndustrial Science, University of Tokyo, 3: Graduate School of Science and Technology, Kobe
University)* Institute of Observational Research for Global Change, 3173-25, Showa-machi, Kanazawa-ku,
Yokohama, Kanagawa 236-0001, Japan e-mail: kimpei@jamstec.go.jp

Abstract
The majority of past precipitation studies investigating stable isotopes and the amount effect have used monthly
data. Only a few such studies have incorporated daily observation data in the Asia-Pacific Region. Daily
rainfall was sampled at Bangkok and Phuket, Thailand, between August and November, 2001. The isotopic
compositions and rainfall amounts at both stations revealed trends similar to each other with respect to monthly
variability, but differed with respect to daily variability. Rainfall δ 18 O decreased as amounts decreased, at both
stations, when the rainfall rate was less than 30 mm/day; however, no clear trend was apparent when the rainfall
rate exceeded 30 mm/day. Composites of 3-hourly rainfall and relative humidity data for light (≤ 30 mm/day)and heavy (> 30 mm/day) rain events were analyzed to determine diurnal variations. Two rainfall peaks, inthe early morning and evening, occurred at Bangkok. Rainfall was more continuous throughout the day, except
in the early morning, at Phuket. In addition, relative humidity showed large diurnal variations during bothlight and heavy rain events at Bangkok. At Phuket, relative humidity during the day for heavy rain events wasmuch higher than during light rain events. Results show that a small number of rain events occurred only in theevening, and followed Rayleigh condensation processes at both stations. However, continuous westerly moistureflux, during heavy rain events at both stations, may violate the closed system assumption that is fundamental toRayleigh condensation processes. Heavy rain events with different isotopic compositions occurred throughout
the day at Phuket. These results suggest that the isotopic composition of rainfall is strongly influenced bycloud systems and formation processes at the collection site.
Keyword: stable isotopes, rainfall, diurnal variation, convective system, Thailand.

1. Introduction
Stable isotopes (δ 2 H, δ 18 O) in precipitation are widelyused to infer temperature and precipitation within the
global hydrological cycle. Dansgaard (1964) analyzed
monthly observational datasets provided by the Global
Network for Isotopes in Precipitation (GNIP) project(IAEA, 2001), of the International Atomic Energy Agency(IAEA) and the World Meteorological Organization(WMO). That study revealed a linear relationship between precipitation amount and isotopic composition, or
the so-called precipitation amount effect, at low-latitude
stations. The precipitation amount effect can be explained by Rayleigh condensation (i.e., enrichment of theatmosphere by evaporation from falling raindrops) andisotopic exchange between falling drops and surrounding vapor. Upwind exchange in organized storm systems is also important for determing isotopic composition(Lawrence et al., 2004).
Araguás-Araguás et al. (1998) revealed the spatial andtemporal variability of long-term monthly means of thestable isotopic composition of precipitation over the AsiaPacific region using GNIP datasets. They highlighted aregion where the isotopic composition responded to precipitation amount because the air mass was modulated
by monsoon activity and seasonal displacement of theIntertropical Convective Zone (ITCZ). They further reported that seasonal changes in precipitation isotopes inthe tropical Pacific were controlled, mainly, by rain-out
processes in deep convective clouds and by isotopic ex-changes beneath the cloud. Datta et al. (1991) examined temporal variations in the stable isotopic composition of precipitation in New Delhi by statistically analyzing GNIP datasets. They suggested that long-term
variability in δ 18 O may be governed by the intensity anddistribution of precipitation, and by the movement of the
moist air mass. They also suggested that evaporation ofraindrops could isotopically enrich monsoon precipitation.
The GNIP datasets contain monthly averaged values
for precipitation amounts and isotopic compositions from1968 to 1995 at Bangkok (IAEA, 2001). Ichiyanagi et al.
(2003) noted a strong negative correlation between longterm monthly means of precipitation amounts and δ 18 O.
These monthly averaged values clearly show the amounteffect. The correlation coefficient of 0.98 has a significance level exceeding 95 %. For individual months, negative correlations occurred in May, July, August, October,
November, and December. No correlation was apparent
in the other six months. To determine the true isotopic
composition of precipitation, further analyses using daily
or other short-term observations are warranted. A few
studies have used event-based isotope data to study variability in the isotopic composition of precipitation. This
study examines the amount effect using daily rainfall data
observed in Thailand, and elaborates on important processes controlling the isotopic composition of rainfall.

2. Data and Methods
Daily rainfall isotopic data were collected at Bang Na
Agromet (13.44◦ N, 100.34◦ E) in Bangkok and at Phuket

3. Results
3.1. Daily Rainfall Variability
Figure 1 shows daily rainfall and isotopic data at
Bangkok and Phuket. Both stations show similar seasonal(monthly) variability trends in rainfall amount and δ 18 O.
Light rain and high δ 18 O occurred in August and November; heavy rain and low δ 18 O were common in September and October. Short-term (daily) variability, however, showed a different pattern. At Bangkok, two rainfall peaks occurred on 18 September and 7 October. At
Phuket, a large peak at the end of September was followed
by intermittent peaks of decreasing magnitude into earlyNovember. Phuket had more rainy days than Bangkok;
however, both stations had approximately the same number of heavy rain days (> 30 mm/day). The δ 18 O value
was near -5 0/00 until early September at Phuket and until
mid-September at Bangkok. Between mid-September andmid-October, sudden δ 18 O drops to less than -10 0/00 appeared more frequently at Bangkok than at Phuket. Subsequently, δ 18 O at Bangkok remained at a higher level
than at Phuket.
a) Bangkok

-5

60

-10

40

-15

20

09/01

09/15

10/01

10/15

11/01

b) Phuket

-5
-10

-15

-20
0

-5

60

-10

40

-15

20

09/01

09/15

10/01

10/15

11/01

20

40
60
80
Rainfall (mm/day)

100b) Phuket

100
80

08/15

0

0
11/15

0

-20
08/01

a) Bangkok
5

5

0
11/15

Fig. 1: Time series of daily rainfall (bar graph) andδ 18 O (line graph) at a) Bangkok and b) Phuket from
August to November, 2001. The solid line indicates
30 mm/day.
Yoshimura et al. (2003) reported similar temporal variability at three stations in Thailand (Bangkok and two
stations in northern Thailand) during the 1998 rainy season. In that study it was found that large-scale moisturetransport influenced the short-term (daily) variability ofrainfall isotopes at all stations. In this study, however,the daily rainfall isotope trends at Bangkok differed fromthose at Phuket. Phuket’s location may explain why its
isotopic variability differs from that of other stations in
Indochina, in that it is located on the Malay Peninsula
and is surrounded by ocean. Ohsawa et al. (2001) also0
Oxygen 18 (permil)

5

08/15

Rainfall (mm/day)

80

-20
08/01

Oxygen 18 (permil)

1000

Rainfall (mm/day)Oxygen 18 (permil)

5

showed the difference in diurnal variation of convective
activity between coastal and inland area over Indochina.
Figure 2 shows the relationship between rainfall amountand δ 18 O at the two stations. Light rain and low δ 18 O
were common at both stations. Njitchoua et al. (1999)
reported that low δ 18 O values accompanying daily rainfallof less than 20 mm were caused either by the condensation of residual vapor in a cloud system or by condensation
at high altitudes (low temperatures). Precipitable water
vapor, which constituted the rainwater source, was characterized by low δ 18 O values. Datta et al. (1991) counted
the number of days with rainfall exceeding 30 mm, andnoted that the monthly δ 18 O composition was affected byboth rainfall amount and temperature.
Heavy rain (about 30 mm/day) and high δ 18 O samples(more than -10 0/00 ) are common at Bangkok. Except forlight rain/low δ 18 O and heavy rain/high δ 18 O samples,decreasing rain amounts are accompanied by decreasingδ 18 O values. Additionally, there is an obvious negative
correlation between rainfall amount and δ 18 O when daily
rainfall rates are less than 30 mm/day at Phuket. The
δ 18 O values ranged from -10 ∼ -5 0/00 when daily rainfall
rates exceed 30 mm/day. In the next section, the amounteffect is examined in relation to light (≤ 30 mm/day) andheavy rain (> 30 mm/day) events.

Oxygen 18 (permil)

Airport (8.07◦ N, 98.19◦ E) from August to November,
2001, and were subsequently analyzed. Diurnal variability was calculated using three-hourly rainfall and relative
humidity data provided by the Thailand Meteorological
Department (TMD). There were no 3-hourly data at BangNa Agromet in Bangkok, so data from Bangkok Metropolis station (13.73◦ N, 100.56◦ E) were used instead.

-5
-10

-15

-20
0

20

40

60

80

100

Rainfall (mm/day)Fig. 2: Scatterplots between isotopic composition
and rainfall amount effect at a) Bangkok and b)
Phuket. The solid line indicates 30 mm/day.

3.2. Diurnal Variation
Diurnal variations in rainfall intensity, frequency of rainy
days, and relative humidity were investigated using 3hourly routine observations at Bangkok and Phuket. The
frequency is defined as the ratio of rainy days to total

days from 1 August to 15 November in 2001. Figure 3
shows the diurnal variations in rainfall intensity, frequencyof rainy days, and relative humidity composites for lightand heavy rain at the two stations. Rainfall intensity
at Bangkok shows two peaks (about 4 mm in the earlymorning and more than 10 mm in the evening) for lightrain events and one peak (about 3 mm in the evening)for heavy rain events. The frequency of rainy days also
shows two peaks (more than 50 % in the early morningand evening) for light rain events and one peak (morethan 30 % in the evening) for heavy rain events. Relative
humidity for both events is less than 70 % and more than
90 % during night and day, respectively.
Rainfall intensity at Phuket is less than 1 mm throughout the day for light rain events and more than 3 mmthroughout the day, except in the early morning, for heavy
rain events. In addition, the frequency of rainy days is lessthan 40 % for light rain events throughout the day andmore than 60 % for heavy rain events throughout the day,except in the early morning. Relative humidity is lessthan 70 % during the day for light rain events, and morethan 80 % throughout the day for heavy rain events.
During heavy rain events at Bangkok, there are two
peaks in rainfall, one in the early morning and one in
the evening. Rainfall rates are relatively high throughout the day, except during the early morning, at Phuket.
Relative humidity shows large diurnal variation for bothheavy and light rain events at Bangkok. Relative humidity at Phuket, in contrast, shows large and small diurnal variation for light and heavy rain events, respectively.
Relative humidity during the day for heavy rain events ismuch higher than for light rain events.

a) Bangkok
Rainfall (mm/hr)

15
Heavy
Light

12
9

9

6

6

3

3
0
1

Frequency (%)

Heavy
Light

12

0
4

7

10 13 16 19 22

100

100

80

80

60

60

40

40

20

20

0

1

4

7

10 13 16 19 22

1

4

7

10 13 16 19 22

1

4

7

10 13 16 19 22
Local Time01

Relative Humidity (%)b) Phuket
15

4

7

10 13 16 19 22

100

100

90

90

80

80

70

70

60

60
1

4

7

10 13 16 19 22
Local Time

Fig. 3: Diurnal variations in rainfall intensity, frequency of rainy days, and relative humidity for lightand heavy rain events at a) Bangkok and b) Phuket.

4. Discussions
These results show that the convective system and rainfall formation process are important for determining theisotopic compositions of rainfall. Light rain and low δ 18 O,
which were caused either by the condensation of residual
vapor in a cloud system or by condensation at high altitudes, were common at both stations. Except for somelight rain and low δ 18 O samples, there is an obvious decreasing trend, at both stations, for light rain events: asrain amounts decrease, so too does δ 18 O. Periods of lightrain are characterized by a small number of rain events,
occurring in the evening, and a discontinuous westerly
moisture flux. The isotopic composition of rainfall at both
stations is affected, therefore, by the Rayleigh condensation process, as the amount effect clearly shows.
At Bangkok, heavy rain (about 30 mm/day) and samples with δ 18 O exceeding -10 0/00 are common. Periodsof heavy rain occurred only during the evening, and each
event resulted from a deep convective system. Isotopic
ratios for convective rains are higher than ratios for stratiform rains (Gedzelman and Arnold, 1994, Fig. 1). Sucha continuous moisture flux may violate the closed system
assumption that is necessary for Rayleigh distillation processes. At Phuket, δ 18 O values range from -10 ∼ -5 0/00 forheavy rain events. Periods of heavy rain are characterizedby many rain events throughout the day, and by continuous westerly moisture flux. Temperature and humidity
conditions in clouds can vary greatly during the day, so
the isotopic composition of rainfall at Phuket may also
differ among individual rain events. The isotopic composition of daily rainfall is the sum of many rain events,
which may have different isotopic compositions. Thus,
the daily isotopic composition may not show the amounteffect during periods of heavy rain.
These results suggest that the type of convective system and rain formation process help to determine the isotopic composition of rainfall. However, Ichiyanagi et al.
(2002) noted that flux and the transport path of moisture
flux are the main influences on stable isotopes in precipitation. Lawrence et al. (2004) analyzed isotopic variations of rainfall and water vapor in detail using trajectory
analysis. They showed that isotope ratios are related tothe intensity and organization of storm systems upwind of
the collection site; ratios decrease as storm activity andorganization increases. Processes controlling rainfall isotopes include rainwater origin, transport processes in the
air mass, and formation processes at the collection site.
In this paper, the convective system and formation processes near the collection site only were considered. Further studies on transport and formation processes upwindof the collection site are needed to clarify the isotopic
composition of rainfall.

5. Conclusions
Daily rainfall samples were collected at two stations inThailand to examine whether the amount effect affected
rainfall isotopes. Rainfall δ 18 O decreased as amounts decreased, at both Stations, when the rainfall rate was lessthan 30 mm/day. There is no clear relationship whenrainfall rates exceed 30 mm/day. Composites of rainfalland relative humidity were analyzed to compare periodsof light (≤ 30 m/day) and heavy (> 30 mm/day) rain.
There are two peaks of rainfall during heavy rain events
at Bangkok. One is in the early morning and one is in theevening. Rainfall rates during heavy rain events at Phuket
are relatively high throughout the day, except in the earlymorning. Large diurnal variations in relative humidity
occur for both light and heavy events at Bangkok, but for
only light rain events at Phuket. The relative humidity
during the day in Phuket for heavy rain events is muchhigher than for light rain events.
For light rain periods, the relatively few rain events occurred only in the evening, and followed Rayleigh condensation processes at both stations. For heavy rain periods,
the relatively frequent rain events occurred throughout
the day, with different isotopic compositions at Phuket.
Furthermore, relatively high humidity condition for theperiods of heavy rain at both stations may violate theclosed system assumption that is necessary for Rayleighcondensation processes. These results suggest that thecloud system and formation processes near the collection
site are important for determining the isotopic composition of rainfall.
Acknowledgments
This study was supported by the Frontier Observational Research System for Global Change (FORSGC)
for project research in FY 2001. Some meteorological
data used in this article are from GEWEX Asia Monsoon
Experiment (GAME) -Tropics observation. The authors
thank Prof. Michio Hashizume of Chulalongkorn University and the staff of the Thailand Meteorological Department (TMD) for their cooperation in sampling rainfall.

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