Seasonal Variation of Precipitation Properties Associated withMonsoon over Palau
Hisayuki Kubota, Ryuichi Shirooka, Tomoki Ushiyama, Takashi Chuda,
Suginori Iwasaki and Kensuke Takeuchi
Institute of Observational Research for Global Change
Japan Agency for Marine-Earth Science and Technology (IORGC, JAMSTEC)
2-15 Natsushima-cho, Yokosuka, Kanagawa, JAPAN 237-0061, email: kubota@jamstec.go.jp
Abstract
Seasonal variation of precipitation properties associated with monsoon wind direction is investigatedfocusing on data of Peleliu Island at Republic of Palau of Western Pacific from Jun. 2001 to Apr. 2002.
Westerly wind monsoon period starts at 18 May. 2001 and easterly wind monsoon starts at 26 Nov. whichdates are affected by intraseasonal oscillations. During westerly wind monsoon period, westerly winds areprevailed and their intensities are correlated with cloud amount. Diurnal variation of precipitation is alsoprevailed with nocturnal maximum during convective active phase. Precipitation has strong intensity withshort duration. On the other hand, during easterly wind monsoon period, zonal winds are weak correlatedwith cloud amount. The properties of precipitations have weak intensity with long duration comparedfrom westerly monsoon. When dry air comes from subtropics, convection is suppressed. However
averaged rainfall rate is same as westerly monsoon period.
Keywords: western Pacific, precipitation, monsoon

1. Introduction
Seasonal variation over western Pacific region isclosely related to the Asian monsoon. Western North
Pacific region is one of the distinct monsoon regions(Murakami and Matsumoto, 1994). Over continental areafrom India to Indochina Peninsula, apparent wet and dry
season appeared. When rainy season does start is criticalfor local agriculture and water resource and so on.
Therefore seasonal march of monsoon was discussed
well for the rainy season. The period of wet and dry
season was determined by many researches usingsatellite OLR (Outgoing Long wave Radiation) data
which is an index of convective activity and rainfall data
(Tanaka, 1992; Lau and Yang, 1997; Wang and LinHo,2002).
The term “monsoon” has been generally defined by aseasonal variation of surface wind (Ramage, 1971).
Western Pacific region has been recognized that
low-level wind changes its direction apparently fromeasterly to westerly during northern hemisphere summer
monsoon (Matsumoto, 1992). Institute of Observational
Research for Global Change has been conducted theobservational project PALAU (Pacific Area Long-term
Atmospheric observation for the Understanding ofclimate change) over Peleliu Island (7.05°N, 134.27 °E)

of Republic of Palau (Fig. 1). In this study we define themonsoon season by using low-level zonal wind at Koror(7.33°N, 134.48°E) (Fig. 1) and focus on the seasonalvariation of precipitation properties over western Pacificassociated with monsoon wind direction using Peleliu
station data which is expected to be less affected fromlarge island.

Fig. 1: Maps of western Pacific region. Left side is theenlarged map around Palau.

2. DataWe use precipitation and equivalent cloud amount data
observed at Peleliu station from 28 June 2001 to 30 April
2002. Equivalent cloud amount is calculated by thefrequency of cloud base measurement of Ceilometerwithin every hour. These values are assumed to be usefulindex for understanding the activity of cloud over Palau.
Upper air sounding is used which is carried out by KororNational Weather Service (NWS) from 1973 to 2003.
Special Sensor Microwave Imager (SSM/I) precipitabledata are used of 1°x 1° averaged data at the same regionof Palau.

3. Monsoon season
We defined the monsoon season over Palau using
zonal wind at 850 hPa height. We used a threshold offive days running mean 5 m/s westerly wind to define thewesterly wind monsoon season. Figure 2 shows the timeseries of zonal wind and westerly wind monsoon period
at Palau. Westerly wind monsoon onset appears aroundMay to July. Withdrawal dates appear around September
to December. The dates are affected by the phase ofintraseasonal oscillation. We can see that westerly wind
does not always blow even during westerly windmonsoon season.

Fig. 2: Seasonal and annual variation of daily averaged
zonal wind at 850 hPa at Koror from 1973 to 2003. The
intensities of zonal wind are indicated by colors.
Westerly wind monsoon onset and withdrawal dates areplotted by triangles

4. Seasonal variability over Palau
In this section we focus on the seasonal variability
observed at Peleliu Island station by dividing westerly
wind monsoon from easterly wind monsoon. Peleliu
Island is a very small island and its length is less than 10km. The effect of land-sea breeze circulation anddaytime convection due solar heating over land surface issmall. Therefore we can assume observation data at
Peleliu Island is representative as over an open ocean.

4.1 Zonal wind and cloud amountFigure 3 shows the time series of equivalent cloudamount and Koror 850 hPa zonal wind from 28 June
2001 to 30 June 2002. Westerly wind monsoon period
continued until 25 November 2001 and the next westerly
wind monsoon onset is 18 May 2002. During westerly
wind phase equivalent cloud amount and zonal wind are
well correlated and correlation coefficient is 0.71. Whenwesterly wind is intensified, equivalent cloud amountincreases, which indicate that convective activity isintensified over Palau. On the other hand, during easterly
wind monsoon equivalent cloud amount does notdecrease as zonal wind direction changes to easterlies.
There is no correlation between equivalent cloud amountand zonal wind during easterly wind monsoon. After
westerly wind monsoon has finished northeasterly tradewind is prevailed over Palau.

Fig. 3: The time series of equivalent cloud amount (solidline) and Koror 850 hPa zonal wind (dotted line) from 28
June 2001 to 30 June 2002.

4.2 Diurnal variation of precipitation
We compared the diurnal variation of precipitation
during westerly wind monsoon from easterly wind. In
this study we also divided the analyses period of active
phase of convection from inactive phase using equivalent
cloud amount data. Threshold of 5 day running meanequivalent cloud amount more than 4 is active phase andless than 3 is inactive phase (see Fig. 3). Diurnal
variation of precipitation has nocturnal maximum duringactive phase on westerly wind phase (Fig. 4a). On theother hand, in inactive phase diurnal variation is weakand has small afternoon maximum (not shown). The
feature of diurnal variation of precipitation during
westerly wind monsoon is consistent with the previous
work over equatorial western Pacific (Sui et al., 1997;
Chen and Houze, 1997; Kubota and Nitta, 2001).
However it was not clear during easterly wind phase (Fig.
4b).
(a)

(b)

Fig. 4: Diurnal variation of precipitation averaged in
active phase during westerly wind monsoon (a) andeasterly wind monsoon (b). Units are mm/h

4.3 Duration and intensity of rain event
Diurnal variation of precipitation was obscureduring easterly wind monsoon. We examine theprecipitation properties further what make this difference
during easterly wind phase from westerly wind phase.
We defined rain event as continuous rain without 3 hours
intermittence of less than 0.1 mm/h using hourly
precipitation data at Peleliu Island. Figure 5 shows thefrequencies and intensities of rain event divided westerly
wind phase from easterly wind. Rain event frequency ofmore than 13 hours during easterly wind phase reaches
more than twice as westerly wind. This means that longrain event is evident during easterly wind monsoon
compared from westerly wind. Almost all duration ofrain event show that the intensity of rain event is strong
during westerly wind monsoon. Tagami (1990) suggested
that diurnal variation of precipitation is apparent at thecase of rain event of short duration within 11 hours andstrong intensity over Japan. Therefore weak diurnal
variation during easterly wind monsoon comes from long
duration and weak intensity of rain event.

5. Summary and discussions
We are conducted the observational project PALAU overPeleliu Island (7.05°N, 134.27 °E) of Republic of Palau.
Precipitation and equivalent cloud amount observed atPeleliu Island station were used from 28 June 2001 to 30

April 2002. We defined the monsoon season using fivedays running mean 850 hPa zonal wind sounding dataover Palau as the period more than 5 m/s. Westerly windmonsoon onset appears around May to July. Withdrawal
dates appear around September to December. The onsetand withdrawal date of westerly wind monsoon depends
on the phase of intraseasonal oscillation.
The averaged characteristics of precipitation properties
and others during westerly wind monsoon and easterly
wind monsoon are listed in Table 1. The values of zonal
wind at 850 hPa height are nearly same with opposite
direction. The difference of precipitation and equivalent
cloud amount compared westerly from easterly is within
10 %. Precipitation and cloud amount does not decrease
even during easterly wind phase. Precipitable water has
difference at westerly wind monsoon from easterly wind.
This is due to the dry air comes from subtropical region
during easterly wind phase (not shown). CAPE also has adifference between westerly and easterly. Diurnal
variation of precipitation has nocturnal maximum during
active phase on westerly wind phase. However it was notclear during easterly wind phase. The intensity ofprecipitation was strong and duration was short during
westerly wind phase. On the other hand, weak intensity
and long duration was observed during easterly windmonsoon. During westerly wind phase, Palau locates
southern region of ITCZ. And during easterly wind phase,
ITCZ moves south of Palau and subtropics dry air
propagates around Palau. Therefore the difference ofprecipitation properties may be related with thedifference of large-scale circulation.

Fig. 5: Frequencies (boxes) and intensities (lines) of rain
event distributed every duration of rain event. Westerly
wind phase is dark boxes and solid line. Easterly windphase is light boxes and dotted line. Unit of frequency isnumbers and intensity is mm/ event. The abscissa isduration (hours) and 13 represents more than 13 hours

zonal wind
precipitation

6/28-11/25
3.7m/s8.8mm/day

11/26-4/30-3.4m/s
8.8mm/daycloud amountPrecipitable water
CAPE

35.6%
55.0mm2824(m/s)²

32.5%
47.8mm1879(m/s)²

diurnal variationof precipitation

Nocturnalmaximum(active phase)strong/ short

Not clearintensity/ duration
weak/ longof precipitation
Table 1 Comparison between westerly wind monsoon
and easterly wind mo nsoon.

Acknowledgements
SSM/I data was obtained from the Remote Sensing
System. The GFD-DENNOU library was used for thedrawing of figures.

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