Diurnal variation of pre ipitable water vapor observed with GPS in Thailand
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Mikiko FUJITA , Fujio KIMURA , Mikio SATOMURA , Teruyuki KATO

4

( 1: IORGC, JAMSTEC, 2: Institute of Geos ien e, University of Tsukuba / FRCGC, JAMSTEC,
3: Fa ulty of S ien e, Shizuoka University and 4: Earthquake Resear h Institute, University of Tokyo )Abstra t

Observation using GPS re eiver was performed to obtain PWV (Pre ipitable Water Vapor) with high time resolution
at six stations in Thailand by GAME-T (GEWEX Asian Monsoon Experiment -Tropi s). Diurnal variation of PWV wasexamined using GPS data obtained during 1998-1999. The results show that the diurnal variation is weak both in thedry season (from January to April, November, and De ember) and the wet season (from May to O tober) over Thailand.
At Bangkok, the amplitude of diurnal variation is less than 1mm. Pre ipitable water vapor de reases in the morning
showing minimum, but its peak is un lear. And PWV at Bangkok have espe ially lear diurnal variations in April 1998and Mar h 1999. The amplitude of PWV is about 5mm, its maxima appear around early morning and minimum appear
in evening. In these periods, westerly wind at 850hPa is signi antly weaker than those in other months. On the otherhand, prominent seasonal variation an be observed in PWV over Thailand. There is lear di eren e in hange of PWV
between dry and wet season. At Bangkok, variation of about 10-15 days y le is prominent in PWV with large amplitude
of about 40mm in the dry season. In ase of the wet season, however, variation of PWV is smaller than that in the dryseason, and mean PWV keeps high value of between 50mm to 60mm. In general, the diurnal variation of PWV in thedry season is learer than that in the wet season. Variation of water vapor is small in the wet season be ause of nearlysaturated water vapor in the atmosphere as the result of ontinuous supply by monsoon. In ase of Thailand, however,the diurnal variation of PWV is not lear even in the dry season, and the lear diurnal variations of PWV appear before
monsoon onset at Bangkok.
Keyword: pre ipitable water vapor, dirunal variation

1. Introdu tion

Around Indo hina Peninsula, large quantities of water vapor are supplied by monsoons. The water vapor produ es onve tive a tivity and pre ipitation, whi h play important role intropi al atmospheri dynami s.
There have been many studies dis ussing about diurnal
variation of onve tive a tivity in this area. Nitta and Sekine(1994) analyzed the diurnal variation of onve tive a tivity in thetropi al western Pa i using by the geostationary meteorologial satellite of Japan (GMS) data. They found that large diurnal
variation of onve tion exits in this area, and over the ontinents
and large island, the peak of onve tion is in late afternoon toevening. Ohsawa et al. (2001) investigated distribution of diurnal variation of onve tive a tivity in detail using by GMS data,
and found that some maxima tend to appear in the windward areas of mountain, in basins and valleys, and in ostal area. Theysuggested that these maxima are losely asso iated with terrain
or terrain-in lude lo al ir ulation.
Convergen e of water vapor intensies onve tive a tivityin general. It is important to get information of water vapor inthe atmosphere for investigation of diurnal variation. However,
it is insuÆ ient with time resolution of observational data su h
as soundings data to dis uss lo al ir ulation.
Global Positioning System (GPS) observation is usuallyused monitoring of the land deformations. Re ently, atmospheri
water vapor information an be obtained from GPS. Many resear hers have investigated study about water vapor using GPS,
however, there are few studies in tropi al Asia.
In this study, the diurnal variation of water vapor in Thailand is investigated by means of pre ipitable water vapor (PWV)
estimated from GPS with high time resolution.
* Institute of Observational Resear h for Global Change (IORGC),
Japan Agen y for Marine-Earth S ien e and Te hnology (JAMSTEC),
237-0061, 2-15 Natsushima- ho, Yokosuka, Kanagawa, Japan. e-mail:fmikijamste .go.jp

20

CHIANG MAINONG KHAISRI SAMRONG
15BANGKOKUBON RATCHATHANI

3200
2800
2400
2000
1600
1200
800
400010

PHUKET

5100105110: The lo ation of GAME-T GPS observation station:
ontours of elevation with 400 m interval.

Fig. 1

2. Observation and data analysisObservation using GPS re eiver has been performed at sixstations (Bangkok, Chiang Mai, Nong Khai, Phuket, Sri Samrong and Ubon Rat hatani) in Thailand sin e 1998 in the partof GAME-T. The lo ations of GPS stations are shown in Fig.1.
GPS antennas are xed on the roof of ea h building of meteorologi al department.
The GPS data were re eived at 30-se ond intervals andre orded to the hard disk automati ally. The atmospheri delay of GPS is estimated at one-hour intervals from GPS data using by analyti software, BERNESE4.2. The zenith hydrostatidelay was estimated from surfa e pressure, latitude and ellipsoidal height at ea h observation station (Elgered et al., 1991).
The zenith wet delay was onverted into pre ipitable water vapor by using  parameter (Askne and Nordius, 1987). The
weighted mean temperature Tm was estimated from surfa e tem-perature Ts observed at Thai Meteorologi al Department (TMD)(Bavis et al., 1992). We estimated PWV from GPS data at espeially four stations (Bangkok, Chiang Mai, Nong Khai and Ubon
Rat hatani) in 1999.
Surfa e pressure, temperature, relative humidity and rain
gauge data were provided from TMD. These data are sampledthree hourly (01, 04, 07, 10, 13, 16, 19, 22LST). The obje tive
analysis data su h as wind and temperature of NCEP/NCAR Reanalysis were used.
3. Results and dis ussion
3.1 Diurnal variation

The analysis term is divided into two periods: the dry season period (from January to April and from November to O tober) and the wet season period (from May to O tober).
Figure 2 shows the time series of daily GPS-PWV and saturated water vapor observed at Bangkok in 1999. There is leardi eren e in hange of PWV between the dry and the wet seasons. In the dry season, variation of about 10-15 days y le isprominent in PWV with large amplitude of about 40mm. In aseof the wet season, however, variation of PWV is smaller than thatin dry season, while mean PWV keeps high value of between
50mm to 60mm. Saturated water vapor in Fig.2, estimated byusing NCEP data, is almost a onstant value during 1999. During the wet season, PWV estimated from GPS be omes nearlysaturated water vapor. It is suggested that water vapor in the atmosphere is saturated as the result of ontinuous supply by monsoon and pre ipitation.
Bangkok 1999

saturaed-PWV

60
40GPS-PWV

200JanFebMar

AprMayJunJulAug

SepOctNov

3.2 Seasonal variation

For more detailed exploration of diurnal variation, we investigated amplitude of monthly averaged diurnal y le of PWV
and the relation between the diurnal variation and low level windspeed. Figure 4 shows the amplitude of the monthly averaged diurnal y le of PWV with monthly mean wind speed (850hPa) atBangkok in 1999. In Mar h, the monthly averaged amplitudeof the diurnal y le of PWV is pe uliar large ompared with inother month. In other month, however, the amplitude is always
small. It is suggested that lear diurnal variation of PWV exists
only in Mar h in 1999 at Bangkok. On the other hand, monthly
mean wind speed at 850hPa is weaker in Mar h than those inother month.

Dec: The time series of daily PWV of GPS (lower line)
and saturated water vapor (Upper line) observed at Bangkokin 1999.
2(a) dry-season

2PWV

00-1

SH-1-2-3-202468 10 12 14 16 18 20 22Local Time

31PWV [mm]Bangkok 1999

86420123456 7 8Month9 10 11 12: Monthly mean wind speed at 850hPa (solid line) andamplitude of monthly averaged diurnal variation of PWV(bar graph) at Bangkok in 1999.
Fig. 42(b) wet-season

21SH

00-1

PWV-1-2-3-202468 10 12 14 16 18 20 22
Local Time

3: The mean diurnal variation of PWV and SH in
the dry and wet season at Bangkok in 1999. All values
are shown in term of deviation from 00LST. The error bars
mean estimated standard deviation of average.
Fig.

10141210864
20

Specific Humidity [k/kg]

PWV [mm]

1
1

Specific Humidity [k/kg]

3

wind speed [m/s]Fig. 2

amplitude [mm]

PWV [mm]80

Figure 3 shows the averaged diurnal y le of PWV in ea hperiod. All values are shown in term of deviation from 00LST.
The error bars mean the standard deviations of the average,
whi h are equal to the standard deviation divided by the squareroot of sampling number. Diurnal y le of PWV at Bangkok isweak both in dry and wet season. The amplitude is less than
1mm. Pre ipitable water vapor de reases in the morning having
a lear minimum, but its maximum peak is un lear. Other twostations in Thailand ex ept for Chiang Mai have similar diurnalvariation.
Takagi et al. (2000) showed the diurnal variation of PWV
al ulated from GPS at Lhasa in Tibet. The diurnal variation atLhasa has a lear minimum around 18LST and 15LST in the premonsoon and the monsoon period, respe tively. Also in Japan,GPS-PWV has prominent diurnal variation. Sasaki and Kimura(2001) suggested that variation of PWV di er in lo ation su h as
mountains, inland and ostal area. Moreover, Wu et al. (2003)
investigated that at Sumatra island, lo ated western part of Indonesia, distin t diurnal variation of pre ipitable water vapor
exits even on days with heavy rain, while a minimum around
18LST.

5: Monthly averaged diurnal variation of PWV in
1999. All values are shown in term of deviation from00LST.
Fig.

Figure 5 shows monthly averaged diurnal variation of PWVin 1999. All values are shown in term of deviation from 00LST.

Referen esFig. 6: Averaged diurnal variation of horizontal wind vorti ity during Mar h 1999. The ontour shows wind speed,ve tor shows wind dire tion and speed. Upward and rightward arrows indi ate southerly and westerly wind. Morethan 5m/s area was shaede.

Pre ipitable water vapor de rease about 4mm in daytime, has aminimum around 20LT in Mar h. Diurnal variations of PWV inother month are weaker than that in Mar h, and their peaks are
un lear. Averaged diurnal variation of horizontal wind velo ity
during Mar h 1999, whi h observed by wind proler at Bankok,
is showed in Figure 6. In night time, strong sea breeze is prominent while its wind speed is about 10m/s. The diurnal variationof horizontal wind is lear only in Mar h. It implies that water
vapor in rease by this sea breeze during this month.
Di eren e of weekly averaged surfa e temperature (Ts) at
Bangkok and weekly averaged sea surfa e temperature (SST) at
Gulf of Thailand (10N, 100E) in 1999 was estimated. There isdi eren e of about 2 degree between Ts and SST, while amplitude of diurnal variation of surfa e temperature is about 4 degree.
It is suggested that this di eren e between Ts and SST is enoughto weaken or intensify the sea breeze.
These results indi ate that the diurnal y le of PWV beome lears as the results of the e e ts of sea breeze, when theambient low -level wind speed is weak and the thermally indu edlo al ir ulations are a tive. The amplitude of diurnal variationof PWV at Bangkok seems to be asso iated with thermal lo al
ir ulations during mild low-level wind speed is weak at the onset period of monsoon. By analysis of observation data, Murataet al. (2002) suggested that the pre ipitation at Kototabang, West
Sumatra Provin e in Indonesia, tend to o ur when the low-level
westerly is weak.
4. Con lusion

In this study, pre ipitable water vapor was estimated fromGPS data in 1999 around Thailand with high time resolution.
Clear di eren e in temporal variation during dry and wet season
was found. In the dry season, about 10-15 days y le is prominent with large amplitude of about 40mm. In the wet season,
however, variation of PWV is mu h smaller than that in dry season, although mean value of PWV keeps high value of between
50mm to 60mm.
Amplitude of monthly averaged diurnal y le of PWV wasestimated. The amplitude in Mar h is espe ially large omparedwith that in other month at Bangkok. In Mar h, the low level
wind is weak, and diurnal variation of surfa e wind is lear. Itis suggested that the amplitude of diurnal variation of PWV atBangkok is asso iated with thermally indu ed lo al ir ulations
generated under mild ambient wind around the onset of monsoon.

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A knowledgments

The authors express their hearty thanks to the member of Thai
Meteorologi al Department, Chulalongkorn University, Royal
Irrigation Department, National Resear h Coun il of Thailand,
National Institute of Information and Communi ations Te hnology and Institute of Industrial S ien e University of Tokyo for
their supports in ollaborative observations and o ering usefuldata.