Study On Precipitable Water Vapor Change (obtained from GPS) and Humidity
Sununtha Kingpaiboon1, Mikio Satomura 2, Mayumi Horikawa 3(1: Associate Professor, 2: Professor, 3: Graduate School of Environmental Studies)
Khon Kaen University, Thailand, e-mail: Sununtha@kku.ac.th, Shizuoka University, Japan,
e-mail: Semsato@ipc.shizuoka.ac.jp and Nagoya University, JapanAbstract
GPS observation was set up in the campus of Khon Kaen University, Northeastern Thailand (under the GAME-T Project) toinvestigate the Precipitable Water Vapor (PWV) change and to understand the characteristics of its seasonal and diurnalchanges. The GPS observation has been performed since August 2001. The data obtained from January 2001 to December
2002 were processed and presented in this study. A Trimble 4000 SSi receiver and a Trimble microcentered L1/L2
antenna are used for the observation. The antenna is fixed on the roof of a building and the data is recorded by every 30
minutes for 24 hours a day. The data obtained were processed by using GAMIT software referring the 7 IGS stations datato obtain the Zenith Tropospheric Delay (ZTD) at every one hour. The three hourly PWV was calculated from the ZTD
using air pressure and temperature data which were measured by the Thai Meteorological Department. The results obtained
showed that PWV change with a large amptitude from 20 mm. to 60 mm. before the monsoon onset (dry season), it wasnoted that the PWV increased before rain and decreased after rain. In the wet season, the PWV was almost constant at about
60 mm. or 70 mm. The PWV was related with the humidity. Daily relative humidity measurements were obtained fromRawinsonde data, observed at standard pressure levels : 1000 mbs 850 mbs 700 mbs 600 mbs 500 mbs 400 mbs 400mbs 300 mbs, or from ground level to 32,000 ft. above ground level. Statistical analysis indicated that PWV and relative
humidity at 300 mbs or at 32,000 ft above ground level were relative with the minimum variance and standard deviation of
125.33 and 11.19.
Keyword: Global Positioning System (GPS), Precipitable Water Vapor (PWV), Relative Humidity (RH), RawinsondeObservation

1. Introduction
The study on the water vapor change in the troposphere isimportant for the meteorology and hydrology, but it isdifficult to measure the vapor change in the upper
atmosphere. On the other hands, GPS observation isusually performed to obtain precise positions for thegeodetic survey or civil engineering. The microwave ofGPS delays when it passes through the ionosphere and thetroposphere. The delay makes errors in the positioning but
they are good signals to get information on atmosphereconditions for the meteorologists and hydrologists. The
excess path delay caused by the ionosphere is depend on
the wavelength and it can be estimated by using the data of
two wavelengths from the satellites. After removing theeffects of the ionosphere, we can get the delay due to thetroposphere. The delay due to the troposphere consists of
the hydrostatic delay by the dry gas and another delay by
the water vapor. The former is called the dry term and thelatter is the wet term. The dry term can be estimated
precisely from the barometer data and we can get the wetterm. The amount of the water vapor can be obtained bytransforming the wet term to precipitable water vapor(PWV). The GPS observation to investigate the PWV had
been carried out at Khon Kaen University since August2001.

2. Observation and Data Processing
A Trimble 4000SSi receiver and a trimble
microcentered L1/L2 antenna are is used in the presentobservation. The antenna was set on a tripod on a roof of abuilding of the Department of Agriculture Engineering,
Khon Kaen University. The data were recorded at every 30
seconds and they were downloaded to a PC once a day.

The data obtained in the period from January to December
in 2002 were processed by using GAMIT software byreferring to seven IGS stations at Shanghai, Yaragadee,
Tsukuba, Guam, Lhasa, Cocos and Singapore. The
coordinates of the Khon Kaen site were obtained first withGLOBK software, and the Zenith Tropospheric Delay(ZTD) were obtained at every one hour by constraining its
coordinates within 3 mm in horizontal components and 5mm in the vertical component. The 3 hourly air pressure
data by Khon Kaen station of the Thai Meteorological
Department (TMD) were used to obtain the dry terms.
They are the values at mean sea level and we calculated thepressure at the antenna site whose elevation is 198.21 m,
which is obtained from the ellipsoidal height by GPS andgeoid height of NIWA EGM geoid. The elevation on thebenchmark in front of the Department of Civil Engineeringof the Khon Kaen University is 195.505 m and this value isproper considering the height of the building.
The relation between PWV and Relative Humidity(RH) were studied. Daily relative humidity (JanuaryDecember, 2002) were obtained from Rawinsonde data, at
Ubonratchathani Meteorological station where is thenearby area representative. Rawinsonde data wereobserved at standard pressure levels of 1,000 mbs 850 mbs
700 mbs 600 mbs 500 mbs 400 mbs and 300 mbs. On theother hand, the Rawinsonde observation were made at theheight of from ground level to 32,000 ft above ground level.
Variance and standard deviation between PWV and RH of
every standard pressure levels were analized.

3. Results
The PWV change from January to December 2002
obtained from the processing is shown in Fig.1. The

precipitation (three hourly) is also shown in this figure.
The PWV periodically change widely between 20 to 60mm, with 1 or 2 weeks duration in the dry season(November-April). It is noted that PWV increases before
rainfall and decreases after rainfall. On the other hand, they
are almost constant to be 60 or 70 mm from May toSeptember (wet season). These features are almost same asother GPS stations in Thailand.

9080
70
60
50
4030
20
100

Preciptation(mm)PWV(mm)PWV an d Prec iptation at Khon Kaen (January-December,2002 )
9080
7060
50
4030
20
100
1- 16- 31- 15- 2- 17- 1- 16- 1- 16- 31- 15- 30- 15- 30- 14- 29- 13- 28- 13- 28- 12- 27- 12- 27Jan Jan Jan Feb Mar Mar Apr Apr May May May Jun Jun Jul Jul Aug Aug Sep Sep Oct Oct Nov NovDec Dec
DataPreciptation

PWV

Fig. 1: PWV change obtained at Khon Kaen in the year of2002.
In orders to investigate the relationship between thePWV and the Relative Humidity (RH). Daily RH by
Rawinsonde observation at every standard pressure levels(have been maintioned above) were statistically analized,
the results have shown in Table 1. It was found that PWVand RH at 300 mbs (32,000 ft height above ground level)
were relative with the minimum variance and standard
deviation (Fig.2.).
Table 1. Statistical analysis of PWV and RH.
StatisticalAnalysis
Average*VarianceStandardDeviation

1000
24.75
262
16.20

Standard Pressure Level At (mbs)
850
700
600
500
400
16.50
18.01
18.60
22.63
24.67
131
149
147
219
173
11.47
12.22
12.16
14.80
13.16

300
15.24
125
11.20

* Average of the difference between PWV and RH.

9080
7060
50
4030
20
100RH(%)

10080
60
40200PWV(mm)PWV at Kho n Kaen and RH Data (January-December,2002)
120

1- 16- 31- 15- 2- 17- 1- 16- 1- 16- 31-15- 30- 15- 30- 14- 29- 13- 28- 13- 28- 12- 27- 12- 27Jan Jan Jan Feb Mar Mar Apr Apr May May MayJun Jun Jul Jul Aug Aug Sep Sep Oct Oct Nov NovDecDec
Data
PWV

RH at 300 mbs

Fig. 2: Change of PWV and RH at 300 mbs.

4. Conclusion
We have performed GPS observation at Khon Kaen
University to investigate the precipitable water vaporchanged. The data observed from January to December
2002 are processed and obtained the following results:
The PWV changes widely in dry season (NovemberApril) and almost constant at high value in wet season(May-September). It is remarked that PWV increased

before rainfall and decreased after rainfall. PWV and RH at
300 mbs (32,000 ft height above ground level) wererelative with the minimum variance and standard deviation.

Acknowledgements
The authors wish to express their sincere thanks to TMD
who are kindly to provide them the pressure, thetemperature data at Khon Kaen and the Rawinsonde
observation data at Ubonratchathani station. They also
express their sincere thanks to Prof. T. Oki at IIS and Prof.
T. Kato at ERI, University of Tokyo, who gave their help
to perform the GPS observation.

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