Earth Planets Space, Vol. 52 (No. 11), pp. 913-919, 2000
Hiroshi Takiguchi1, Teruyuki Kato2, Hiromichi Kobayashi3, and Toshiyuki Nakaegawa4
1Department of Geophysics, Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Japan
2Earthquake Research Institute, University of Tokyo, Japan
3Kajima Corporation, Japan
4Meteorological Research Institute, Japan Meteorological Agency, Japan
(Received January 17, 2000; Revised September 26, 2000; Accepted September 29, 2000)
Abstract: We report the delineation of the onset of the Asian Monsoon based on GPS sensing of water vapor in Thailand. We conducted GPS observations at five sites in Thailand since March 1998 under the hydrological project called GAME-T. The objective of the project is to clarify the water and energy cycle system in the Asian Monsoon area. As a preliminary analysis, we used data from March to June 1998 and estimated the water vapor content in the zenith direction (PWV) every 30 minutes using GIPSY software (GPS-PWV). A comparison of the resultant PWV with those estimated from rawinsonde data (Sonde-PWV) suggested that, generally, the long term trends of both GPS-PWV and Sonde-PWV are consistent and a rapid increase of water vapor content is visible in May, which corresponds to the onset of the Monsoon. However, systematic differences between GPS-PWV and Sonde-PWV are eminent. The RMS of the difference (RMSD) between Sonde-PWV and GPS-PWV reaches about 8.7 mm. This large RMSD can be reduced to about 5 mm by removing some unreliable sonde data and making a linear correction to Sonde-PWV. In addition, a comparison of GPS-PWV with other meteorological data (temperature, humidity, and rainfall) showed that there is a strong correlation between a rapid increase of GPS-PWV and heavy rainfall in Bangkok and in Chiang Mai, which may be used to judge the onset of the Monsoon in the area accurately.