A comparison of Global Positioning System retrieved precipitable water vapor with the numerical weather prediction analysis data over the Japanese Islands

Behavior of precipitable water vapor (PWV) routinely retrieved from the nat ionwide array of the Global Positioning System (GPS) established by Geograp hical Survey Institute (GSI) of Japan are compared with the Japan area obje ctive analysis data for numerical weather prediction (NWP) of the Japan Met eorological Agency (JMA). The array used here has a spatial resolution of a bout 50 km for monitoring crustal deformation. The 3-hourly zenith troposph eric delay (ZTD) data obtained in GSI's routine analysis system are convert ed into PWV data by using the 12-hourly NWP data. While a front accompanyin g with heavy rainfall moved eastward across the Japanese Islands from 0900 local standard time (LST) September I to 0900 LST September 3, 1996, the GP S array successfully detected the temporal anomalies of GPS PWV moving alon g with the front, in which the internal errors are estimated to be less tha n 3 mm. The results reveal that GSI's GPS array can work as an all weather giant array sensor of PWV over the Japanese Islands. It is found, however, that the GPS, shows systematically fewer PWV than NWP data in mountainous a reas reaching about 10 mm. This bias results mainly from the fact that most GPS sites located at bottom of valley in mountainous areas. After removing the topographical effects, there still remain significant differences amou nting to 2 - 4 mm associated with errors of GPS observations and/or NWP obj ective analyses data.