A Study of Near Real-time Water Vapor Analysis Using a Nationwide Dense GPS Network of Japan

In order to use the nationwide dense receiver network of the Global Positioning System (GPS) in Japan to contribute to water vapor monitoring and numerical weather prediction, a near real-time (NRT) analysis trial was performed at the Meteorological Research Institute. In this paper, the NRT analysis procedure is described along with some validation results.In view of the computational load involved in analyzing more than 1,300 GPS stations in Japan, the precise point positioning (PPP) procedure was adopted. The PPP procedure requires accurate information of GPS satellites’ positions and clock offsets. International GNSS Service (IGS) has been routinely providing ultra rapid ephemeris (IGU) that includes satellite orbits and clock offsets with latency of about 3 hours. We found the accuracy of satellite clock offsets in IGU was insufficient for the retrieval of precipitable water vapor (PWV) through the PPP procedure. Therefore, we applied correction to the IGU clock using the predicted clock offset at an IGS station “USUD”. The hydrogen maser atomic clock at USUD also had some differences with GPS time. However, we found it could be fitted and predicted by a linear equation for a period of several days.The resulting satellite clock offsets exhibited some biases toward the IGS final ephemeris, but the time constant biases of satellite clock offsets did not affect the PWV retrieval at all. The retrieved PWV data agreed well with those obtained from radio-sonde observations. The root mean square differences in summer and in winter were around 3.4 mm and 1.6 mm, respectively. The results were comparable with those obtained by preceding studies using the final ephemeris. The Retrieved spatial and temporal variation of PWV in a heavy rainfall case demonstrated the usefulness of the NRT PWV retrieval for weather monitoring. We could capture the water vapor increase that preceded torrential rain.