Comparisons of GPS-derived precipitable water vapors with radiosonde observations in Japan

We investigated the accuracy and features of precipitable water vapor (PWV) obtained from the Japanese Global Positioning System (GPS) network. We com pared PWVs estimated from the Geographical Survey Institute (GSI) routine g eodetic analysis using the Bernese software with those derived from the nea rest 10 radiosonde stations over the Japanese islands. The comparisons for a half year showed that the agreement of the CPS-derived PWV was 3.7 mm in terms of rms difference, with a standard deviation of 2.6 mm and a mean bia s of -2.7 mm, which is less precise than those obtained in central North Am erica. This may reflect the high temporal and spatial variability of water vapor over the Japanese islands, causing differences between PWVs if GPS an d radiosonde launch stations are not colocated. We also found systematic bi ases in the GPS-derived PWV. The absolute value of the mean bias at 1200 UT was systematically larger than that at 0000 UT. Their difference reached a s much as 2 mm. Moreover, the mean biases tended to increase as PWV increas ed. We also confirmed that there were similar systematic biases in the PWV obtained from our GPS analysis using the GIPSY software with high temporal resolution. This fact indicates that the systematic biases depend neither o n analysis software nor on temporal resolution of PWV estimation. We analyz ed GPS data collected at the Tsukuba station using GIPSY for 2 years and sh owed a possibility that the systematic biases also found at the Tsukuba sta tion were attributed to ocean tidal loading and seasonal variation of the m apping function both of which were not taken into account in the GPS analys is with the routine analysis using Bernese and our analysis using GIPSY. Th ese results suggest the importance of implementing them in the analysis for accurate estimates of absolute value of PWV from the Japanese GPS network.