In this study, the sensing of water vapor using the global positioning system (GPS) was investigated in Tehran. Water vapor mean temperature T m is a key parameter in conversion of GPS zenith wet delay to precipitable water vapor (PWV). Then, by using 8 years of radiosonde and surface temperature data, we achieved a new site-specific T m model in Tehran. After the comparison of Bevis et al. (J Geophys Res 97(D14):15787-15801, 1992) and the site-specific models, a mean bias error of -1.3 K was found for Bevis model, while this is less than 0.1 K for the new local model. Therefore, PWV time series were generated for the ground-based GPS site in Tehran from the beginning of 2005 to the end of 2013. Comparing the GPS with the radiosonde PWV, it was shown that the GPS can be used to measure the PWV with high precision. The root mean square error for differences between the GPS and radiosonde was derived as 1.5 mm. The PWV has significant relationship with precipitation in our case study. The thresholds of the relative humidity anomaly with different PWV anomaly related to rainfall occurrences are also considered in this research. The analyses show that use of PWV anomaly condition together with surface meteorological parameters reduces the number of false rainfall recognitions significantly.

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In this study, the performance of different processing software and strategies on the estimation of Zenith Wet Delay (ZWD) from Iran Permanent GPS Network (IPGN) was evaluated. For this purpose, GAMIT (version 10.4) and Bernese (version 5.0) software were used to estimate the ZWD values in baseline and Precise Point Positioning (PPP) mode, respectively. Then, the GPS ZWD time series in Tehran, which is the only International GNSS Service (IGS) station in Iran, were validated with the corresponding values derived from the measurements of the nearby radiosonde site at Mehrabad airport. Based on one year of estimates in both network (or baseline) and PPP mode, the GPS and radiosonde ZWD were consistent with a mean bias of 6 mm and standard deviation of 12 mm. furthermore, the IGS final tropospheric products over 2011 were used to validate our GPS data processing in this study. The Results showed that there is a good agreement between our estimates and those obtained from IGS with a mean bias of less than 1 mm. Comparing PPP with the network GPS ZWD solutions in 15 stations of IPGN over one year, showed that two methods are consistent with mean bias and standard deviation of less than 2 and 5 mm. Finally, to examine the operational usage of IPGN tropospheric products, using the IGS ultra-rapid orbits the near real time ZTD were estimated and compared with final post processed solutions. For all stations, the near real time ZTD estimation results were comparable with the corresponding post processed estimates in terms of bias and standard deviation. The obtained correlation coefficient between final and near real time solutions was more than 0.95. These results suggest that ZTD derived from Iranian regional GPS network has the potential to incorporate in different meteorological applications.