Recommend citations:
◊ Zhang W., Zhang H., Liang H., Lou Y., Cai Y., Cao Y., Zhou Y., Liu W. (2019) On the suitability of ERA5 in hourly GPS precipitable water vapor retrieval over China. Journal of Geodesy. 93, 1897-1909.
◊ Zhou Y., Lou Y., Zhang W., Kuang C., Liu W., Bai J. (2020) Improved Performance of ERA5 in Global Tropospheric Delay Retrieval. Journal of Geodesy. 94:103.
Introduction
The original idea to create the GMET online service is actually to provide an interface to calculate the air pressure and weighted mean temperature (Tm) at GNSS stations from ERA5 reanalysis product for precipitable water vapor (PWV) retrieval.
Ground-based GNSS has been proved to be a powerful tool in measuring atmospheric water vapor compared to other techniques in the past decades, with advantages of high accuracy, high temporal resolution, low cost, good homogeneity and continuity in all weather conditions. The number of ground-based GNSS stations are continuously increasing, making the ground-based GNSS meteorology promising in many applications. However, a fact is that most of the ground-based stations were not originally built for meteorological use. We therefore do not have collocated meteorological sensors at these stations to measure the air temperature and pressure, which are indispensable for PWV retrieval. Even at many GNSS stations with collocated meteorological sensors, the measurements are not reliable enough due to the calibration and drift issues in the sensors.
According to our assessment (Zhang et al., 2019), the latest ECWMF global reanalysis, ERA5, with temporal and spatial resolution of 1 h and 31 km, respectively, can well support the retrieval of PWV by providing the meteorological parameters. The PWV error contributed by ERA5-derieved pressure and Tm is generally less than 1mm over China, and the PWV diurnal variations can be well maintained. However, when we tried to estimate pressure and Tm from ERA5 at GNSS stations, we found that the hourly ERA5 data are very large to be downloaded and archived, and it is somewhat time-consuming and boring to make necessary studies on the data before using them. People who are interested in calculating meteorological parameters at the same time epoch may repeatedly download the same huge dataset. We are therefore very eager to have an online interface to facilitate the calculation of meteorological parameters without the time-consuming data preparation and data manipulation, which motivates us to create this website. Along with the air temperature, pressure and Tm, we also support the calculation of e (water vapor pressure), ZHD (Zenith Hydrostatic Delay), ZWD (Zenith Wet Delay), ZTD (Zenith Total Delay) and PWV (Precipitable Water Vapor) based on the reanalysis.
Based on the RADIATE ray-tracing software open sourced by TU Wien (https://vmf.geo.tuwien.ac.at/), we have also made investigations on the ray-traced tropospheric delays from the ERA5, and found some improvements compared to delays from ERA-Interim (Zhou et al. 2020), the predecessor of ERA5 and the data source for the latest VMF3 mapping function. Therefore, this website also provides mapping function derived from ERA5 at user input sites.
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