Development of time-varying global gridded T s -T m model for precise GPS-PWV retrieval

Abstract

Water-vapor-weighted mean temperature, T m , is the key variable for estimating the mapping factor between GPS zenith wet delay (ZWD) and precipitable water vapor (PWV). For the near-real-time GPS-PWV retrieval, estimating T m from surface air temperature T s is a widely used method because of its high temporal resolution and fair degree of accuracy. Based on the estimations of T m and T s at each reanalysis grid node of the ERA-Interim data, we analyzed the relationship between T m and T s without data smoothing. The analyses demonstrate that the T s -T m relationship has significant spatial and temporal variations. Static and time-varying global gridded T s -T m models were established and evaluated by comparisons with the radiosonde data at 723 radiosonde stations in the Integrated Global Radiosonde Archive (IGRA). Results show that our global gridded T s -T m equations have prominent advantages over the other globally applied models. At over 17% of the stations, errors larger than 5K exist in the Bevis equation (Bevis et al., 1992) and in the latitude-related linear model (Y. B. Yao et al., 2014), while these large errors are removed in our time-varying T s -T m models. Multiple statistical tests at the 5% significance level show that the time-varying global gridded model is superior to the other models at 60.03% of the radiosonde sites. The second-best model is the 1°×1° GPT2w model, which is superior at only 12.86% of the sites. More accurate T m can reduce the contribution of the uncertainty associated with T m to the total uncertainty in GPS-PWV, and the reduction augments with the growth of GPS-PWV. Our theoretical analyses with high PWV and small uncertainty in surface pressure indicate that the uncertainty associated with T m can contribute more than 50% of the total GPS-PWV uncertainty when using the Bevis equation, and it can decline to less than 25% when using our time-varying T s -T m model. However, the uncertainty associated with surface pressure dominates the error budget of PWV (more than 75%) when the surface pressure has an error larger than 5hPa. GPS-PWV retrievals using different T m estimates were compared at 74 International GNSS Service (IGS) stations. At 74.32% of the IGS sites, the relative differences of GPS-PWV are within 1% by applying the static or the time-varying global gridded T s -T m equations, while the Bevis model, the latitude-related model and the GPT2w model perform the same at 37.84%, 41.89% and 29.73% of the sites. Compared with the radiosonde PWV, the error reduction in the GPS-PWV retrieval can be around 1-2mm when using a more accurate T m parameterization, which accounts for around 30% of the total GPS-PWV error.