On the use of GPS zenith total delays to monitor NWP systems

A system to retrieve automatically the zenith total delay (ZTD) values at European GPS sites in a near real time (NRT) mode has been developed, with the purpose of retrieving these observables from the emerging dense networks of GPS receivers like SUOMINET. To test the concept, we have used data streams collected using a NRT European network. We used Predicted and Rapid orbits from the Center of Orbit Determination in Europe (CODE) and the GIPSY/OASIS-II software package to estimate the ZTD values. This software uses a stochastic filter to provide time-dependent estimates for each site.

In addition, an unattended system for the comparison of the GPS-derived ZTD estimates with the simulated ZTD values carried out with the HIRLAM NWP model has been established in collaboration with the Spanish Instituto Nacional de Meteorologia (INM). The HIRLAM model is a complete analysis and forecast system over a limited area. The model, at the INM, is run at two different horizontal resolutions: OPR (low-resolution run) and HIR (high-resolution run), both with the same 31 p-sigma, hybrid levels and vertical resolution. The HIR and OPR variables have been interpolated both horizontally and vertically to the locations of the GPS sites. Forward operators of zenith wet delay (ZWD), zenith hydrostatic delay (ZHD) and ZTD have been developed.

In this presentation we discuss the selected approach as well as the results of the comparison obtained during the last year. In addition, initial results on the impact of the assimilation (in a variational way) of these GPS data into NWP models will be given.