Developments in Ground-Based GPS Meteorology

The use of ground-based GPS observations for the estimation of preciptiable water vapor (PWV) is now a well proven, tested, and widely applied technique. The strengths of ground-based GPS meteorology lie in its high accuracy of about 1.5 mm in PWV, its high temporal resolution ranging from seconds to hours, its all-weather capabilities, and relatively low costs. In spite of its undeniable success, ground-based GPS meteorology also has some disadvantages. It does not generally provide atmospheric profile information, does presently not provide data over the oceans, and most importantly GPS results have not yet yielded consistent and strong positive impact on weather models. This presentation will review the current status, and focus on current activities to overcome some of these shortcomings. In order to obtain information on the atmospheric profile, GPS slant observations have to be combined in tomographic or data assimilation analysis. Since low-elevation GPS observations contain the most atmospheric information improved GPS mapping functions are needed. We will describe a new "direct " mapping function that utilizes time and location specific profiles of refractivity from the NCEP AVN atmospheric analysis, and report results from its use in the routine analysis of SuomiNet at UCAR. To address the issue of GPS in the oceans we conducted a GPS experiment during a Caribbean cruise. The goal of this experiment was to determine the feasibility of low-resolution refractivity profiling with GPS from a moving platform and the estimation of PWV from a ship in the open ocean. Results depend strongly on the attainable position quality of the ship. Analysis of data from this experiment are presented. Finally, scientists at NCAR and UCAR are now collaborating to assimilate GPS PWV estimates from SuomiNet into the NCAR MM5 model. This work is now starting with the long term goal of a closed loop system where GPS helps improve the MM5 model, which in turn will provide improved refractivity fields for use in the GPS data analysis