Ground-Based GPS Meteorology at the NOAA Forecast Systems Laboratory

The ability to use the Global Positioning System (GPS) to make accurate refractivity measurements under all weather conditions has, in the last few years, led to the development of a promising new meteorological observing system for NOAA. The first application of ground-based GPS meteorology involves the measurement of integrated (total column) precipitable water vapor in the atmosphere. This technique, called GPS-IPW, has several advantages over conventional water vapor observing systems including low cost, high measurement accuracy, all weather operability, and long-term measurement stability. In addition, GPS-IPW requires no external calibration, operates unattended for long periods with high reliability, and is easily maintained. Since GPS-IPW measurements are compatible with those made by satellites, they provide a convenient and independent method for calibrating and validating global satellite observations.

The major disadvantage of GPS-IPW is that it provides no information about the vertical distribution of the water vapor. Since knowledge about the vertical distribution is essential to meteorologists, research into the best ways to use GPS-IPW in weather forecasting is ongoing at government laboratories and universities around the world. Studies of the impact of GPS-IPW data on weather forecast accuracy conducted at the NOAA Forecast Systems Laboratory (FSL) in Boulder, Colorado over the past two years have been encouraging. Even more encouraging is the fact that these results have been achieved with a limited number of sites. As a consequence, NOAA meteorologists expect significant improvements in short-term forecasts of clouds, precipitation, and severe weather when high-resolution numerical weather prediction models routinely use data from a nationwide network of GPS-IPW systems.

The current status of the NOAA/FSL GPS Water Vapor Demonstration Network is described and some observations from the network are presented. Plans for expanding the network and improving data availability are described. When techniques such as measuring line-of-sight refractivity are combined with promising new data processing and analysis techniques, it may be possible to recover a significant amount of information about the vertical and horizontal structure of the atmosphere from ground-based GPS observations.