Recent computer simulations by MacDonald and Xie (2001) indicate that it may be possible to retrieve the vertical distribution of moisture in the atmosphere from Global Positioning System (GPS) line-of-sight (or slant-path) observations using 3-dimensional variational analysis techniques. Since information about the vertical distribution of moisture is always desirable, there is great interest in GPS slant-path observations to support next-generation weather forecasting and climate monitoring activities at NOAA. To make the best use of these measurements in numerical weather prediction, it is necessary to understand their observational error covariances, and be able to assess their accuracy under the full range of conditions encountered in nature. As atmospheric scientists, we are not expert in GPS measurement techniques, and we rely on the developers of these techniques to help us gain a realistic view of this potentially significant extension of ground-based GPS meteorology. Lingering questions about the limitations of the various techniques, assumptions used to make the measurements, and the lack of a clear way to validate the results, led the Forecast Systems Laboratory to convene a workshop on slant-path GPS meteorology in Boulder, Colorado on June 21, 2001. The meeting was attended by many of the leaders in GPS measurement science, including: Chris Rocken and Randolph Ware from the University Corporation for Atmospheric Research; Yoaz Bar Sever from NASA’s Jet Propulsion Laboratory; Yehuda Bock and David Chadwell from the Scripps Institution of Oceanography; Pedro Elosequi and Jim Davis from the Harvard Smithsonian Center for Astrophysics; and Tom Herring from the Massachusetts Institute of Technology.

In this presentation, I will attempt to summarize the various techniques used by the presenters to make slant-path measurements or retrieve information about the vertical refractivity structure using slant-path techniques. I will then present some of the major conclusions and recommendations regarding making and validating slant-path GPS measurements.

Reference

MacDonald, A.E and Y. Xie, 2001. On the use of slant observations from GPS to diagnose three dimensonal water vapor using 3DVAR, A. E. MacDonald and Yuanfu Xie, Fourth Symposium on Integrated Observing Systems, AMS 2000, Long Beach, California. 62-73.