GPS Water Vapor Monitoring

Figure 36. Existing and future GPS precipitable water vapor (GPS-IPW) sites.

Of the four significant GPS-IPW system failures in 1996, three were associated with lightning strikes. Demonstration Division staff are currently working with UNAVCO to evaluate various in-line lightning surge suppressors at NOAA profiler sites, and they plan to install the selected units at the sites in 1997.

In January 1996, ERL installed the GAMIT geodetic processing software package, codeveloped by the Massachusetts Institute of Technology (MIT) and Scripps Institution of Oceanography (SIO), on a workstation in the Profiler Control Center in Boulder, Colorado. After attending a class on GAMIT fundamentals, ERL staff began processing their own GPS data in February 1996. Demonstration Division staff have been using this capability to evaluate many of the issues associated with real-time data processing and data quality, and SIO and the University NAVSTAR Consortium (UNAVCO) have been handling routine daily processing of the water vapor data. Division staff expect to assume responsibility for this activity in 1997, which will provide the university collaborators more time to concentrate on other critical research and development activities.

More than six months of experience with the demands of GPS data processing has enabled division staff to better define the near real-time data processing requirements for networks of GPS-IPW systems. As a result, a new and more capable [dedicated] workstation has been ordered and will replace the original unit. This workstation is expected to yield significant improvements in performance during 1997.

GPS-IPW Data and System Performance Monitoring - The results of data processing were reviewed on a regular basis to evaluate system performance and identify features of meteorological or climatological interest for further study. Some interesting examples of the data acquired in 1996 are presented in Figures 37 and 38.

Figure 37 depicts data acquired by the radar wind profiler and GPS-IPW systems located at the Haskell, Oklahoma, NPN site during a spring storm in April 1996. Radiosonde data acquired at the nearby ARM CART Site Boundary Facility in Morris, Oklahoma, is also shown. Of particular interest is the generally excellent correlation between high signal power from the vertical beam of the profiler, usually associated with [clouds and] rainfall, and peaks in precipitable water vapor measured by the GPS. With the notable exception of a small GPS-IPW peak at about 1200 UTC on day 112 (21 April), there is almost a one-to-one correlation between the radar signal power and precipitable water vapor. Observations such as this are made possible for the first time because of the high temporal resolutions of the profiler and GPS, and the all-weather capability of the GPS-IPW system.

Figure 37. Radar wind profiler and GPS-IPW observations during a spring storm observed at Haskell, Oklahoma, 21-23 April 1996.

Figure 38 captures what is believed to be a very unusual occurrence. On 22-23 July 1996, total precipitable water vapor observed by the GPS-IPW system at the Lamont, Oklahoma, NPN site rose more than 3.5 cm in about 6 hours, and then fell almost as much in the next 6 hours. This event is the result of thunderstorm outflow associated with the passage of a late spring storm, as confirmed by surface measurements showing an increase in surface pressure of 5 mb in 3 hours, a temperature drop of about 15 degrees in the same period, winds gusting to 15 m s-1, and rainfall at the rate of 3 mm per hour. Radiosonde data acquired at the nearby Atmospheric Radiation Measurement Central Facility (also shown in Figure 38) confirms the event, but not in the detail provided by GPS. As a contributor to the CORS GPS network, division staff sent FSL GPS data and surface meteorological observations to the NOAA National Geodetic Survey every day. These data are available to the public through the following Internet location:

Ftp cors.ngs.noaa.gov
login: anonymous
password: your complete e-mail address.

Plans are to assimilate the GPS data into FSL's Mesoscale Analysis and Prediction System (MAPS), the research version of the Rapid Update Cycle (RUC) numerical weather prediction model running at the National Centers for Environmental Prediction, using the three-dimensional variational analysis. Options identified by the Forecast Research Division include direct analysis of the precipitable water vapor field and/or use a forward model to estimate the zenith tropospheric signal delay (ZTD) from the forecast background, and use the ZTD residual (difference between the forecast and observation) in variational analysis. Collocation studies, sensitivity tests, and verification and evaluation studies are planned for 1997.

In addition, processed GPS precipitable water vapor data are sent to several organizations every day for analysis and collaborative research and development efforts. These organizations include the National Environmental Satellite, Data, and Information Service (NESDIS) Satellite Research Laboratory, the FSL Forecast Research Division, the ETL System Demonstration and Development Division, and the Department of Energy's Atmospheric Radiation Measurement program. NCEP and the FSL Facility Division will also start receiving these data early in 1997.

A necessary step in the development of any new observing system involves an assessment of the impact of new data on weather forecast accuracy. The FSL Forecast Research Division is conducting a study of the effects of surface-based GPS precipitable water vapor data on mesoscale numerical forecasts.

Recent comparisons of GPS-IPW measurements with temporally and spatially sampled Geostationary Operational Environmental Satellite (GOES) and Polar Orbiting Environmental Satellite (POES)/Television and Infrared Operational Satellite (TIROS) Operational Vertical Sounder (TOVS) precipitable water vapor suggest the potential of using surface-based GPS data to correct wet biases in the satellite precipitable water vapor in an operational environment. A paper on this topic will be presented at the American Geophysical Union meeting in December 1996, and plans are being made to extend this line of investigation to different seasons and climate regimes in 1997.

FSL is working with the National Data Buoy Center to develop a small surface meteorological sensor package, the GPS Surface Observing System (GSOS), that can be deployed at GPS sites with power and communications, such as the U.S. Coast Guard/U.S. Army Corps of Engineers Differential GPS (DGPS) sites along the coasts and major river systems of the United States. The first GSOS was installed at the U.S. Coast Guard Electronics Engineering Center at Wildwood, New Jersey, in February 1996, and a second unit has been operating continuously at ERL in Boulder since August 1996.

Figure 38. Very rapid change in total precipitable water vapor observed at the Lamont, Oklahoma, NOAA Profiler Network site in July 1996.