Why monitor atmospheric water vapor? How do GPS receivers measure total integrated water vapor? Impact on Forecasting NOAA/OAR GPS Network (GPS-IPW) Publications GPS Total Precipitable Water Vapor Data Displays Comments or questions: David Welsh

GPS Antenna mounted on the corner fence post at one of NOAA/FSL's 404-MHz profiler sites.

Why monitor atmospheric water vapor? Water vapor is one of the most significant constituents of the atmosphere since it is the means by which moisture and latent heat are transported to cause "weather". Water vapor is also a greenhouse gas that plays a critical role in the global climate system. This role is not restricted to absorbing and radiating energy from the sun, but includes the effect it has on the formation of clouds and aerosols and the chemistry of the lower atmosphere. Despite its importance to atmospheric processes over a wide range of spatial and temporal scales, it is one of the least understood and poorly described components of the Earth's atmosphere. How do GPS receivers measure total integrated water vapor? GPS satellite radio signals are slowed by the Earth's atmosphere, which results in a delay in the arrival time of the transmitted signal from that expected if there were no intervening media. It is possible to correct for the ionospheric delay, which is frequency dependent, by using dual-frequency GPS receivers. The delays due to the neutral atmosphere are not frequency dependent, but depend on the constituents of the atmosphere that are a mixture of dry gasses and water vapor. The vertically scaled signal delays introduced by these components are called the zenith hydrostatic delay (ZHD) and the zenith wet delay (ZWD), respectively. In practice, the total signal delays measured by the GPS receiver from all satellites in view are mapped to the vertical using the function 1/sin(elevation angle of the satellite), and combined to give the Zenith Total (or Tropospheric) Delay (ZTD). At sea level, ZTD has a magnitude of about 250cm to which the hydrostatic and wet components contribute about 97% and 3%: approximately proportional to the ratio of the total mass of dry air to water vapor in the atmosphere. A record of the total delay, surface pressure, surface temperature, and total precipitable water vapor (PWV) is available at each site every 30-minutes. The actual calculation is performed by NOAA in this fashion. The ZTD is estimated by constraining the positions of widely-spaced GPS receivers and measuring the apparent error in position every 30 minutes. When all system related errors are accounted for, the residual error is presumed to come from the neutral atmosphere. The ZHD is calculated by measuring the surface pressure and applying a mapping function. The ZHD is then subtracted from the ZTD to give the ZWD which is related to the PWV directly above the GPS antenna through a factor that is proportional to the mean temperature of the atmosphere. The mean air temperature is currently estimated from a surface temperature measurement and its relationship to the climatological history of the temperature profile for that region based on radiosonde data. Impact on Forecasting While an important goal in modern weather prediction is the improvement of short-term cloud and precipitation forecasts, our ability to do so is severely limited by the lack of timely and accurate water vapor data. Prior to the development of GPS atmospheric remote sensing by the the University NAVSTAR Consortium and North Carolina State University (key individuals are now at University of Hawaii at Manoa), water vapor observing systems included radiosondes, surface-based microwave radiometers, satellite-based microwave and infrared radiometers, and some research systems. Each of these systems has limitations. Radiosondes provide information about the vertical distribution of water vapor in the atmosphere, but are labor intensive and only launched twice-daily. Surface-based radiometers are capable of high temporal resolution, but are costly and do not function well in all weather conditions, especially rain. Satellites provide global coverage, but have limited accuracy over land, are not capable of observing low in the atmosphere (where the water vapor is concentrated) and, in the case of infrared radiometers, can provide data only under clear sky conditions. Research aircraft-based measurements are limited and expensive, and routine observations using commercial aircraft (CASH) are just now being demonstrated, but these will provide data along flight paths and during ascent and descent near major airports. While GPS does not yet provide information about the vertical distribution of water vapor, it is inexpensive, accurate, reliable, operates under all weather conditions, and is capable of being widely deployed. NOAA/ERL GPS Network (GPS-IPW) In an effort to develop an operational surface-based GPS integrated precipitable water vapor (GPS-IPW) monitoring system for the National Oceanic and Atmospheric Administration (NOAA), the Environmental Technology Laboratory (ETL) and it's sister organization the Forecast Systems Laboratory (FSL) Demonstration Division (DD), currently operate and collect data from the GPS receivers and surface meteorological sensors seen in this station location map. This network should help us better understand the advection of water vapor and the latent heat associated with the phase change in water as it relates to the Earth's energy balance. Data collected at these sites are also part of the Continuously Operating Reference Station (CORS) network managed by NOAA's National Geodetic Survey (NGS), providing position information to the geodetic community. This project is a collaboration between aaa NOAA OAR, NGS, and National Data Buoy Center, the Scripps Institution of Oceanography, the University of Hawaii at Manoa, the University NAVSTAR Consortium, and the USCG. Publications Bevis, B.G., S. Bussinger, T.A. Herring, C. Rocken, R.A. Anthes, and R.H. Ware, 1992, GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning System, J. Geophys. Res., 97, 15787-15801. Businger, S., S.R. Chiswell, M. Bevis, J. Duan, R.A. Anthes, C. Rocken, R.H. Ware, M. Exner, T. Van Hove, and F.S. Solheim, 1996, The Promise of GPS in Atmospheric Monitoring, Bull. Amer. Meteor. Soc., 77, 5-18. Gutman, S.I., R.B. Chadwick, D.E. Wolfe, A. M. Simon, T. Van Hove, and C. Rocken, 1994, Toward an Operational Water Vapor Remote Sensing System using GPS. FSL Forum, Sept. 1994, 13-19. Gutman, S.I., D.E. Wolfe, and A. M. Simon, 1995, Development of an Operational Water Vapor Remote Sensing System Using GPS; A Progress Report. FSL Forum, Dec. 1995, 21-32. Kuo, Y., Y. Guo, and E.R. Westwater, 1993, Assimilation of Precipitable Water Vapor Measurments into a Mesoscale Numerical Model, Mon. Wea. Rev., 121, 1215-1238. Rocken, C., R.H. Ware, T. Van Hove, F. Solheim, C. Alber, J. Johnson, and M.G. Bevis, 1993, Sensing Atmospheric Water Vapor with the Global Positioning System, Geophys. Res. Lett., 20, 2631-2634. Yuan, L.L., R.A. Anthes, R.H. Ware, C. Rocken, W.D. Bonner, M.G. Bevis, and S. Bissinger, 1993, Sensing Climate Change Using Global Positioning System, J. Geophys. Res., 98, 14925-14937. Anyone interested in these data should contact either Daniel E. Wolfe, (303) 497-6204 or Seth I. Gutman, (303) 497-7031 National Oceanic and Atmospheric Administration | Environmental Technology Laboratory Updated: March 21, 2000 Webmaster