Bevis M., S. Businger, 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, Journal of Geophys. Research, Vol. 97, No. D14, pp 15,787-15,801.
Abstract: We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground-based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time-varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground-based GPS networks could be analyzed in concert with observations of GPS satellite occulations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.Bevis, M., S. Businger, S. Chiswell, T.A. Herring, R.A. Anthes, C. Rocken and R.H. Ware, 1994. GPS Meteorology: Mapping zenith wet dealys onto precipitable water, Journal of Applied Meteorology, 379-386.
Abstract: Emerging networks of Global Positioning System (GPS) receivers can be used in the remote sensing of atmospheric water vapor. The time-varying zenith wet delay observed at each GPS receiver in a network can be transformed into an estimate of the precipitable water overlying that receiver. This transformation is achieved by multiplying the zenith wet delay by a factor whose magnitude is a function of certain constants related to the refractivity of moist air and of the weighted mean temperature of the atmosphere. The mean temperature varies in space and time and must be estimated a priori in order to transform an observed zenith wet delay into an estimate of precipitable water. We show that the relative error introduced during this transformation closely approximates the relative error in the predicted mean temperature. Numerical weather models can be used to predict the mean temperature with an rms relative error of less than 1%.Businger, S., S.R. Chiswell, M. Bevis, J. Duan, R. A. Anthes, C. Rocken, R. H. Ware, T. VanHove, and F. S. Solheim, 1996. The Promise of GPS in Atmospheric Monitoring, Bull. Am. Meteor. Soc., 77, pp 5-18.
Abstract: This paper provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earth's atmosphere. Microwave radio signals transmitted by GPS satellites are delayed (refracted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites. The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to measurements by ground-based water vapor radiometers. Increased spatial and temporal resolution of the water vapor distribution provided by the GPS/STORM network proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated water vapor associated with the passage of a midtropospheric cold front, both of which triggered severe weather over the area during the course of the experiment. Given the rapid growth in regional networks of continuously operating Earth-based GPS receivers currently being implemented, an opportunity exists to observe the distribution of water vapor with increased spatial and temporal coverage, which could prove valuable in a range of operational and research applications in the atmospheric sciences. The first space-based GPS receiver designed for sensing the Earth's atmosphere was launched in April 1995. Phase measurements of GPS signals as they are occluded by the atmosphere provide refractivity profiles (see the companion article by Ware et al. in this issue). Water vapor limits the accuracy of temperature recovery below the tropopause because of uncertainty in the water vapor distribution. The sensitivity of atmospheric refractivity to water vapor pressure, however, means that refractivity profiles can in principle yield information on the atmospheric humidity distribution given independent information on the temperature and pressure distribution from NWP models or independent observational data. A discussion is provided of some of the research opportunities that exist to capitalize on the complementary nature of the methods of active atmospheric monitoring by GPS and other observation systems for use in weather and climate studies and in numerical weather prediction models.Cahine, M, 1997. GEWEX News.
Duan, J., M. Bevis, P. Fang, Y.Bock, S. Chiswell, S. Businger, C. Rocken, F. Solheim, T. VanHove, R. Ware, S. Mc Clusky, T. A. Herring, and R. W. King, 1996. GPS Meteorology: Direct Estimation of the Absolute Value of Precipitable Water, J. of Applied Met ., Vol. 35, No. 6, pp 830-838.
Abstract: A simple approach to estimating vertically integrated atmospheric water vapor, or precipitable water, from Global Positioning System (GPS) radio signals collected by a regional network of ground-based geodetic GPS receivers is illustrated and validated. Standard space geodetic methods are used to estimate the zenith delay caused by the neutral atmosphere, and surface pressure measurements are used to compute the hydrostatic (or ``dry'') component of this delay. The zenith hydrostatic delay is subtracted from the zenith neutral delay to determine the zenith wet delay, which is then transformed into an estimate of precipitable water. By incorporating a few remote global tracking stations (and thus long baselines) into the geodetic analysis of a regional GPS network, it is possible to resolve the absolute (not merely the relative) value of the zenith neutral delay at each station in the augmented network. This approach eliminates any need for external comparisions with water vapor radiometer observations and delivers a pure GPS solution for precipitable water. Since the netural delay is decomposed into its hydrostatic and wet components after the geodetic inversion, the geodetic analysis is not complicated by the fact that some GPS stations are equipped with barometers and some are not. This approach is taken to reduce observations collected in the field experiment GPS/STORM and recover precipitable water with an rms error of 1.0-1.5 mm.Gutman, S., R. Chadwick, D. Wolfe, A. Simon, T. VanHove, and C. Rocken, 1994. Toward an Operational Water Vapor Remote Sensing System Using GPS, FSL Forum.
Abstract: Significant progress has been made over the past decade in the development of remote-sensing instruments to profile wind and temperature. However, the current technology of profiling water vapor remotely is still far from perfect. Although some promising optical research systems, such as the Raman lidar, can provide high vertical resolution profiles of water vapor, it may be years before they are generally available. Currently, there are several systems that can measure the vertically integrated water vapor (i.e., precipitable water) with a high degree of accuracy. In this paper we use a simple method to assimilate precipitable water measurements (possibly from a network of dual-channel ground-based microwave radiometers or a satellite-based system) into a mesoscale model. The basic idea is to relax the predicted precipitable water toward the observed value, while retaining the vertical structure of the model humidity field. We test this method with the special 3-h soundings available from the Severe Environmental Storms and Mesoscale Experiment. The results show that the assimilation of precipitable water into a mesoscale model recovers the vertical structure of water vapor with an accuracy much higher than that from statistical retrieval based on climatology. The improved analysis due to assimilation also leads to improved short-range precipitation forecasts.Melbourne, W., E. Davis, C. Duncan, G. Hajj, K. Hardy, E. Kursinski, T. Meehan, L. Young, and T. Yunck, The Application of Spaceborne GPS to Atmospheric Limb Sounding and Global Change Monitoring, JPL Pub. 94-18, 1994.
Rocken, C. T. Van Hove, J. Johnson, F. Solheim, R. H. Ware, M. Bevis, S. Businger, S. Chiswell, GPS/STORM - GPS Sensing of Atmospheric Water Vapor for Meteorology, Journal of Atmos. and Ocean. Tech., Vol. 12, No. 3, pp 468-478, June 1995.
Abstract: Atmospheric water vapor was measured with six Global Positioning System (GPS) receivers for 1 month at sites in Colorado, Kansas, and Oklahoma. During the time of the experiment, from 7 May to 2 June 1993, the area experienced severe weather. The experiment, called ``GPS/STORM,'' used GPS signals to sense water vapor and tested the accuracy of the method for meteorological applications. Zenith wet delay and precipitable water (PW) were estimated, relative to Platteville, Colorado, every 30 min at five sites. At three of these five sites the authors compared GPS estimates of PW to water vapor radiometer (WVR) measurements. GPS and WVR estimates agree to 1-2 mm rms. For GPS/STORM site spacing of 500-900 km, high-accuracy GPS satellite orbits are required to estimate 1-2-mm-level PW. Broadcast orbits do not have sufficient accuracy. It is possible, however, to estimate orbit improvements simultaneously with PW. Therefore, it is feasible that future meteorological GPS networks provide near-real-time high-resolution PW for weather forecasting.Rocken, C., T. VanHove. R. Ware, Near real-time GPS sensing of atmospheric water vapor, Geophys. Res. Let., submitted, June, 1997.
Rocken, C., R. Anthes, M. Exner, R. Ware, D. Feng, M. Gorbunov, B. Herman, D. Hunt, Y.-H. Kuo, W. Schreiner, S. Sokolovskiy, and X. Zou, Verification of GPS/MET Data in the Neutral Atmosphere, Journal of Geophysical Research, in press, 1998.
Ware, R., M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, H. K. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, GPS Sounding of the Atmosphere from low Earth orbit: Preliminary Results, Bulletin of the American Meteorological Society, 77, 19-40, 1996.
Abstract: This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750 km altitude and 70 degrees inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research. Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5-7 km), temperature may be estimated directly from refractivity. This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5-7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2 degrees C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5 degrees C at the tropopause, degrading to about 1 degrees C at 40 km altitude. Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.Ware, R.H., C. Alber, C. Rocken, F. Solheim, Sensing integrated water vapor along GPS ray paths, Geophys. Res. Let., Vol. 24, No. 4, pp 417-420, Feb. 15, 1997.
Abstract: We demonstrate sensing of integrated slant-path water vapor (SWV) along ray paths between Global Positioning System (GPS) satellites and receivers. We use double differencing to remove GPS receiver and satellite clock errors and 85-cm diameter choke ring antennas to reduce ground-reflected multipath. We compare more than 17,000 GPS and pointed radiometer double difference observations above 20 degrees elevation and find 1.3 mm rms agreement. Potential applications for SWV data include local and regional weather modeling and prediction, correction for slant wet delay effects in GPS surveying and orbit determination, and synthetic aperture radar (SAR) imaging. The method is viable during all weather conditions.Yuan L., R. A. Anthes, R.H. Ware, C. Rocken, W. Bonner, M. Bevis and S. Businger, Sensing Climate Change Using the Global Positioning System, Journal of Geophys. Res., Vol. 98, No. D8, pp 14,925-14,937, 1993.
Abstract: Using simulated atmospheric data from the National Center for Atmospheric Research (NCAR) community climate model (CCM), we test the hypothesis that the global positioning system (GPS) can be used to detect global and regional climate change. We examine how the fundamental GPS variables (wet and total delays and vertical profiles of refractivity) as well as precipitable water as estimated by ground-based GPS receivers would change in a climate with 2 times the present concentrations of carbon dioxide (CO<<SUB 2>>). Because of the higher water vapor content in the doubled CO<<SUB 2>> simulation the wet delay and the precipitable water show a significant increase in the tropics and middle latitudes. Refractivity also shows an increase in the lower troposphere. We also simulate the changes in the GPS signal delay in a doubled CO<<SUB 2>> climate as would be measured by a ratio occultation technique using low Earth-orbiting (LEO) satellites equipped with GPS receivers. Increases in temperature and water vapor in the lower troposphere of the model atmosphere produce opposite effects on the occultation delay. Increased temperature tends to decrease the delay, while increased water vapor increases the delay. Amplified by the long "lever arms'' of the LEO-atmosphere-GPS link, a strong "greenhouse warming'' signal is simulated, with increases in occultation delay of nearly 100 m using the globally averaged data. This increase indicates that globally the effect of increased water vapor dominates. However, significant regional differences are present in the occultation delay response. In the tropics. where the temperature increase is smallest and the water vapor increases are largest, increases in delay of about 300 m are simulated. In contrast, in the polar regions where the increased temperatures are greatest and the increases in water vapor are smallest, the temperature effect dominates and a decrease in occultation delay of nearly 70 m is simulated. When compared to expected errors in measuring the occultation delay of about 1 m, these results indicate that monitoring trends in occultation delays would be a practical way to detect global and regional climate change.