Sensing Climate Change Using the Global Positioning System

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 concentration of carbon dioxide (CO2). Because of the higher water vapor content in the doubled CO 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 2 climate as would be measured by a radio occultation technique using low Earthorbiting (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 This paper describes model simulations of a new approach to measuring atmospheric water vapor in the troposphere and temperature in the upper atmosphere for use in operational weather forecasting and weather and climate research. The approach involves using the global positioning system (GPS). GPS was originally designed as a highly accurate navigational system but has become a widely used tool by geodesists, geophysicists, and surveyors because of its capability for very precise positioning. In GPS geodesy the distances between GPS satellites and GPS receivers are determined by measuring the time of propagation of radio signals from satellites to receivers. This time is affected by the temperature and the water vapor content of the atmosphere along the path of propagation. For the purposes of geodesy the effects of the atmosphere must be removed to the extent possible. However, this "noise" to the geodesist is a "signal" to the atmospheric scientist [Hardy et al., 1992; Bevis et al., 1992, 1993]. Using GPS, researchers could measure vertical profiles of refractivity, a characteristic of the atmosphere depending upon the local pressure, temperature, and moisture. From this refractivity information it is possible to extract temperature or moisture data which could be incorporated into regional or global numerical weather prediction (NWP) models, ultimately improving forecasts. Another promising application of GPS data is the detection of global and regional climate change by the