THE PROMISE OF CPS IN ATMOSPHERIC MONITORING

This paper provides an overview of applications of the Global Position ing System (GPS) for active measurement of the Earth's atmosphere. Mic rowave radio signals transmitted by GPS satellites are delayed (refrac ted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites. The delay in G PS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrate d along the signal path. Measurement of atmospheric water vapor by Ear th-based GPS receivers was demonstrated during the GPS/STORM field pro ject 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 n etwork proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated waiter 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. Give n the rapid growth in regional networks of continuously operating Eart h-based GPS receivers currently being implemented, an opportunity exis ts to observe the distribution of water vapor with increased spatial a nd temporal coverage, which could prove valuable in a range of operati onal and research applications in the atmospheric sciences. The first space-based GPS receiver designed for sensing the Earth's atmosphere w as launched in April 1995. Phase measurements of GPS signals as they a re occluded by the atmosphere provide refractivity profiles (see the c ompanion article by Ware et al. on page 19 of this issue). Water vapor limits the accuracy of temperature recovery below the tropopause beca use 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 atmos pheric humidity distribution given independent information on the temp erature and pressure distribution from NWP models or independent obser vational data. A discussion is provided of some of the research opport unities that exist to capitalize on the complementary nature of the me thods of active atmospheric monitoring by GPS and other observation sy stems for use in weather and climate studies and in numerical weather prediction models.