Our quest towards a deeper understanding of weather and climate and a significantly improved skill to predict their future behavior depends critically on our knowledge of the present structure of atmospheric water and its variations. Here we present and discuss a particular implementation of the spacecraft radio occultation technique designed to characterize the thermodynamic and compositional structure of the atmosphere referred to as BRIGHTOC (Bi-static Radar Imaging of Geopotential, Humidity, Temperature, Ozone and Clouds). The concept works by transmitting tones near the 22 and 183 GHz water lines from one LEO across the limb of the atmosphere to another LEO. The measured delay and attenuation of the signal experienced as it propagates through the atmosphere can be inverted to recover atmospheric moisture, temperature and the geopotential of pressure surfaces. In clear sky conditions, our analysis indicates this limb-viewing system will retrieve horizontally-averaged water vapor concentrations to ~1-2% precision with ~250 m or better diffraction-limited vertical resolution from near the surface well into the mesosphere (Kursinski etr al., 2002a). Absolute accuracy of better than 1% will be achieved through the use of calibration tones and on orbit spectroscopic calibration. Relative humidity can be retrieved to a few % limited primarily by the retrieved temperature accuracy. In cloudy conditions, the performance will be within a factor of ~2 of clear sky performance and allow characterization of cloud liquid water to ~0.04 g/m3 or better near the surface and significantly better at higher altitudes (Kursinski et al., 2002b). The technique complements nadir viewing sounders which provide better horizontal resolution. Its unique features are well suited to characterizing the present distribution of atmospheric moisture and its variations at unprecedented levels of precision and accuracy. The technique will provide substantial observational constraints on the mechanisms controlling the distribution of atmospheric moisture. A portion of our concept is presently being considered for demonstration in the ESA ACE+ mission to launch in 2007. We will describe the technique and summarize its expected capabilities.

Kursinski, E. R., S. Syndergaard, D. Flittner, D. Feng, G. Hajj, B. Herman, D. Ward, and T. Yunck, A microwave occultation observing system optimized to characterize atmospheric water, temperature and geopotential via absorption, JTECH, in press, 2002a.

Kursinski, E. R., D. Flittner, B. Herman, D. Feng, S. Syndergaard, and D. Ward, "An Active Microwave Limb Sounder for Profiling Water Vapor, Ozone, Temperature, Geopotential of Pressure Surfaces and Clouds", NASA's Earth System Technology Conference (ESTC), Pasadena, CA, June 12, 2002b.