An Overview of the University of Arizona ATOMS Project

Abstract

Results from the now well known GPS/MET experiment have demonstrated the capabilities of radio occultation techniques for remotely sensing certain atmospheric properties (Ware et al. 1996; Rocken et al. 1997; Kursinski et al. 1997). The GPS/MET experiment was designed to provide vertical atmospheric profiles of temperature, pressure, density, and geopotential height from determinations of the vertical refractivity profiles. Indeed, for altitudes from a few km above the surface to about 40 km, extremely accurate profiles were obtained with very good vertical resolution. At altitudes below about 5–8 km, an ambiguity may exist however, when using just the GPS frequencies, due to the presence of water vapor. Water vapor, as well as dry air, affects the refractivity of the atmosphere, and the two variables cannot be separated without additional assumptions, or additional information. In polar regions where water vapor amounts are generally quite low, accurate temperatures can usually be retrieved by assuming the water vapor to be zero. In tropical regions, where low tropospheric temperature profiles are quite constant from day to day, water vapor profiles may be recovered from the refractivity profiles by assuming the temperature profile is known. It is in mid-latitudes where this ambiguity is most important. Techniques have been developed to overcome some of this difficulty, but uncertainties still exist in the recovered water vapor profiles, especially in data sparse regions. The ATOMS (Active Tropospheric Ozone and Moisture Sounder) project was conceived in order to provide a totally independent measurement to use for water vapor retrievals. A preliminary study indicated that the use of phase measurements as in the GPS/MET experiment, but near a water vapor absorption line where the refractivity undergoes a rapid variation, would not provide the necessary sensitivity due to the small mixing ratio of water vapor. However amplitude measurements at various frequencies within and near the line would provide a method to retrieve vertical water vapor profiles. In this paper we report on the progress we have made to date in developing this technique. By using various frequencies within both the 22 GHz and 183 GHz water vapor absorption lines, it will be shown that usable profiles may be recovered over a wide range of altitudes. A similar study will be presented using the 195 GHz ozone absorption line to recover ozone profiles from the mid-troposphere to well up into the stratosphere.