Application of GPS radio occultation data to weather analysis and prediction

Bill Kuo

MMM Division
National Center for Atmospheric Research
Boulder, CO 80307, USA

Atmospheric limb sounding technique making use of the radio signals transmitted by Global Position System (GPS) has emerged as a promising approach for global meteorological observations. As demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment, the GPS radio occultation sounding data are of high accuracy and high vertical resolution. Results from recent GPS radio occultation missions of CHAMP and SAC-C confirm this assessment. In late 2005, the joint U.S.-Taiwan COSMIC mission will be launched and is expected to collect approximately 3,000 radio occultation soundings per day. These data will be available in near real-time for global weather analysis and prediction.

The raw measurements of GPS radio occultation soundings are phase and amplitude of radio signals. It takes a number of steps to reduce the data to the traditional meteorological variables of temperature, pressure and water vapor. Because of the ray traversing geometry, the GPS radio occultation data have unique "pencil-like" measurement characteristics, which is very different from the point measurement of a radiosonde or an "area-average" measurement of a microwave sounder. To assimilate GPS radio occultation data effectively into a weather prediction model, one needs to correctly process the data and to properly account for the measurement characteristics and measurement errors. In this presentation, we will present the analysis of GPS radio occultation data from GPS/MET, CHAMP and SAC-C. We will discuss scientific and technical issues related to the assimilation of GPS radio occultation data into weather prediction models.