Assimilation of GPS radio occultation data for numerical weather prediction

With the availability of approximately 4,000 radio occultation soundings pe r day within three hours of observation, COSMIC has the potential to contri bute significantly to global and regional weather analysis and prediction. However, the basic radio occultation measurements (phase delays) are very d ifferent from traditional meteorological measurements (i.e., temperature, w ater vapor), and to effectively assimilate them into weather prediction mod els is a challenging task. Over the past five years, considerable progress has been made in the development of an effective strategy for the assimilat ion of GPS radio occultation data. In this paper, we (1) review the measure ment and data reduction procedures, (2) discuss the error characteristics o f the GPS radio occultation data, (3) discuss the various strategies for da ta assimilation, (4) review results from recent data assimilation research, and (5) provide suggestions for future research. Results from recent studies have led to the conclusion that the best strate gy to assimilate GPS radio occultation data is a mixture of bending angles below 10 km and refractivity above 10 km using a variational approach. The assimilation of GPS radio occultation data is likely to have a significant positive impact on global and regional weather prediction through improved definition of water vapor, temperature and wind fields. Although refractivi ty and bending angles are not directly related to the winds, the assimilati on of GPS data leads to improvements in the wind analysis through internal model dynamic adjustments. In order to make optimal use of GPS radio occultation data in weather analy sis and prediction, considerable research is needed in: (1) better characte rization of GPS measurement errors, particularly in the lower troposphere, (2) improving the computational efficiency and optimizing the strategg of b ending angle and refractivity assimilation,and (3) performing a set of obse rving system simulation experiments with realistic simulation of GPS radio occultation data. These research tasks should be conducted prior to the lau nch of the COSMIC satellites, so that we can fully realize the potential of COSMIC data in global and regional weather prediction.