Assimilation of GPS Radio Occultation Data for Numerical Weather Prediction

With the availability of approximately 4,000 radio occultation sound ings per day within three hours of observation, COSMIC has the potential to contribute significantly to global and regional weather analysis and pre diction.However, the basic radio occultation measurements (phase delays) are very different from traditional meteorological measurements (i.e., tem perature, water vapor), and to effectively assimilate them into weather pre diction models is a challenging task.Over the past five years, considerable progress has been made in the development of an effective strategy for the assimilation of GPS radio occultation data.In this paper, we (1) review the measurement and data reduction procedures, (2) discuss the error charac teristics of the GPS radio occultation data, (3) discuss the various strategies for data assimilation, (4) review results from recent data assimilation re search, and (5) provide suggestions for future research.Results from recent studies have led to the conclusion that the best strategy to assimilate GPS radio occultation data is a mixture of bending angles below 10 km and refractivity above 10 km using a variational ap proach.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 refractivity and bending angles are not directly related to the winds, the assimilation ofGPS data leads to improvements in the wind analy sis through internal model dynamic adjustments.In order to make optimal use of GPS radio occultation data in weather analysis and prediction, considerable research is needed in: (1) better char acterization of GPS measurement errors, particularly in the lower tropo sphere, (2) improving the computational efficiency and optimizing the strat-