With the ability to release dropsondes as often as once per hour over targeted areas of interest, the driftsondes are becoming an important observing facility for research and operational applications. In the upcoming T-PARC field experiment, the driftsonde will be deployed to study tropical cyclones and their extratropical transformation. The driftsondes provide high-vertical resolution measurements of temperature, moisture and wind fields over a specific region of interest. With the launch of COSMIC in April 2006, GPS radio occultation (RO) soundings are being collected in real-time to support research and operational NWP. COSMIC provide high vertical resolution measurements of atmospheric refractivity (which is a function of temperature, moisture and pressure), uniformly distributed around the globe. Typically, about 2,500 GPS RO soundings are collected on a daily basis. These two types of measurements have different characteristics, and are complementary to each other.

In this study, we perform an observing system simulation experiments to assess the relative impact of driftsonde and COSMIC GPS RO soundings on the prediction of Typhoon Shan Shan (2006) over the western Pacific. We first conduct a 12-km MM5 simulation of Typhoon Shan Shan, initialized with the ECMWF global analysis. This serves as the “nature” run for the OSSE experiment. We then simulate observations from seven drifting balloons released from Hawaii and a realistic configuration of COSMIC GPS RO soundings. These simulated observations are assimilated into the WRF 3D-Var system. Forecast experiments are conducted to assess the relative importance of these two types of observations on the prediction of Typhoon Shan Shan. Our results show that both driftsonde and COSMIC GPS RO soundings contribute to the improvement of typhoon prediction, with driftsonde contributing more to intensity forecast, and COSMIC GPS RO more to track forecast. Additional experiments are conducted to assess the importance of wind observations provided by driftsondes. These results will be presented at the conference.