Abstract

The Formosa Satellite Mission #3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC), hereafter referred to as COSMIC, is a joint Taiwan-U.S. mission launched in April 2006. The COSMIC mission consists of six small satellites that employ the Global Positioni... Show moreThe Formosa Satellite Mission #3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC), hereafter referred to as COSMIC, is a joint Taiwan-U.S. mission launched in April 2006. The COSMIC mission consists of six small satellites that employ the Global Positioning System (GPS) radio occultation (RO) technique to sound the neutral atmosphere and ionosphere with uniform global coverage. As of January 2008, COSMIC is providing approximately 2,000 RO soundings per day to support the research and operational communities. For East Asian countries, the COSMIC soundings are particularly valuable for the study of typhoons and Mei-yu convective systems, as they provide observations over the datasparse Western Pacific Oceans and the South China Sea. In this study, we assimilate COSMIC GPSRO soundings and examine their impact on the prediction of Typhoon Shanshan (2006). The assimilation is first carried out using the WRF-Var (3D-Var) system. We find that in order for COSMIC GPSRO soundings to have an impact, it is critical to perform continuous assimilation through cycling. With one-hour cycling over a one-day period, COSMIC GPSRO soundings significantly improve the track forecast. However, the assimilation of only seven COSMIC GPSRO soundings in a cold-start experiment produces virtually no impact. The continuous cycling assimilation is able to incorporate 110 GPSRO soundings, and has a profound impact. We also find that the assimilation of typhoon bogus soundings improves the typhoon intensity and track forecast, particularly during the first two days. To assess the impact of data-assimilation systems, we compare the performance of the WRF 3D-Var system with the WRF/DART ensemble filter system for the assimilation of COSMIC GPSRO soundings. The results show that the WRF/DART ensemble filter system can assimilate the GPSRO data more effectively than the WRF 3D-Var method. In particular, the WRF/DART ensemble filter system is able to produce a storm with more coherent typhoon structure after one day of continuous assimilation, while a much weaker and less coherent storm is produced by WRF 3D-Var. In addition to Typhoon Shanshan (2006), we assimilate GPSRO soundings from COSMIC during the two-week period of 1 to 14 June 2007, associated with a Mei-yu system, using the WRF/DART ensemble filter data-assimilation system. We find that the assimilation of COSMIC data significantly strengthens the Western Pacific Subtropical High, and consequently improves the prediction of Mei-yu precipitation over southern China and Taiwan. Show less