Impact of Formosat3Cosmic Data on Typhoon and Meiyu Prediction

Ying-Hwa Kuo, Hui Liu and Yong-Run Guo National Center for Atmospheric Research, Boulder, Colorado, USA

Chuen-Teyr Terng and Yun-Tien Lin Central Weather Bureau, Taipei,, Taiwan

The Formosa Satellite Mission #3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC), hereafter referred to as COSMIC, is a Taiwan-US mission launched in April 2006. COSMIC 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 provides 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 data-sparse Western Pacific Ocean 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 1-14 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.

1. Introduction

The radio occultation (RO) technique, which makes use of radio signals transmitted by Global Position System (GPS) satellites, has emerged as a powerful and relatively inexpensive approach to sound the global atmosphere with high precision, accuracy and vertical resolution in all weathers and over both land and ocean. This was first demonstrated by the proof-of-concept GPS/MET (GPS Meteorology) experiment in 1995-1997 (Ware et al., 1996), and substantiated by the CHAMP (CHAllenging Minisatellite Payload; Wickert et al., 2001) and SAC-C (Satellite de Aplicaciones Cientificas-C; Hajj et al., 2004) missions. In April 2006, the Formosa Satellite Mission #3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC, hereafter referred to as COSMIC) was successfully launched into initial orbits of 512 km from the Vandenberg Air Force Base (Kuo et al., 1999; Rocken et al., 2000; Cheng et al., 2006). COSMIC started collecting GPSRO soundings eight days after the launch, and three months later started providing data to the international science communities. By the end of 2007, COSMIC satellites had already been deployed to their operational orbits at 800 km elevation, evenly spaced with 30° separation. This allows COSMIC soundings to be distributed uniformly around the globe. As of 5 March 2008, the COSMIC Data Analysis and Archive Center (CDAAC) has processed 941,933 neutral atmospheric profiles and 1,227,682 ionospheric profiles, and delivered them to the operational and research communities. Even though COSMIC was launched less than two years ago, it has already contributed significantly to global weather prediction, climate monitoring, and ionospheric research (Anthes et al., 2008).

In comparison with previous GPSRO missions, COSMIC offers three major advantages. First, COSMIC uses the advanced open-loop (OL) tracking technique (Sokolovskiy, 2001). All previous GPSRO missions (with the exception of SAC-C, which was used to test the OL tracking) used phase-lock-loop (PLL) tracking. With the use of PLL tracking, only a small fraction of GPSRO soundings penetrate to below 1 km. Those that do penetrate to the lower troposphere are often affected by tracking errors, especially over the tropical atmospheric boundary layer (ABL). With OL tracking, 70% of COSMIC soundings penetrate to the lower tropical troposphere, while those over the higher latitudes, more than 90% of COSMIC soundings, penetrate to below 1 km (Sokolovskiy et al., 2006a; Anthes et al., 2008). The deep penetration of GPSRO soundings allows us to monitor the variation of the height of the ABL (Sokolovskiy et al., 2006b), and atmospheric river events over the Eastern Pacific Ocean (Neiman et al., 2008).

COSMIC represents the world's first constellation designed to provide GPSRO soundings with uniform global distribution. The six satellites were launched with one single rocket. The differential precession technique is used to deploy the satellites to their final orbits at 800 km using the on-board propulsion system, with 30° separation (Yen et al., 2008). The evenly spaced orbital plans allow the COSMIC GPSRO soundings to be distributed uniformly around the globe in local solar time. This is very important for observing the diurnal cycle in the neutral atmosphere and ionosphere (where it is especially strong) and for preventing aliasing of the diurnal cycle in climate signals (Zeng et al., 2008). Moreover, with each of the six satellites capable of performing both rising and setting occultation, COSMIC provides an-order-of-magnitude more soundings than a single satellite mission, such as CHAMP.

The third advantage of COSMIC is the availability of GPSRO data in near real time to support operational applications. With the support of two high-latitude ground stations, each COSMIC satellite can download its data once every orbit (100 min). After 5 min of data transfer and another 15 min of data processing, these data are available to support global operational numerical weather prediction through the Global Telecommunication System (GTS; Rocken et al., 2000). After a few months of testing, the ECMWF (European Centre for Medium Range Forecasts) began the operational assimilation of COSMIC GPSRO data on 12 December 2006, the NCEP (National Centers for Environmental Prediction) on 1 May 2007, the UKMO (United Kingdom Meteorological Office) on 15 May 2007, and Meteo France on 1 September 2007. All these operational centers have reported positive impact with the operational assimilation of COSMIC GPSRO data (Cucurull and Derber, 2008; Healy, 2008; Poli et al., 2008).

Taiwan is located at subtropical latitudes, and is affected by severe weather in every season. In particular, the heavy rainfall events during late spring and early summer, a period known as the Mei-yu (Kuo and Chen, 1990), and the typhoons in the summer (Wu and Kuo, 1999) are major weather-related disasters that can cause significant loss of lives and property. With the availability of the GPSRO observations from COSMIC, this provides an opportunity to improve the forecasting of these severe weather events, and to improve our understanding of the regional climate in the vicinity of Taiwan (Wu et al., 2000). In this article, we examine the impact of COSMIC GPSRO soundings on the prediction of Typhoon Shanshan (2006) and a Mei-yu heavy rainfall event that took place in June 2007.

In Sec. 2, we describe the Typhoon Shanshan (2006) case and the assimilation of COSMIC GPSRO soundings using the WRF-Var (3D-Var) system. We examine how the details of assimilation procedures could influence the impact of GPSRO soundings on typhoon prediction. In Sec. 3, we compare the performance of WRF-3D-Var and the WRF/DART (Data Assimilation Research Testbed) ensemble filter data-assimilation system in the assimilation of GPSRO soundings and their impact on Typhoon Shanshan prediction. In Sec. 4, we examine the impact of COSMIC GPSRO soundings on the prediction of a Mei-yu system, and its associated heavy rainfall over Taiwan. A summary and closing remarks are given in the final section.

2. WRF 3D-Var Assimilation of COSMIC GPSRO Data and Its Impact on the Prediction of Typhoon Shanshan (2006)

2.1. Typhoon Shanshan case

Typhoon Shanshan formed on 9 September 2006, about 500 km north-northeast of Yap, near 14°N, 139°E (Fig. 1). The storm moved northwestward and went through rapid development, becoming a Category 4 storm by 12 September 2006. It then moved westward and northward, skirting to the east of Taiwan on 15 and 16 September. It reached its peak intensity near 0000 UTC 16 September, with a central pressure of 919 mb, and a peak wind speed of about 60 ms-1. Typhoon Shanshan made landfall on the island of Kyushu on 17 September. It was downgraded from a Category 4 storm to a Severe Tropical Storm by 0000 UTC 18 September, just before it crossed the island of Hokkaido. Later, it was transformed into an extratropical cyclone after it interacted with a midlatitude system. Figure 1 shows the best track and storm central pressure as a function of time. For this study, we will focus on the period from 0000 UTC 13 September through 17 September. In particular, we will assimilate the COSMIC GPSRO soundings over a one-day period, from 0000 UTC 13 to 0000 UTUC 14 September, and assess their impact on the prediction of Typhoon Shanshan.

2.2. Assimilation of COSMIC data using WRF 3D-Var

Through collaboration between NCAR and the Central Weather Bureau (CWB), a WRF

Wrf Typhoon

Figure 1. (a) Best track and (b) central pressure time series of Typhoon Shanshan (courtesy of Japan Meteorological Agency). The color of the points represents the storm intensity: blue — tropical depression; green — tropical storm; yellow — severe tropical storm; red — typhoon; magenta — extratropical cyclone.

Figure 1. (a) Best track and (b) central pressure time series of Typhoon Shanshan (courtesy of Japan Meteorological Agency). The color of the points represents the storm intensity: blue — tropical depression; green — tropical storm; yellow — severe tropical storm; red — typhoon; magenta — extratropical cyclone.

(weather research and forecasting model) and WRF 3D-Var (three-dimensional variational data assimilation) system (Barker et al., 2004) has been established for operation, starting from July 2007. The operational WRF model consists of three nested domains, with grid sizes of 45, 15 and 5 km, respectively. In this study, we perform experiments only on the 45 km domain (without nesting). Additional information on the WRF-Var system can be found at http://www.mmm.ucar.edu/wrf/WG4/wrfvar/ wrfvar-tutorial.htm

Figures 2(a) and 2(b) show the distribution of COSMIC GPSRO soundings, both in time and in space, over the one-day period of 0000 UTC 13 to 0000 UTC 14 September 2006. During this 24 h period, there are very few GPSRO soundings in the vicinity of Typhoon

Wrf Typhoon
Figure 2. (a) The spatial and (b) temporal distribution of COSMIC GPSRO soundings, and (c) the loci of CWB global and typhoon bogus soundings.
Table 1. List of WRF 3D-Var experiments.

Name

Cycling

Bogus data

COSMIC GPSRO data

Remarks

0 0

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