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Figure 15. (a) 850 mb moisture flux in the GPS experiment, and (b) differences in the 850 mb moisture flux between the GPS and NoGPS experiments averaged over the two-week period from 1 to 14 June 2007.

The location and intensity of precipitation over southern China and the Taiwan Strait are very different. In particular, the WRF model initialized with the WRF/DART analysis that assimilates GPSRO soundings produces more intense precipitation over the Taiwan Strait. The difference field shows that more than 30 mm additional precipitation falls over western and southern Taiwan. In comparison with the available precipitation analysis over Taiwan (Fig. 16), we find that the more intense precipitation as a result of COSMIC GPSRO data assimilation compares more favorably with the observed rainfall. The 24h accumulated rainfall over mainland China ending at 0000 UTC 9 June (Fig. 20) also compares more favorably with the GPS experiment, which gives precipitation further to the south than the NoGPS experiment.

4. Summary and Conclusions

The successful launch of the FORMOSAT-3/ COSMIC mission marked the beginning of a new era in GPS atmospheric remote sensing. By providing more than 2,000 GPS radio occultation soundings per day uniformly distributed around the globe in near real time, COSMIC provides much needed data over data-sparse regions of the world, including the tropical oceans and the polar regions. For weather forecasting over Taiwan and East Asia, COSMIC provides valuable data over the western Pacific and the South China Sea. The assimilation of COSMIC GPSRO soundings can contribute to improved forecasting of typhoons and heavy precipitation associated with the Mei-yu front. In this article, we have examined the impact of COSMIC GPSRO soundings on the prediction

Accumulated Rainfall 2007060700 Accumulated Rainfall 2007060800

Accumulated Rainfall 2007060700 Accumulated Rainfall 2007060800

(a) (b)

Accumulated Rainfall 2007060900 Accumulated Rainfall 2007061000

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Figure 16. 24 h accumulated precipitation over Taiwan ending at 0000 UTC on 7, 8, 9, and 10 June 2007.

Figure 16. 24 h accumulated precipitation over Taiwan ending at 0000 UTC on 7, 8, 9, and 10 June 2007.

of Typhoon Shanshan (2006) and the heavy precipitation event associated with a Mei-yu front in early June 2007. Our study has led to the following conclusions:

(1) It is essential to perform continuous assimilation through cycling in order for COSMIC GPSRO soundings to have a significant impact on typhoon track prediction. One needs to realize that even with 2,000 GPSRO soundings per day, the data density is still relatively low. Over the CWB 45 km domain, there are approximately 100 GPSRO soundings over a 24 h period, or

25 GPSRO soundings over a 6 h period. Continuous assimilation with a relatively narrow assimilation window (1 h) allows more COSMIC soundings to be assimilated at the time close to observations. Cold-start-type data assimilation is usually not effective, since only a limited number of soundings are used.

(2) The assimilation of COSMIC GPSRO soundings using the WRF/DART ensemble filter method is found to be more effective than the WRF 3D-Var assimilation method for the Typhoon Shanshan case. The WRF/DART system produces a much

Figure 17. (a) 850 mb moisture flux in the GPS experiment, and (b) differences in the 850 mb moisture flux between the GPS and NoGPS experiments, ending at 0000 UTC 8 June 2007.

stronger typhoon than the WRF 3D-Var system with or without the assimilation of COSMIC GPSRO data. Moreover, the assimilation of COSMIC GPSRO data with the WRF/DART system produces much more profound changes than the WRF 3D-Var system. In other words, the WRF/DART ensemble system can extract more information from the same GPSRO data than the WRF 3D-Var system, and subsequently has a larger analysis increment. The superior performance of the WRF/DART ensemble system is attributed to the fact that WRF/DART uses flow-dependent background error covariances, and takes into consideration the forecast multivariate error correlations among moisture, temperature, and surface pressure. On the other hand, WRF 3D-Var uses background error covariances derived from historical forecasts, which do not contain information directly related to the case at hand.

Of course, we should also recognize that the improved performance of WRF/DART ensemble filter assimilation is obtained at an increased computational cost. For the Typhoon Shanshan case, the WRF/DART system requires more than an order of magnitude more computing resources than the WRF 3D-Var system.

(3) The assimilation of typhoon bogus soundings is found to be quite important for improved typhoon track forecasting, particularly for cold-start experiments. Because the NCEP AVN global analysis often underestimates the intensity of the storm, forecasts without the assimilation of typhoon bogus data usually give large track errors. However, for cycling experiments, we found that the assimilation of bogus soundings does not always give better results, particularly for longer-range forecasting. We conclude that improved typhoon bogusing procedures should be developed. Better yet,

1 OSE 1 1 DE 115E 120E 125E 130E

Figure 18. 850 mb specific humidity for (a) the NoGPS run, (b) the GPS run, and (c) the difference between the GPS and the NoGPS run (in gkg^1) at 0000 UTC 8 June 2007.

1 OSE 1 1 DE 115E 120E 125E 130E

Figure 18. 850 mb specific humidity for (a) the NoGPS run, (b) the GPS run, and (c) the difference between the GPS and the NoGPS run (in gkg^1) at 0000 UTC 8 June 2007.

we should achieve improved forecasting by assimilating more real observations and avoiding the use of typhoon bogus soundings, which is the current practice at the ECMWF.

(4) The assimilation of COSMIC GPSRO soundings with the WRF/DART system over a two-week period has made a profound impact on the analysis of the Western Pacific Subtropical High. In particular, the

COSMIC GPSRO assimilation strengthens the WPSH, increases the moisture content of the Mei-yu front, and increases moisture flux convergence along the Mei-yu front. These changes made a significant positive impact on short-range precipitation forecasts over both Taiwan and southern China.

Although these results are very encouraging, they need to be verified with additional case studies. Moreover, the assimilation of GPSRO soundings should be tested over an extended period before it can be used operationally. These efforts are currently being carried out at Taiwan's Central Weather Bureau. Given the importance of the COSMIC GPSRO soundings for the typhoon and Mei-yu prediction, it is important that we continue the operation of the COSMIC mission through its life. We should also begin planning for a follow-on mission to replace COSMIC, which is expected to last through 2011 with a five-year mission life.

Figure 20. Observed rainfall over China ending at 0000 UTC 9 June 2007.

Figure 19. 24 h accumulated rainfall ending at 0000 UTC 9 June for (a) the NoGPS experiment, (b) the GPS experiment, and (c) their differences.

Figure 19. 24 h accumulated rainfall ending at 0000 UTC 9 June for (a) the NoGPS experiment, (b) the GPS experiment, and (c) their differences.

Acknowledgements

We thank Jeff Anderson for fruitful discussions on the results of WRF/DART assimilations. The research results presented in this article are supported by the Central Weather Bureau, the National Space Organization, the National Science Foundation under awards #ATM-0410018 and INT-0129369, and the National Aeronautics and Space Administration under grant NNX08AI23G.

[Received 10 February 2008; Revised 12 March 2008; Accepted 14 March 2008.]

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