[1] Typhoon Morakot (2009), a devastating tropical cyclone (TC) that made landfall in Taiwan from 7 to 9 August 2009, produced the highest recorded rainfall in southern Taiwan in the past 50 years. This study examines the factors that contributed to the heavy rainfall. It is found that the amount of rainfall in Taiwan was nearly proportional to the reciprocal of TC translation speed rather than the TC intensity. Morakot's landfall on Taiwan occurred concurrently with the cyclonic phase of the intraseasonal oscillation, which enhanced the background southwesterly monsoonal flow. The extreme rainfall was caused by the very slow movement of Morakot both in the landfall and in the postlandfall periods and the continuous formation of mesoscale convection with the moisture supply from the southwesterly flow. A composite study of 19 TCs with similar track to Morakot shows that the uniquely slow translation speed of Morakot was closely related to the northwestward-extending Pacific subtropical high (PSH) and the broad low-pressure systems (associated with Typhoon Etau and Typhoon Goni) surrounding Morakot. Specifically, it was caused by the weakening steering flow at high levels that primarily resulted from the weakening PSH, an approaching short-wave trough, and the northwestward-tilting Etau. After TC landfall, the circulation of Goni merged with the southwesterly flow, resulting in a moisture conveyer belt that transported moisture-laden air toward the east-northeast. Significant mesoscale convection occurred on a long-lived east-west-oriented convergence line and on the mountain slope in southern Taiwan. This convective line was associated with large low-level moisture flux convergence caused by the northwesterly circulation of Morakot and the southwesterly flow. It is thus suggested that the long duration of Typhoon Morakot in the Taiwan area, the interaction of southwest monsoon and typhoon circulation, the mesoscale convection, and the presence of terrain are the key factors in generating the tremendous rainfall.

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This paper presents an observational and numerical study of Typhoon Mindulle (2004) as it affected Taiwan. Mindulle made landfall on the east coast of Taiwan at 1500 UTC 1 July 2004, and after 13 h, it exited Taiwan from the north coast. Severe rainfall (with a maximum amount of 787 mm) occurred over central-southwestern Taiwan on 2 July 2004. During the landfall of Mindulle's main circulation, a secondary low formed over the Taiwan Strait. However, the secondary low, after it developed significantly (vorticity exceeded 5 ϫ 10 Ϫ4 s Ϫ1 over a 30-km radius), did not replace the original center as was observed in many other storms. Instead, it moved inland and dissipated after the original center redeveloped near the north coast of Taiwan. In this study, the evolution of the secondary low, the redevelopment of the primary center, and the processes leading to the severe rainfall were examined. Results showed that the processes leading to the formation and the development of the secondary low were similar to those described in previous studies. These processes include the leeside subsidence warming, the horizontal transport of vorticity around the northern tip of the Central Mountain Range (CMR), and the overmountain upper-level vorticity remnant. However, because of the northward track, Mindulle preserved some strong vorticity on the eastern slope of the CMR. This strong vorticity remnant was steered northward over the ocean offshore from the north coast where the redevelopment of the primary center occurred. This "quasi-continuous track" of Mindulle has not been documented in previous studies. The vortex interaction between the redeveloped primary center and the secondary low resulted in the northeastward movement of the secondary low, which then dissipated after making landfall. Analyses also showed that even though heavy rainfall would occur over the mountain area when only the southwesterly flow prevailed, as on 3 July 2004, Typhoon Mindulle and the secondary low provided extra convergence that resulted in the west-east-oriented convective bands. These convective bands and the orographic lifting of the circulation associated with the secondary low resulted in the heavy rainfall over the central-western plains area.

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The impact of GPS radio occultation (RO) data assimilation on severe weather predictions in East Asia is introduced and reviewed. Both the local observation operator that assimilates the retrieved refractivity as local point measurement, and the nonlocal observation operator that assimilates the integrated retrieved refractivity along a straight raypath have been utilized in WRF 3DVAR to improve the initial analysis of the model. A general evaluation of the impact of these approaches on Asian regional analysis and daily prediction is provided in this paper. In general, the GPS RO data assimilation may improve prediction of severe weather such as typhoons and Mei-yu systems when COSMIC data were available, ranging from several points in 2006 to a maximum of about 60 in 2007 and 2008 in this region. Based on a number of experiments, regional model predictions at 5 km resolution were not significantly influenced by different observation operators, although the nonlocal observation operator sometimes results in slightly better track forecast. These positive impacts are seen not only in typhoon track prediction but also in prediction of local heavy rainfall associated with severe weather over Taiwan. The impact of 56 GPS RO soundings on track prediction of Cyclone Gonu (2007) over the Indian Ocean is also appealing when compared to other tracks assimilated with different observations. From a successive evaluation of skill scores for real-time forecasts on Mei-yu frontal systems operationally conducted over a longer period and predictions of six typhoons in 2008, assimilation of GPS RO data appears to have some positive impact on regional weather predictions, on top of existent assimilation with all other observations.