Correlating the transport of precipitable water vapor in a complex orographic environment before, during and after a typoon: case study of typhoon morakot (2009)

On August 7, 2009, Typhoon Morakot struck Taiwan producing 1,504 mm of rain in a 24- hour period, and 2874 mm over a 96-h period. The flooding and landslides associated with Typhoon Morakot claimed more than 700 lives and caused major damage in Taiwan. In-situ observations of precipitable water vapor (PWV) from 76 GPS stations and 450 rain gauges operated by the Taiwanese Central Weather Bureau captured the water vapor evolution and rainfall distribution as the storm moved through the island nation. Recently, Fang et al (2010) performed a high-resolution (4-km) simulation of Morakot using the Advanced Weather Research and Forecasting (WRF-ARW) model. This research compared model generated PWV and rainfall to in-situ data to evaluate the performance of the WRF-ARW during the passage of Typhoon Morakot. In general, the model captured time evolution of the water vapor distribution as Typhoon Morakot passed through the island. However, there are also subtle differences. Although the model 96-h accumulated rainfall reproduced many salient features of the orographic precipitatoin, accurate prediction of rainfall evolution at high temporal and spatial resolution proves to be challenging due to complex topography and its interaction with the typhoon circulation. Comparison of PWV observation from the climatology of August 2008 suggests that the pre-typhoon environment is abnormally moist over Taiwan. This might provide an explanation for the extra-ordinary rainfall amount for this case. This study illustrates that the PWV observations from a dense ground-based GPS network, coupled with a rainfall observations from a high-resolution rain gauge network, can be quite valuable in verifying high-resolution model prediction for a significant weather event such as Typhoon Morakot.