A Study on the Typhoon Precipitation Forecast in Taiwan

The continuous torrential rain associated with a typhoon often caused flood, landslide or debris flow, leading to serious damages to Taiwan. Therefore the quantitative precipitation forecast (QPF) during typhoon period is highly needed for disaster preparedness and emergency evacuation operation in Taiwan. A typhoon rainfall climatology model (Lee et al. 2006) using the rain-gauge observations thus was developed and has been used in real-time operation practice. The model considers mainly the topographical lifting of typhoon circulation. The possible effects of typhoon intensity, size, track and the satellite rainfall potential are considered in a newer version of the model. Results show that the model can provide reasonable total rainfall estimate for each river basin during the typhoon period for many cases but perform poorly for some cases. Therefore, the Taiwan Typhoon and Flood Research Institute (TTFRI) started the typhoon quantitative precipitation forecast ensemble forecast experiment in 2010. The ensemble members include various models and consider different setups in the model initial conditions, data assimilation processes and model physics.

Results show that the ensemble mean provides valuable information on typhoon track forecast and quantitative precipitation forecasts around Taiwan. For example, the ensemble mean track captured the sharp northward turning when Typhoon Megi (2010) moved westward to the South China Sea. The model rainfall also continued showing that the total rainfall at the northeastern Taiwan would exceed 1,000 mm, before the heavy rainfall occurred. Similar situation occurred for Typhoon Nalgae (2011) which caused 1,621 mm accumulated rainfall during a 72-hours period (1~3 Oct) in I-lan area. Although the ensemble forecast performs reasonably well for the total rainfall amount during the typhoon period, it still has limitation especially the short-duration extremely heavy rainfall occurred over a small area which will be discussed in the presentation