A study on the landfall typhoon with compact structure in Taiwan

On the average, Taiwan is affected by about four typhoons every year. The Central Mountain Range (CMR) whose average elevation is about 2,000 m often has significant impact on typhoons crossing the island, including changes in rainfall distribution and track deflection. However, the structure and motion of typhoon would play important roles regarding how the typhoon is affected by the topography. The purpose of this research is placed on the landfall typhoon with compact structure.

Among those typhoons making landfall on Taiwan, Typhoon Toraji (2001) appears to be quite special because it has an intense and compact structure. Observations taken by the Doppler radar show that deep convection of Toraji occurred only within 25 km from the center. Such concentrated convection resulted in 146.5 mm (390 mm) maximum hourly (3-hourly) rainfall at a nearby station when Toraji was making landfall. In addition, the measured wind speed was nearly 50 knots at few kilometers radius and dropped to about 10 knots at 50 kilometers radius. Besides Toraji, Typhoon Longwang (2005) appears to have a compact structure too. The observation data shows that the radius of maximum wind is 50 km. Before Longwang made landfall on Taiwan, Aerosonde ever penetrated through the eye and eye wall of Longwang and took continuous measurements along the flight path.

Numerical simulations using MM5 and WRF were performed in order to further study the landfall of Toraji and Longwang, including the structural changes and the processes responsible for the maintenance of the compact structure. Results show that different parameterization schemes would lead to different structures. After a series of test, the model can reproduce reasonably well the track and structures of typhoons, including the radar echoes. However, for Toraji, WRF can give better simulation while MM5 give a better simulation of Longwang.