Orographic effects on typhoon Herb (1996)

In average, there is at least one typhoon passing through the northern Taiwan every year and cause tremendous damage. From 1996, a dual Doppler radar set up with a baseline of 50 km started to operate routinely. Within this area one ISS (integrated sounding system) and many surface stations and raingauges form a good mesoscale observation network. The northern dual Doppler lobe is mainly over the open sea while the southern one is over land with high mountains. A new terrain following coordinate Doppler wind synthesis algorithm is adopted, hence the detail kinematic and thermodynamic structure changes during the landfall can be revealed even over the complicated topography.

Within the northern domain, the rainbands and the eye wall structures were quite similar to the literatures. However once the circulation of typhoon entered the southern domain and encountered the terrain, due to the friction, blocking and the forced convection on the slope, the three dimensional flows and reflectivity structures changed dramatically. These changes happened not only at the lowest level but the whole depth of the system. While the rainband arrived, the total wind speed decreased from 45m/s to 35 m/s at 1.5km height level. But the depth of the radial inflow was increased. In the same time the strong wind beneath 4km height (isotach> 35m/s) was elvated from 4km to 8km along the mountain slope. As for the change of the eye wall, the erected tangential wind maximum pattern near the eye wall collapsed, it became a wider and lower jet like flow along the mountain slope. The vertical wind shear at lower levels was also reversed. The strongest tangential wind at 1.5km height decreased from 55m/s to 35m/s while it approached the mountain range about 1km height. The pressure gradient reduced to the half of the magnitude retrieved in the northern domain in the same time. There are many interesting features about the effects of the interactions between the typhoon and terrain. The terrain following Doppler synthesis proved to be very useful. Through more diagnostic studies and numerical simulation, the dynamic processes of the spin down of typhoon during its landfall will be clearer.