Influence of topography on tropical cyclone motion and rainfall

The influence of topography on tropical cyclone (TC) motion crossing over Taiwan is studied. Tracks and rainfall data of 55 typhoons between 1977-2009 are used to analyze the features of TC moving speed and rainfall amount. The observations reveal two distinct regions. Typhoons reaching the southern portion of Taiwan usually move quickly and experience acceleration after landfall. On the contrary, typhoons making landfall in northern Taiwan often move slowly and experience deceleration after landfall. This kind of variation is not affected by the intensity and moving speed before landfall but rather by the landfall position. The rainfall amount is inversely proportional to the moving speed over land and neither rainfall amount nor intensity show a clear long-term trend.

Idealized land-free WRF experiments demonstrate that the acceleration and deceleration phenomenon can be produced in a calm environment with Gaussian type topography and TCs initialized as Rankine vortices. The topography was created by elevating a portion of the sea surface, making a landless island. TCs making “landfall” at about 24 degrees latitude decelerate and become trapped by the topography. The simulation also shows a track deflection and maximum rainfall on the windward side before “landfall”. Afterwards, a local rainfall maximum appears in the southern portion of the island. The increase of potential temperature at the southeast coast is produced while the vortex approaches and crosses the topography. A TC making “landfall” on the southern end of the island accelerates while crossing over the obstacle. The features of secondary low formation and potential temperature increase at the northwest coast are also shown in this simulation.