Orographic Influences on Tropical Cyclone Tracks in the Vicinity of Taiwan

Jared-Milano – University at Albany

Dr. Ming-Jen Yang – National Taiwan University

NSF/ Partnership for International Research and Education (PIRE)

September 2018

Orographic Influences on Typhoon Tracks in the Vicinity of Taiwan

Taiwan Island’s unique geographical placement in the western Pacific Ocean sees itself interacting with, on average, three to four tropical cyclones, TCs, each year. The west coast of the island is occupied by a central mountain range, CMR, which has high peaks comparable to those found in the Colorado Rocky Mountains, only with a significantly higher topographical gradient.. This work examines how the terrain of Taiwan affects the tracks of TCs as they move in its vicinity, and if the terrain causes phenomena which can be observed using radar. Additionally, this study draws other TC relevant connections between Taiwan and Puerto Rico; though smaller in size, Puerto Rico has a similar shape and central mountain range feature.

The CWB, Taiwan’s Central Weather Bureau, has identified nine climatologically-based, categories of TC tracks which often lead to Taiwan’s severe weather events (CWB FAQ for Typhoon fig. 6). The most common of these TC tracks either pass over the northern tip of the island or bifurcate the CMR. Terrain affects these TCs well before any rain bands appear over the island and influence their tracks. The unique topography of the island also contributes to vortex shearing which has been shown to break down the TC eye-wall at landfall. In some cases, the eye-wall will reconstruct itself after clearing the terrain, then intensify as the TC moves over the strait of Taiwan (Yang et al. 2018). It was observed that the CMR has localized effects on the intensity of precipitation as well. Two typhoon case studies were used to conduct this research: Typhoon Maria (2018), which tracked over the northern tip of Taiwan but did not make landfall, and Typhoon Fanapi (2010), the eye-wall of which made landfall over Hualien, on the island’s central west coast.

The goal of this exploratory study was to learn about how, and why, Taiwan’s terrain may influence possible track outcomes of inbound typhoons, as they approach the island itself. Factors which tend to influence how a TC tracks over the island include the storms size ratio to the landmass, the Froude number, propagation speed on approach, and Brunt-Väisälä frequency experienced when moving directly over the CMR. The steep terrain also highlights possible limitations to be expected when collecting radar data around it, and conversely, where radar excels at observing resultant weather phenomena such as locally enhanced precipitation or a TC eye-wall replacement cycle.

This study took place in Taiwan, over a seven-week period, from July to August 2018, as part of NSF funded Partnerships for International Research and Education (PIRE). The research itself was partly conducted in the Mesoscale and Orographic Precipitation Laboratory at National Taiwan University and as well as in the Radar Meteorology Laboratory at National Central University. The general purpose of this particular research project was for me to gain international collaboration experience with scientists whom are conducting cutting edge research in their respective fields. Overall, the University at Albany’s PIRE grant in Taiwan seeks to build extreme weather resiliency, and global community resiliency, through improved weather and climate prediction, and improved emergency response strategies.