The Effects of Orography on Typhoon Intensity in COAMPS-TC

The change in structure and intensity of three typhoons were investigated just before their landfalls over Taiwan. Typhoon Sinlaku (2008) made landfall closer to northern Taiwan, while Typhoon Nanmadol (2011) made landfall over the southern tip of Taiwan. Typhoon Jangmi (2008) made landfall over central Taiwan, decreasing in intensity the most rapidly of the three. One of the most important factors in the intensity decrease is anticipated to be the orography of Taiwan.

COAMPS-TC simulations were run for one iteration time (in which landfall occurs after 12 to 24 hours after initialization) for each of these three storms to represent a range of landfall locations. The model results were compared to a variety of available observations, a particularly important group being the rain gauge network over Taiwan. This allows the location and strength of rain shadows to be identified, which is a sign of the dry, downslope flow which could dramatically reduce the intensity. Diagnostics are also run for other factors which could cause weakening, such as cold SSTs, strong wind shear, and moisture variations. Preliminary findings indicate that environmental factors play less of a role in the intensity of the storm than orography when in close proximity to the island, such as counteracting the effect of strong vertical wind shear.