The effects of orography on tropical cyclone structure

Idealized numerical simulations using the Naval Research Laboratory's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS®) are used to study the interaction of an initially axisymmetric Rankine vortex moving in a uniform mean flow and an elliptical, sinusoid mountain island. The parameters that are varied in this set of experiments include: the cross-flow dimension of the island and its orientation with respect to the mean flow, the landfall location of the tropical cyclone, the radius of maximum winds, and the mean stability. The analysis of the simulation results focuses on three areas. First is the track of the tropical cyclone as impacted through interaction with the island. Secondly, the surface wind field over the idealized island is studied to identify preferred areas of strong winds as a function of the distance from the storm center. These results are also applied to the problem of orographic precipitation enhancement. Finally, changes in the storm structure and intensity as a result of ingesting downslope flow with increased potential vorticity, lower moisture content, and/or higher stability are examined.

For an island with similar dimensions to Taiwan (325 x 125 x 3 km) and a storm with a small radius relative to the cross-flow island width, there is a consistent southward deflection of the track upstream of the island. Storms approaching the southern end of the island follow continuous tracks over the terrain, while storms approaching the northern end of the island have discontinuous tracks as the upper-level circulation becomes phased with a secondary low which develops on the northwest leeside of the island. For northern landfalling storms, the east-central coast south of the eye is located in a low-wind speed wake to the lee of the island. For southern landfalling storms, a weak downslope windstorm forms in the northwestern quadrant of the island. Additional experiments with different island sizes, tropical cyclone sizes, and mean flow characteristics are also presented.