The Orographic Effects Induced by an Island Mountain Range on Propagating Tropical Cyclones

Simon Wei-Jen Chang Science Applications, Inc., McLean, VA 22102

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Abstract

Numerical simulations with a primitive equation model which includes parameterized physics are conducted to study the effects of an island mountain range on translating tropical cyclones. The idealized topography with a 200 m peak is introduced over a 12 h growth period. The initial state contains a nonlinearly balanced vortex embedded in a uniform, unsheared, tropical easterly flow.

Many orographic effects are produced similar to those observed for typhoons passing over mountain ranges. The storm tends to translate at about twice the speed of the basic flow near the mountain, while its intensity is reduced. Air flows mostly around the mountain range instead of over it, forming a ridge on the windside and a trough on the leeside slopes. The tropical cyclone's passage induces a mean cyclonic circulation around the mountain with strongest amplitudes at low levels. As a result, the model tropical cyclone makes a cyclonic curvature in its path around the north end of the island mountain.

Further numerical experiments suggest that cumulus heating which maintains the tropical cyclone forces the cyclonic circulation around the mountain. In the experiment with an unforced, quasi-barotropic vortex we found that the lower level circulation is blocked by the mountain range. As the original low-level center fails to pass the mountain range, a secondary low-level circulation center forms in the induced lee trough. The secondary low-level center develops as the upper level center comes into phase.

A vorticity budget is performed for the 700 mb airflow prior to landfall and confirms the importance of diabatic processes in producing the observed orographic effects. Diabatic processes generate convergence to maintain the vorticity of the tropical cyclone. The horizontal advection of positive vorticity in conjunction with the leeside vortex stretching, results in the mean positive vorticity around the mountain.

Abstract

Numerical simulations with a primitive equation model which includes parameterized physics are conducted to study the effects of an island mountain range on translating tropical cyclones. The idealized topography with a 200 m peak is introduced over a 12 h growth period. The initial state contains a nonlinearly balanced vortex embedded in a uniform, unsheared, tropical easterly flow.

Many orographic effects are produced similar to those observed for typhoons passing over mountain ranges. The storm tends to translate at about twice the speed of the basic flow near the mountain, while its intensity is reduced. Air flows mostly around the mountain range instead of over it, forming a ridge on the windside and a trough on the leeside slopes. The tropical cyclone's passage induces a mean cyclonic circulation around the mountain with strongest amplitudes at low levels. As a result, the model tropical cyclone makes a cyclonic curvature in its path around the north end of the island mountain.

Further numerical experiments suggest that cumulus heating which maintains the tropical cyclone forces the cyclonic circulation around the mountain. In the experiment with an unforced, quasi-barotropic vortex we found that the lower level circulation is blocked by the mountain range. As the original low-level center fails to pass the mountain range, a secondary low-level circulation center forms in the induced lee trough. The secondary low-level center develops as the upper level center comes into phase.

A vorticity budget is performed for the 700 mb airflow prior to landfall and confirms the importance of diabatic processes in producing the observed orographic effects. Diabatic processes generate convergence to maintain the vorticity of the tropical cyclone. The horizontal advection of positive vorticity in conjunction with the leeside vortex stretching, results in the mean positive vorticity around the mountain.

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