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Numerical Modeling Studies of a Process of Lee Cyclogenesis

Yuh-Lang LinDepartment of Marine, Earth and Atmospheric Sciences, North Carolina State University. Raleigh. North Carolina

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Donald J. PerkeyDepartment of Physics and Atmospheric Science, Drexel University, Philadelphia, Pennsylvania

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Abstract

A process of lee cyclogenesis associated with backsheared baroclinic flow is studied using a fully nonlinear, primitive equation numerical model. A region of low pressure and a narrow baroclinic zone develop to the southwest of the mountain in the model for surface northerly wind with constant backshear. These major features are consistent with theoretical results of other authors. The low-level flow splits around the mountain for low Froude number such as investigated in this study. The splitting process is less pronounced for flow over a lower mountain. The positive pressure deviation has its center displaced northeast with respect to the mountaintop because of the relatively strong ageostrophic advection of cold air. Our numerical simulation indicates that a theory of lee cyclogenesis proposed by Smith is valid, at least in the early stage of cyclogenesis.

The lee cyclogenesis is affected by both the low-level sensible heating and the turning of the wind associated with boundary-layer processes. The development of the low is weakened in the early stage, which is the result of the weakened warm advection associated with the mountain-induced anticyclone. Weak upward motion, instead of strong downward motion, is found near the lee low-pressure region. The upward motion is produced by the mountain acting as an elevated heat source. This upward motion may accelerate and strengthen the cyclogenesis if the low-level flow were saturated and latent heating were to become important.

Abstract

A process of lee cyclogenesis associated with backsheared baroclinic flow is studied using a fully nonlinear, primitive equation numerical model. A region of low pressure and a narrow baroclinic zone develop to the southwest of the mountain in the model for surface northerly wind with constant backshear. These major features are consistent with theoretical results of other authors. The low-level flow splits around the mountain for low Froude number such as investigated in this study. The splitting process is less pronounced for flow over a lower mountain. The positive pressure deviation has its center displaced northeast with respect to the mountaintop because of the relatively strong ageostrophic advection of cold air. Our numerical simulation indicates that a theory of lee cyclogenesis proposed by Smith is valid, at least in the early stage of cyclogenesis.

The lee cyclogenesis is affected by both the low-level sensible heating and the turning of the wind associated with boundary-layer processes. The development of the low is weakened in the early stage, which is the result of the weakened warm advection associated with the mountain-induced anticyclone. Weak upward motion, instead of strong downward motion, is found near the lee low-pressure region. The upward motion is produced by the mountain acting as an elevated heat source. This upward motion may accelerate and strengthen the cyclogenesis if the low-level flow were saturated and latent heating were to become important.

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