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Weiming Sha, Takeshi Kawamura, and Hiromasa Ueda

Abstract

It is demonstrated in this numerical study that prefrontal perturbations may be triggered by a penetrating sea-breeze head into an existing nocturnal temperature inversion. The perturbations consist of the lower-layer wavelike perturbation and the upper-wave motion, both of which are manifested as rotor streaming. However, these prefrontal gravity waves take the form of the weak transient waves of a depression trapped in the ambient compensating flow field.

The sea-breeze head dissipates as it penetrates inland into the nocturnal temperature inversion. At midnight, a horizontal vortex is completely detached from the feeder flow of the sea breeze. This isolated horizontal vortex is identified as the sea-breeze cutoff vortex. It is shown that the sea-breeze cutoff vortex may be evolved from a dissipating sea-breeze head. After the sea-breeze cutoff vortex is formed, it propagates farther inland as an isolated wave-type disturbance. Examination of the force balance suggests that the inertia and radiative energy loss are dominant in the processes of the sea-breeze cutoff vortex.

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Weiming Sha, Takeshi Kawamura, and Hiromasa Ueda

Abstract

A two-dimensional, nonhydrostatic, compressible, and dry numerical model was developed. By using high spatial resolution and an appropriate turbulence model, fine structure and dynamics of the sea-breeze head were investigated.

Kelvin–Helmholtz (KH) instability was found to occur in the foremost part of the head. Consequently KH billows were generated there and grew in amplitude while travelling backward relative to the advancing front along the “zero-velocity boundary.” The KH billows entrained the upper air into the sea breeze. The resultant turbulent mixing and wave perturbations induced a top friction force acting on the sea-breeze head. Structure of the KH billow and the wavelike motion induced by it near the ground were also investigated and compared with the results obtained in laboratory density currents, linear theories, and observations.

The KH instability (KHI) did not occur at all stages of the sea breeze, and this resulted in a diurnal variation of the fine structure and dynamics of the head. Rather, the KHI was produced only in the middle part of the day and the induced top friction force decelerated the inland penetration of the sea-breeze front. By estimating all the controlling forces acting on the head, it was concluded that the increasing top friction force is associated with the slowing of the sea-breeze penetration.

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