A Numerical Study on Sea/Land Breezes as a Gravity Current: Kelvin–Helmholtz Billows and Inland Penetration of the Sea-Breeze Front

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.