The effect of the land breeze on the mesoscale wind field off the Oregon coast Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1v53k031m

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  • Two land breeze events occurred off the Oregon coast on the nights of April 19th and 20th, 1973. An array of four moored toroid buoys and one land station recorded the effect of the land breeze event on the surface mesoscale wind and temperature fields. The land breezes may have resulted from the premature summerly conditions of fair weather and southward coastal winds that were caused by an early northeastward extension of the North Pacific High. The main features of the events were as follows: 1) A cooling period of a few hours after sunset established an air temperature gradient of -0.1° C km⁻¹ in the nearshore 10 km region. 2) The advance of the land breeze-front produced a 5° C temperature drop at the land station and a 1° C temperature drop at the buoy stations. 3) Simultaneously, the front also caused a decrease in wind speed by about an order of magnitude at each of the stations. During the passage of the front the wind veered from southward at 10 m sec⁻¹ to westward at 2 to 3 m sec⁻¹. 4) At dawn the temperature gradient was rapidly reversed, but there was a 2 hour lag before the wind speed began to increase. No frontal return flow was observed, instead the wind backed to the south and increased gradually over the array. Horizontal divergence and vertical vorticity were calculated using a simplified program. The land breeze produced spans of positive vorticity (5 x 10⁻⁴ sec⁻¹) over the array, possibly due to the horizontal wind shear in the offshore direction. The land breeze also caused a zone of convergence over the nearshore 10 km. The convergence was preceeded by a brief period of intense divergence. There was no convergence zone beyond the nearshore region. Instead there appeared alternating bands of convergence and divergence with a period of around 37 minutes. The same periodicity was observed in the offshore wind velocity. These features can be explained by a model of horizontal roll vortices migrating seaward from the nearshore convergence zone. The roll wavelength is inferred to be 4.7 km, the westward migration speed is 2 m sec⁻¹, and the height of the PBL is estimated to be 1. 5 km. This leads to a PBL Reynolds number of 370 ± 80, which is lower than previous observations and suggests that the rolls are produced by buoyancy and parallel instability. A model which is compatible with all the above is presented.
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