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. / Graduation date: 1975
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28994 |
| Date | 24 June 1974 |
| Creators | Poole, Stephen Lynn |
| Contributors | Burt, Wayne V. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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