High frequency bottom stress variability and its prediction in the CODE region
High frequency bottom stress variability and its prediction in the CODE region
Date
1983-06
Authors
Grant, William D.
Williams, Albert J.
Glenn, Scott M.
Cacchione, David A.
Williams, Albert J.
Glenn, Scott M.
Cacchione, David A.
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DOI
10.1575/1912/8879
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Keywords
Ocean waves
Shear waves
Boundary layer
Ocean circulation
Shear waves
Boundary layer
Ocean circulation
Abstract
High quality bottom boundary layer measurements obtained in the CODE
region off Northern California are described. Bottom tripod velocity
measurements and supporting data obtained during typical spring and early
summer conditions and during a winter storm are analyzed to obtain both
velocity profiles and mean bottom stress and bottom roughness estimates. The
spring/summer measurements were taken in June, 1981 during CODE-1 at C3 (90 m)
by Grant and Williams, WHOI; the winter storm data was taken in November 1980
prior to CODE-1 at the R2 (80 m) site by Cacchione and Drake, USGS.
The mean near-bottom (< 2m) velocity profiles are logarithmic (R2 > 0.993)
much of the time for everyday flows; deviations are primarily due to kinematical
effects induced by unsteadiness from internal waves. Stress profiles show
the logarithmic layer corresponds to a constant stress layer as expected for
the inertial region of a boundary layer. Stress estimates made from dissipation
and profile techniques agree at the 95 percent confidence level. Typical z0
values estimated from measurements greater than 30 cm above the bottom
have magnitudes of approximately 1 cm; an order of magnitude larger than the
physical bottom roughness. Corresponding u* values have typical magnitudes
of 0.5-1.0 cm/sec; more than twice as large as expected from a usual drag law
prediction (corresponding to over a factor of four in mean stress). These
values are demonstrated to be consistent with those expected for combined wave
and current flows predicted theoretically by Grant and Madsen (1979) and Smith
(1977). The u* values estimated from the CODE-1 data and predicted by the
Grant and Madsen (1979) model typically agree within 10-15 percent. Similar
results are demonstrated for the winter storm conditions during which large
sediment transport occurs. (Typical z0 values are 4-6 cm; typical u*
values are 3-6 cm/sec). The waves influencing the mid-shelf bottom stress estimates are 14-20
second swell associated with Southern and Western Pacific storms. These waves
are present over most of the year. The results clearly demonstrate that waves
must be taken into account in predicting bottom stress over the Northern
California Shelf.
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Grant, W. D., Williams, A. J., Glenn, S. M., & Cacchione, D. A. (1983). High frequency bottom stress variability and its prediction in the CODE region. Woods Hole Oceanographic Institution. https://doi.org/10.1575/1912/8879