engleski

Frictional bottom boundary layers for tides: observations, theory, and modeling from the northern Adriatic

Vertical variation of the M2, K1, and S2 tides near the bottom are strong and qualitatively agree with the structure expected from the elegant theory of time-dependent, tidally-forced, bottom Ekman layers using a constant eddy viscosity. A best-fit to this theory was made for data from the twelve deeper sites by using different eddy viscosity values and computing the resulting velocity errors. The results for M2, K1, and S2 were vertical eddy viscosity values of 8.7, 7.7, and 9.6 cm2/s, with a typical siteto- site variability of 2, 4, and 4 cm2/s, respectively. However, despite relatively low best-fit residual errors (e.g., 0.38 cm/s average for M2), the best fit suggests that the basic theory is too simple, since the amount of tidal ellipse shortening and broadening relative to the amount of tilt and phase rotation with depth does not match well. Therefore, two different existing numerical simulations of the Adriatic were compared with the ADCP results. The models are the finite-difference Navy Coastal Ocean Model (NCOM) and the finite-element QUODDY model, which use Mellor-Yamada Level 2 and 2.5 closure schemes respectively. Early results from the NCOM comparisons suggest that these more complicated frictional dissipation schemes do indeed better represent the observed vertical structure of the bottom boundary layer.

Bottom friction; tide; Adriatic Sea

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