Modeling internal tides over Fieberling Guyot: resolution, parameterization, performance |
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Authors: | Robin Robertson |
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Institution: | (1) Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades,, NY 10987, USA;(2) Present address: University of New South Wales, ADFA (Australian Defence Force Academy), Canberra, ACT 2600, Australia |
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Abstract: | Terrain-following ocean models are being used to simulate baroclinic tides and provide estimates of the tidal fields for circulation
and mixing studies. These models have successfully reproduced elevations with most of the remaining inaccuracies attributed
to topographic errors; however, the replication of barotropic and baroclinic velocity fields has not been as robust. Part
of the problem is the lack of an adequate observational dataset in the simulated regions to compare the models. This problem
was addressed using a dataset collected during the Flow over Abrupt Topography initiative at Fieberling Guyot. To evaluate
the capability of the Regional Ocean Model System (ROMS) to simulate baroclinic tidal velocities, the combined tides for four
constituents, M2, S2, K1, and O1, were modeled over Fieberling Guyot. Model inputs, numerical schemes, and parameterizations were varied to improve agreement
with observations. These included hydrography, horizontal resolution, and the vertical mixing parameterization. Other factors
were evaluated but are not included in this paper. With the best case, semidiurnal baroclinic tides were well replicated with
RMS differences between the model estimates and the observations of 1.85 and 0.60 cm s−1 for the major axes of the tidal ellipses for M2 and S2, respectively. However, diurnal K1 baroclinic tides were poorly simulated with RMS differences of 4.49 cm s−1. In the simulations, the K1 baroclinic tides remained bottom-trapped unlike the observed fields, which had free waves due to the contribution of the
mean velocity to the potential vorticity. The model did not adequately simulate the mean velocity, and the K1 tides remained trapped. A resolution of 1 km most accurately reproduced the major axes and mean velocities; however, a 4-km
resolution was sufficient for a qualitative estimate of where baroclinic tidal generation occurred. Nine vertical mixing parameterizations
were compared. The vertical mixing parameterization was found to have minor effects on the velocity fields, with most effects
occurring over the crown of guyot and in the lower water column; however, it had dramatic effects on the estimation of vertical
diffusivity of temperature. Although there was no definitive best performer for the vertical mixing parameterization, several
parameterizations could be eliminated based on comparison of the vertical diffusivity estimates with observations. The best
performers were Mellor–Yamada and three generic length scale schemes. |
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Keywords: | Terrrain-following models Baroclinic tides Vertical mixing parameterization |
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