Jinyu Sheng

Vertical Profiles of Suspended Sand Concentration and Size From Multifrequency Acoustic Backscatter

Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three-frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4–6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5- to 10-cm height, over time scales ranging from 6.5 min to 4–6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5–20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time-averaged concentration profiles exhibit an exponential decrease with height above an O(10)-cm-thick near-bottom region of nonexpo-nential decrease. In contrast, the time-averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.

An examination of the spherical scatterer approximation in aqueous suspensions of sand

The available data for scattered acoustic intensity and attenuation in dilute aqueous suspensions of sand are compared with theory. In theoretical calculations, the scatterer is assumed to be spherical and elastic, or rigid and movable, or rigid and immovable. The rigid movable model provides the best fit to the data. The failure of the elastic model in comparison to the rigid sphere models indicates that resonance excitation does not occur in natural sand grains, probably because of irregularities in shape. The fact that better agreement with experiment is obtained with the rigid movable model than with the rigid immovable model indicates that the inertia of the particles is important. Additional approximate expressions for the form factor and attenuation coefficient have been constructed based on a modified form of the so‐called high‐pass model introduced by Johnson [J. Acoust. Soc. Am. 61, 275–277 (1977)]. The modified high‐pass model provides a fit to the data that is as good as, or better than, the r...

CANDIE: A New Version of the DieCAST Ocean Circulation Model

Abstract The development and verification of a new version of the DieCAST ocean circulation model to be referred to as CANDIE (Canadian Diecast) are considered. Both CANDIE and DieCAST have many features in common with the well-known Modular Ocean Model (MOM) of the Geophysical Fluid Dynamics Laboratory. Of particular relevance to the present study are the rigid-lid approximation and the use of standard Cartesian coordinates. The DieCAST formulation in terms of the surface pressure, rather than the volume transport streamfunction, is also used in CANDIE to reduce numerical sensitivity to ocean depth variations. The major difference between MOM and DieCAST is the use of a mixed C and A grid formulation in DieCAST rather than the B grid formulation used in MOM. CANDIE differs from DieCAST in the use of a standard C grid formulation and a reduction in the magnitude of the time truncation error associated with the implicit treatment of the Coriolis force. The implementation of the rigid-lid approximation is r...

Barotropic waves generated by storms moving rapidly over shallow water

[1] In the falls of both 1999 and 2000, waves with characteristics similar to tsunami hit the southeast coast of Newfoundland, Canada. The waves were large enough to cause local flooding, damage to docks, and other destruction. There is, however, no evidence of seismic events, underwater landslides, or slumping events on either occasion. Other explanations, such as storm surge, also appear unlikely, and local weather conditions at the coast were not exceptional at the time. On both occasions, tropical storms moved rapidly across the Grand Banks of Newfoundland from southwest to northeast, with a translation speed of � 30 m s � 1 . A significant, nonisostatic response to atmospheric pressure forcing can be expected over the shallow water of the banks since the translation speed of the storms is comparable to the local shallow water gravity wave speed. We speculate that the atmospheric pressure forcing associated with the storms generated a barotropic wake, and we use a numerical model to argue that as the storm moved back over the deep ocean, the wake was refracted and/or reflected by the variable bathymetry at the edge of the banks and that it was the refraction of the wake toward the coast that led to the unusual sea level events in southeastern Newfoundland. The numerical model results are in general agreement with the eye witness reports. The model-computed wave activity hits the southeast coast of Newfoundland at about the right time and in the right areas for both events, although for the 1999 event the model response is weaker than is observed at Port Rexton in Trinity Bay. The reason for the poorer model performance in the 1999 case is not known, although we do find that the model results are sensitive to uncertainty in the exact track taken by the storm across the banks. The model results demonstrate that the period and wavelength of the gravity waves comprising the wake are, in general, proportional to the length scale of the pressure forcing, an exception being the model response in Conception Bay, Newfoundland, where a resonant seiche response is found to dominate. INDEX TERMS: 4504 Oceanography: Physical: Air/sea interactions (0312); 4564 Oceanography: Physical: Tsunamis and storm surges; 4219 Oceanography: General: Continental shelf processes; 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 4255 Oceanography: General: Numerical modeling; KEYWORDS: tsunami, barotropic, tropical storm, modeling, air-sea

Numerical study of circulation, dispersion, and hydrodynamic connectivity of surface waters on the Belize shelf

A nested grid ocean circulation modeling system is used to examine the circulation, dispersion, and hydrodynamic connectivity of surface waters on the Belizean shelf. The nested grid system consists of a coarse-resolution (�19 km) outer model of the western Caribbean Sea, an intermediate-resolution (�6 km) middle model of the southern Meso-American Barrier Reef System (MBRS), and a fine-resolution (�2 km) inner model of the Belizean shelf. The nested system is forced by climatological monthly mean surface forcing and integrated over 5 years. The near-surface circulation on the Belize shelf produced by the inner model is characterized by a strong and persistent northwestward flow as a direct influence of the Caribbean Current on the northwestern shelf and a weak and spatially variable flow on the inner and southern shelf. The monthly mean model currents are used to calculate retention and dispersion of conservative, near-surface particles carried by the ocean currents. The near-surface dispersion is relatively higher in areas seaward (east) of Lighthouse and Glovers Reef atolls and lower on the inner shelf, particularly within the Inner Channel and in the vicinity of South Water Cay. To examine hydrodynamic connectivity of reefs in the surface waters of the Belize shelf, we calculate upstream and downstream retention areas for coral reefs at Turneffe Islands and Glovers Reef atolls. The potential sources of passive, near-surface particle supply reaching these two reef atolls within 30 days include both the shallow waters surrounding the two sites, the deep waters between them, and the coastal waters of the Bay Islands (Honduras). The 30-day downstream retention areas of the Turneffe and Glovers Reef atolls cover the central and southern Belize shelf, respectively.

A Numerical Study of Circulation in the Western Caribbean Sea

Abstract A three-dimensional ocean circulation model is used to study circulation and month-to-month variability in the western Caribbean Sea. The domain covers the area between 72° and 90°W and between 8° and 24°N, with a horizontal resolution of roughly 18 km. The western Caribbean Sea model is forced by the monthly mean Comprehensive Ocean–Atmosphere Data Set (COADS) wind stress and surface heat flux and monthly mean volume transports through the model open boundaries calculated by a (1/3)° Atlantic Ocean model. The model sea surface salinity is restored to the monthly mean climatology. The semiprognostic method suggested by Sheng et al. is used to reduce the model errors by assimilating the hydrographic data into the momentum equations. The model reproduces many well-known circulation features in the region, including the warm and persistent throughflow known as the Caribbean Current, the highly variable Panama–Colombia Gyre, and moderate seasonal variations of temperature and salinity in the surface ...

Dynamics of a Buoyancy-Driven Coastal Jet: The Gaspé Current

Abstract A primitive equation ocean model is applied to the process study of the Gaspe Current and cyclonic circulation over the northwestern Gulf of St. Lawrence (NWG). The model is driven by river discharge and barotropic boundary flows. Two types of model domains are used: an idealized basin with a flat bottom and piecewise coastline, and a realistic basin with model-resolved NWG bathymetry. The model domains are initially filled with horizontally uniform but vertically stratified waters. The river discharge is expressed in terms of lower salinity and a weak barotropic inflow in the upper waters at the estuary head. The early developments of the estuarine plume and coastal current system driven by the river discharge are qualitatively similar in both basins. After a short-period adjustment, a buoyant plume is developed near the estuary mouth, with a surface-intensified coastal current advecting the estuarine water seaward in the direction of Kelvin wave propagation. The coastal current initially follow...

Subtidal circulation on the Scotian Shelf: Assessing the hindcast skill of a linear, barotropic model

Currents measured during the winter of 1985–1986 by four moorings on the inner Scotian Shelf are used to assess the hindcast skill of a three-dimensional circulation model forced by local wind stress and coastal sea level. The model is linear and barotropic. The integration scheme is based on the modification of the Galerkin spectral method proposed recently by Sheng and Thompson [1993]. The skill of the model is measured by the variance of the hindcast errors divided by the variance of the observations (henceforth γ2). The model is most effective within 30 km of shore (0.51≤γ2≤0.66). At the mooring in the Nova Scotia Current, a surface intensified southwestward jet with its center approximately 50 km from shore, the model fails to capture the bulk of the variance at current meters within 70 m of the surface (0.81≤γ2≤0.87). The skill of the model is lowest at the mooring 65 km from shore (0.92≤γ2≤0.94). To put these measures of skill into perspective, the currents are hindcast using a linear statistical model with the same inputs as the circulation model. The statistical model is optimal in the sense no other linear model with these inputs can achieve a lower hindcast error variance. For half of the current meters within 30 km of shore and two in the Nova Scotia Current the skill of the circulation model is not significantly lower than that of the statistical model. The largest discrepancies in the skill of the two models are found at the offshore mooring. We show that the suboptimal performance of the circulation model is due in part to the assumption of a spatially uniform wind field. We speculate that another contributing factor is the assumed form of the sea level profile along the open boundary that is upstream in the sense of coastal trapped wave propagation. Finally, the statistical model is used to estimate the increase in skill of the circulation model that may result from the assimilation of hydrographic data and additional coastal sea levels.

Sediment eddy diffusivities in the nearshore zone, from multifrequency acoustic backscatter

Abstract Acoustic backscatter measurements made on the seaward face of a submerged nearshore bar are used to determine the vertical eddy diffusivity profile for sand in suspension. Suspended sand size and concentration estimates, obtained from the multifrequency backscatter inverse, permit the sediment eddy diffusivity to be determined by assuming a balance between settling and vertical diffusion after correcting for the vertical wave flux. The resulting diffusivity profiles exhibit a nearbed linear region scaling approximately with the mean maximum friction velocity. Above this region, the data do not collapse to a universal form using a single vertical length scale.

A robust method for diagnosing regional shelf circulation from scattered density profiles

We present a straightforward method for estimating surface circulation on an f plane from a set of irregularly spaced vertical density profiles. The first step is to express bottom density ρh as the sum of a mean ρ˜h for a given water depth h and an anomaly, ρ′h=ρh−ρ˜h. Sea level η can then be decomposed into a dynamic height η˜ relative to a deep reference level and a correction term, η′=η−η˜. The dynamic height is estimated using a generalization of the method of Helland-Hansen [1934] for diagnosing flow through a cross-shelf section under the assumption of zero bottom geostrophic flow. The correction η′ satisfies a two-dimensional elliptic partial differential equation forced by the bottom density anomaly, wind stress and the open boundary conditions. Before calculating the density-driven component of η′ we first test if the ρ′h are statistically different from uncorrelated noise. If they are not, the correction associated with the bottom density anomaly is set to zero. Thus the method has a degree of robustness to errors in the density observations. If the ρ′h have well-defined spatial structure the elliptic equation is solved for η′ and sea level is equated to η˜+η′. Note that even if this last step is required the only gridding of the density data is two-dimensional. This makes the proposed method simpler to use than many of the existing diagnostic models which require a three-dimensional gridding of the observed density profiles. To test the method, we use it to diagnose the flow from an idealized density field overlying an isolated topographic feature. The predicted sea level and flow fields are then compared, and shown to be in good agreement, with results from the Princeton Ocean Model. The method is then used to diagnose the winter surface circulation on the Scotian Shelf from observed density profiles. The reliability of the diagnosed flow pattern is assessed by comparing it against all available near-surface current measurements. The differences between the observed and diagnosed currents are used to estimate the remotely forced circulation on the Scotian Shelf. It is shown that the main features of the circulation can be explained by gradients in the density field. Remote forcing is important near the coast and the shelf break. The effect of local wind is relatively weak.