Search Results

You are looking at 1 - 10 of 22 items for

  • Author or Editor: Dong-Ping Wang x
  • Refine by Access: All Content x
Clear All Modify Search
Dong-Ping Wang

Abstract

A three-dimensional primitive-equation model was used to study gravity currents produced by instantaneous releases of a buoyant fluid in a rectangular channel. Without rotation, the gravity current passes through two distinct phases: an initial adjustment phase in which the front speed is constant, and an eventual self-similar phase in which the front speed decreases with time. With rotation, the gravity current is confined to the right-hand wall, forming a coastal jet. The initial front-speed is constant; however, the front speed decreases rapidly due to strong mixing at the horizontal edge of the gravity current. Also, with rotation, part of the buoyant fluid is trapped near the source region, forming an anticyclonic vortex.

Full access
Dong-Ping Wang

Abstract

Csanady's (1978) theory on the mean shelf circulation in a homogeneous ocean was re-examined by including effects of a continental slope. The results suggested that the mean southwestward flow on the Mid-Atlantic Blight is driven by an inflow from the Georges Bank.

Full access
Dong-Ping Wang

Abstract

A three-dimensional, finite-difference model is developed to study limited-area (island) shelf circulation. The model uses a semi-implicit scheme in the cross-shore dimension and a mode-splitting technique in the vertical dimension, to achieve superior computing efficiency.

The model was applied to a circular island to test its response under the joint effects of bottom topography and density stratification. For the homogeneous ocean case, model simulations agree well with analytical shelf wave theory. For the stratified ocean case. model results indicate formation of temperature fronts associated with coastal upwelling and downwelling.

The model also was applied to the study of the transient shelf circulation off Peru with idealized shelf geometry and wind forcing. At the onset of equatorward (upwelling-favorable) wind, model simulation indicates equatorward flow throughout the water column. After ∼30 h, the equatorward flow propagates poleward out of the forcing zone with a phase speed of 200 km day−1. In the meantime, a poleward current which is induced by the alongshore pressure gradient propagates into the forcing zone with the same phase speed. With the exception of the near-surface current, all flows in the forcing zone are eventually in the direction opposite to the wind stress. These results show good agreement with the observed features of the Peruvian upwelling.

Full access
Dong-Ping Wang

Abstract

Diffraction of continental shelf waves by irregular alongshore geometry, such as ridges, canyons and bumps, is examined. The full barotropic, shelf-wave equation is treated, and the solutions include forward and back scattering, and a description of the near-field circulation around the topographic feature.

Reflection of long waves by the convergence/divergence of depth contours is small. On the other hand, velocity amplitude of the transmitted wave can become much larger (smaller) than the incident amplitude, in the case of convergence (divergence). Back scattering becomes important, when the incident wave approaches critical frequency (zero group speed). Above critical frequency, the incident long wave is totally reflected as a short wave.

Wave diffraction by ridges or canyons leads to both forward and back scattering. Local amplitude amplification occurs near the depth convergence zone. The amplitude amplification is more intense when higher modes are excited. The reflected short wave is also likely to be trapped immediately upstream of the ridge or canyon, due to bottom dissipation. Consequently, strong localized disturbances will be generated near the ridge or canyon.

The results suggest that topographic irregularities on the continental shelf are the energy sink of long waves. Through diffraction, the large-scale, predominantly alongshore motion transforms to the intense, small-scale, cross-shore motion in the vicinity of sharp depth convergence.

Full access
Dong-Ping Wang

Abstract

Subtidal sea level variations in the Chesapeake Bay were examined over a one-year period for evidence of wind-driven barotropic circulation. The major transport occurred at time scales of 3–5 days, whose magnitude was larger than the river runoff. It was driven by the east-west wind, as part of the coupled coastal ocean-estuary response. At shorter time scales, there was also large barotropic motion which, however, was driven by the local, north-south wind.

The variance of barotropic fluctuation was larger by a factor of 4 in winter than in summer, due to the increased cyclone activities. The coupled coastal ocean-estuary response was also more pronounced in winter. In contrast, the summer season was dominated by local forcing at time scales of 3–7 days.

The results suggest that the barotropic motion is an important component of the net circulation. The corresponding subtidal sea level change contributes significantly to the storm surge. Thus, the nature of barotropic response, particularly the coupled response, must be carefully examined for better understanding of the dispersion processes and storm surges in Chesapeake Bay.

Full access
Dong-Ping Wang

Abstract

Nontidal circulation in Chesapeake Bay was examined from one-month current records at 50 and 200 km from the entrance. The monthly mean flow was basically a two-layered circulation; in addition, there were large wind-driven velocity fluctuations at several-day time scales. Corresponding to velocity changes, the salinity distribution had large variations, comparable to its seasonal change.

Bay water responded to longitudinal (local) wind and coastal (nonlocal) Ekman flux. The response was barotropic in the lower Bay, and baroclinic (frictional) in the upper Bay. The difference in response characteristic appears to be due to the counter-effects of the near-surface windstress shear and the depth-independent surface slope. A frictional model accounts for most of the observed features.

Results of this study provide further evidence of large, atmospherically induced exchange between the estuary and coastal ocean. The importance of wind on upstream salt intrusions is also clearly demonstrated.

Full access
Dong-Ping Wang

Abstract

Coastal-trapped waves are studied in a two-layered, non-flat shelf model. Internal Kelvin wave and quasi-geostrophic waves appear as eigenmodes of the system. The latter reduce to the familiar barotropic shelf waves only in the limit of vanishing stratification. With strong stratification, i.e., where the internal Kelvin wave phase speed is larger than the phase speed of the quasi-geostrophic wave, quasi-geostrophic waves are bottom-trapped. Resonant coupling occurs when the two types of waves have compatible phase speeds; in this case, the relative amplitude distribution of the resonant modes is very sensitive to the change of the baroclinic radius of deformation. Implications of this work for the study of shelf water response to external disturbances are briefly discussed.

Full access
Dong-Ping Wang
and
Lie-Yauw Oey

Hurricane Katrina caused extensive damage to offshore oil and gas production facilities. In this study, the state-of-the-art ocean circulation (the Princeton Ocean Model) and surface wave (Wave Watch III) models, together with high-resolution analyzed winds from NOAA/Hurricane Research Division, are used to simulate the current and wave conditions during Katrina. The model simulation shows large (>15 m) surface waves and strong (>2 m s−1) wind-driven and inertial currents superposed on the Loop Current and Loop Current eddy. The simulated wave fields are verified with surface buoy and satellite altimetry observations; the agreement generally is better than 0.5 m, and the correlation coefficient is above 0.95. Also, while the observed 55-ft significant wave heights on National Data Buoy Center (NDBC) buoy 42040 surpassed the previous record in the Gulf of Mexico, circumstantial evidence suggests that waves as large as 70 ft might have occurred in the storm path. Comparison with the operational analysis suggests that the current NCEP model system tends to underestimate the spatial extent of the serious wave impact.

Full access
Wen-Ssn Chuang
and
Dong-Ping Wang

Abstract

A numerical model is developed to study the internal tidal motion on the continental margin. The system includes irregular bottom topography and a horizontal density stratification maintained by a mean geostrophic current. Both the propagation and generation processes are examined.

In the propagation process, the topographic effect alone will scatter a significant part of the incident wave energy into higher modes, resulting in a beamlike structure for the transmitted wave. For a density front over a flat bottom, energy in the transmitted waves remains largely in the original wave mode, though the wavelength, and hence wave amplitude, varies with local density stratification. When the density front is located over a sloping bottom, the topographic effect is reduced, whereas the frontal effect is not affected. Thus, scattering into higher modes becomes more restricted.

In the generation process, internal tides are produced in the upper and lower parts of the slope region due to the interaction between surface tides and bottom topography. With the presence of a density front, internal tide energy also can be derived from the surface frontal layer. In addition, the topographic effect will be modified by the front. The net effect is destructive (constructive) when the isopycnals tilt upward in the shoreward (seaward) direction.

These results indicate that on the continental margin, the density front has strong effects on the generation and propagation of internal tides, particularly when the density front is located above the continental slope. Since the internal tide provides an important energy source for mixing on the shelf, the density fronts will have a strong effect on the mixing processes.

Full access
Dong-Ping Wang
and
David W. Kravitz

Abstract

A semi-implicit, two-dimensional (in a vertical plane) model is developed for circulation in the partially mixed estuary. Comparisons between the semi-implicit and explicit method are made in the simulation of tidal, wind-driven and density-driven circulations. In general, the two model results are in good agreement in velocity and density computation; the semi-implicit method, however, fails to simulate the surface seiche oscillation. On the other hand, the semi-implicit method is more efficient; depending on the horizontal space resolution, the semi-implicit method can result in orders of magnitude saving in computer time. Application of the semi-implicit model to the Potomac River indicates large longitudinal and vertical changes in tidal, density-driven and wind-driven circulations, which suggests that two-dimensional (in a vertical plane) modeling is essential in the transport and mixing study.

Full access