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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.

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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.

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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.

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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.

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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.

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Li Dong and Bin Wang

Abstract

A previous Lagrangian linear advection scheme (trajectory-tracking scheme) is modified to achieve local mass conservation in this paper, which is more favorable to climate modeling. The discretized tracer parcels are volumes with interfaces instead of centroids. In 2D problems, the parcels are polygons and the interfaces are described by polygonal edges with a finite number. Because polygons will deform under the background wind field, a curvature-guard algorithm (CGA) is devised to retain accurate representation of the deformed interfaces among parcels. The tracer mass carried by parcels is mapped onto the regular latitude–longitude mesh by a first-order conservative remapping algorithm that can handle concave polygons. Several standard test cases have been carried out to show the effectiveness of the new scheme.

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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.

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Li Dong and Bin Wang

Abstract

A Lagrangian linear advection scheme, which is called the trajectory-tracking scheme, is proposed in this paper. The continuous tracer field has been discretized as finite tracer parcels that are points moving with the velocity field. By using the inverse distance weighted interpolation, the density carried by parcels is mapped onto the fixed Eulerian mesh (e.g., regular latitude–longitude mesh on the sphere) where the result is rendered. A renormalization technique has been adopted to accomplish mass conservation on the grids. The major advantage of this scheme is the ability to preserve discontinuity very well. Several standard tests have been carried out, including 1D and 2D Cartesian cases, and 2D spherical cases. The results show that the spurious numerical diffusion has been eliminated, which is a potential merit for the atmospheric modeling.

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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.

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Dong-Hyun Cha and Yuqing Wang

Abstract

To improve the initial conditions of tropical cyclone (TC) forecast models, a dynamical initialization (DI) scheme using cycle runs is developed and implemented into a real-time forecast system for northwest Pacific TCs based on the Weather Research and Forecasting (WRF) Model. In this scheme, cycle runs with a 6-h window before the initial forecast time are repeatedly conducted to spin up the axisymmetric component of the TC vortex until the model TC intensity is comparable to the observed. This is followed by a 72-h forecast using the Global Forecast System (GFS) prediction as lateral boundary conditions. In the DI scheme, the spectral nudging technique is employed during each cycle run to reduce bias in the large-scale environmental field, and the relocation method is applied after the last cycle run to reduce the initial position error. To demonstrate the effectiveness of the proposed DI scheme, 69 forecast experiments with and without the DI are conducted for 13 TCs over the northwest Pacific in 2010 and 2011. The DI shows positive effects on both track and intensity forecasts of TCs, although its overall skill depends strongly on the performance of the GFS forecasts. Compared to the forecasts without the DI, on average, forecasts with the DI reduce the position and intensity errors by 10% and 30%, respectively. The results demonstrate that the proposed DI scheme improves the initial TC vortex structure and intensity and provides warm physics spinup, producing initial states consistent with the forecast model, thus achieving improved track and intensity forecasts.

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