Xu Qichun

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Process modeling studies of physical mechanisms of the formation of an anticyclonic eddy in the central Red Sea

(John Wiley & Sons, 2014-02-25) Chen, Changsheng ; Li, Ruixiang ; Pratt, Lawrence J. ; Limeburner, Richard ; Beardsley, Robert C. ; Bower, Amy S. ; Jiang, Houshuo ; Abualnaja, Yasser ; Xu, Qichun ; Lin, Huichan ; Liu, Xuehai ; Lan, Jian ; Kim, Taewan

Surface drifters released in the central Red Sea during April 2010 detected a well-defined anticyclonic eddy around 23°N. This eddy was ∼45–60 km in radius, with a swirl speed up to ∼0.5 m/s. The eddy feature was also evident in monthly averaged sea surface height fields and in current profiles measured on a cross-isobath, shipboard CTD/ADCP survey around that region. The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was configured for the Red Sea and process studies were conducted to establish the conditions necessary for the eddy to form and to establish its robustness. The model was capable of reproducing the observed anticyclonic eddy with the same location and size. Diagnosis of model results suggests that the eddy can be formed in a Red Sea that is subject to seasonally varying buoyancy forcing, with no wind, but that its location and structure are significantly altered by wind forcing, initial distribution of water stratification and southward coastal flow from the upstream area. Momentum analysis indicates that the flow field of the eddy was in geostrophic balance, with the baroclinic pressure gradient forcing about the same order of magnitude as the surface pressure gradient forcing.
Using MM5 to hindcast the ocean surface forcing fields over the Gulf of Maine and Georges Bank Region

(American Meteorological Society, 2005-02) Chen, Changsheng ; Beardsley, Robert C. ; Hu, Song ; Xu, Qichun ; Lin, Huichan

The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) is applied to the Gulf of Maine/Georges Bank (GoM/GB) region. This model is configured with two numerical domains with horizontal resolutions of 30 and 10 km, respectively, and driven by the NCAR-Eta weather model through a nested grid approach. Comparison of model-computed winds, wind stress, and heat flux with in situ data collected on moored meteorological buoys in the western GoM and over GB in 1995 shows that during the passage of atmospheric fronts over this region, MM5 provides a reasonable prediction of wind speed but not wind direction, and provides a relatively accurate estimation of longwave radiation but overestimates sensible and latent fluxes. The nudging data assimilation approach with inclusion of in situ wind data significantly improves the accuracy of the predicted wind speed and direction. Incorporation of the Fairall et al. air–sea flux algorithms with inclusion of Advanced Very High Resolution Radiometer (AVHRR)-derived SST improves the accuracy of the predicted latent and sensible heat fluxes in the GoM/GB region for both stable and unstable weather conditions.
Reply to comment on “Current separation and upwelling over the southeast shelf of Vietnam in the South China Sea”

(John Wiley & Sons, 2013-03-31) Chen, Changsheng ; Lai, Zhigang ; Beardsley, Robert C. ; Xu, Qichun ; Lin, Huichan ; Viet, Nguyen Trung ; Yang, Ding
Extratropical storm inundation testbed : intermodel comparisons in Scituate, Massachusetts

(John Wiley & Sons, 2013-10-07) Chen, Changsheng ; Beardsley, Robert C. ; Luettich, Richard A. ; Westerink, Joannes J. ; Wang, Harry ; Perrie, Will ; Xu, Qichun ; Donahue, Aaron S. ; Qi, Jianhua ; Lin, Huichan ; Zhao, Liuzhi ; Kerr, Patrick C. ; Meng, Yanqiu ; Toulany, Bash

The Integrated Ocean Observing System Super-regional Coastal Modeling Testbed had one objective to evaluate the capabilities of three unstructured-grid fully current-wave coupled ocean models (ADCIRC/SWAN, FVCOM/SWAVE, SELFE/WWM) to simulate extratropical storm-induced inundation in the US northeast coastal region. Scituate Harbor (MA) was chosen as the extratropical storm testbed site, and model simulations were made for the 24–27 May 2005 and 17–20 April 2007 (“Patriot's Day Storm”) nor'easters. For the same unstructured mesh, meteorological forcing, and initial/boundary conditions, intermodel comparisons were made for tidal elevation, surface waves, sea surface elevation, coastal inundation, currents, and volume transport. All three models showed similar accuracy in tidal simulation and consistency in dynamic responses to storm winds in experiments conducted without and with wave-current interaction. The three models also showed that wave-current interaction could (1) change the current direction from the along-shelf direction to the onshore direction over the northern shelf, enlarging the onshore water transport and (2) intensify an anticyclonic eddy in the harbor entrance and a cyclonic eddy in the harbor interior, which could increase the water transport toward the northern peninsula and the southern end and thus enhance flooding in those areas. The testbed intermodel comparisons suggest that major differences in the performance of the three models were caused primarily by (1) the inclusion of wave-current interaction, due to the different discrete algorithms used to solve the three wave models and compute water-current interaction, (2) the criterions used for the wet-dry point treatment of the flooding/drying process simulation, and (3) bottom friction parameterizations.
Studies of the Canadian Arctic Archipelago water transport and its relationship to basin-local forcings : results from AO-FVCOM

(John Wiley & Sons, 2016-06-25) Zhang, Yu ; Chen, Changsheng ; Beardsley, Robert C. ; Gao, Guoping ; Lai, Zhigang ; Curry, Beth ; Lee, Craig M. ; Lin, Huichan ; Qi, Jianhua ; Xu, Qichun

A high-resolution (up to 2 km), unstructured-grid, fully coupled Arctic sea ice-ocean Finite-Volume Community Ocean Model (AO-FVCOM) was employed to simulate the flow and transport through the Canadian Arctic Archipelago (CAA) over the period 1978–2013. The model-simulated CAA outflow flux was in reasonable agreement with the flux estimated based on measurements across Davis Strait, Nares Strait, Lancaster Sound, and Jones Sounds. The model was capable of reproducing the observed interannual variability in Davis Strait and Lancaster Sound. The simulated CAA outflow transport was highly correlated with the along-strait and cross-strait sea surface height (SSH) difference. Compared with the wind forcing, the sea level pressure (SLP) played a dominant role in establishing the SSH difference and the correlation of the CAA outflow with the cross-strait SSH difference can be explained by a simple geostrophic balance. The change in the simulated CAA outflow transport through Davis Strait showed a negative correlation with the net flux through Fram Strait. This correlation was related to the variation of the spatial distribution and intensity of the slope current over the Beaufort Sea and Greenland shelves. The different basin-scale surface forcings can increase the model uncertainty in the CAA outflow flux up to 15%. The daily adjustment of the model elevation to the satellite-derived SSH in the North Atlantic region outside Fram Strait could produce a larger North Atlantic inflow through west Svalbard and weaken the outflow from the Arctic Ocean through east Greenland.
Tidal dynamics in the Gulf of Maine and New England Shelf : an application of FVCOM

(American Geophysical Union, 2011-12-10) Chen, Changsheng ; Huang, Haosheng ; Beardsley, Robert C. ; Xu, Qichun ; Limeburner, Richard ; Cowles, Geoffrey W. ; Sun, Yunfang ; Qi, Jianhua ; Lin, Huichan

The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was used to simulate the tides in the Gulf of Maine (GoM) and New England Shelf (NES) for homogeneous and summer stratified conditions. FVCOM captures the near-resonant nature of the semidiurnal tide and energy flux in the GoM and the complex dynamics governing the tide in the NES. Stratification has limited impact on tidal elevation, but can significantly modify the tidal current profile. Internal tides are energetic in the stratified regions over steep bottom topography, but their contribution to the total tidal energy flux is only significant over the northeast flank of Georges Bank. The model suggests that the tidal flushing-induced eddy east of Monomoy Island is the dynamic basis for the locally observed phase lead of the M2 tide. The southward propagating tidal wave east of Cape Cod encounters the northeastward propagating tidal wave from the NES south of Nantucket Island, forming a zone of minimum sea level along a southeast-oriented line from Nantucket Island. These two waves are characterized by linear dynamics in which bottom friction and advection are negligible in the momentum balance, but their superposition leads to a strong nonlinear current interaction and large bottom stress in the zone of lowest sea elevation.
A model-dye comparison experiment in the tidal mixing front zone on the southern flank of Georges Bank

(American Geophysical Union, 2008-02-09) Chen, Changsheng ; Xu, Qichun ; Houghton, Robert ; Beardsley, Robert C.

A process-oriented model-dye comparison experiment was conducted to examine the ability of a numerical ocean model to simulate the observed movement of dye across the tidal mixing front on the southern flank of Georges Bank during 22–26 May 1999. The experiment was made using the unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) with varying horizontal resolution. The results indicate that the observed cross-isobath movement of the dye patch was primarily controlled by meso-scale temporal and spatial variability of the water temperature and salinity fields. Onset of vertical stratification tended to slow down an upward stretching of the dye column and trapped the dye within the bottom mixed layer. To reach a convergent numerical solution that reproduced the observed lateral turbulent dispersion of dye, the FVCOM grid required a horizontal resolution of ∼500 m in the dye study region. Within the tidal mixing front of Georges Bank, the movement of the center of the dye patch was mainly driven by the ensemble velocity integrated over the dye volume, with a first-order contribution from vertical shear of the dye's horizontal velocity.
Comparison of observed and model-computed low frequency circulation and hydrography on the New England Shelf

(American Geophysical Union, 2008-09-09) Cowles, Geoffrey W. ; Lentz, Steven J. ; Chen, Changsheng ; Xu, Qichun ; Beardsley, Robert C.

The finite volume coastal ocean model (FVCOM) is configured to study the interannual variability of circulation in the Gulf of Maine (GoM) and Georges Bank. The FVCOM-GoM system incorporates realistic time-dependent surface forcing derived from a high-resolution mesoscale meteorological model (MM5) and assimilation of observed quantities including sea surface temperature and salinity and temperature fields on the open boundary. An evaluation of FVCOM-GoM model skill on the New England shelf is made by comparison of computed fields and data collected during the Coastal Mixing and Optics (CMO) Program (August 1996–June 1997). Model mean currents for the full CMO period compare well in both magnitude and direction in fall and winter but overpredict the westward flow in spring. The direction and ellipticity of the subtidal variability correspond but computed magnitudes are around 20% below observed, partially due to underprediction of the variability by MM5. Response of subtidal currents to wind-forcing shows the model captures the directional dependence, as well as seasonal variability of the lag. Hydrographic results show that FVCOM-GoM resolves the spatial and temporal evolution of the temperature and salinity fields. The model-computed surface salinity field compares well, except in May when there is no indication of the fresh surface layer from the Connecticut River discharge noted in the observations. Analysis of model-computed results indicates that the plume was unable to extend to the mooring location due to the presence of a westward mean model-computed flow during that time that was stronger than observed. Overall FVCOM-GoM captures well the dynamics of the mean and subtidal flow on the New England shelf.
Observational and modeling studies of oceanic responses and feedbacks to typhoons Hato and Mangkhut over the northern shelf of the South China Sea

(Elsevier, 2021-01-01) Dong, Wenjing ; Feng, Yanqing ; Chen, Changsheng ; Wu, Zhongxiang ; Xu, Danya ; Li, Siqi ; Xu, Qichun ; Wang, Lu ; Beardsley, Robert C. ; Lin, Huichan ; Li, Ruixiang ; Chen, Junkun ; Li, Jiahui

Meteorological and oceanic responses to Typhoons Hato and Mangkhut were captured by storm-monitoring network buoys over the northern shelf of the South China Sea. With similar shelf-traversing trajectories, these two typhoons exhibited distinctly different features in storm-induced oceanic mixing and oceanic heat transfer through the air-sea interface. A well-defined cold wake was detected underneath the storm due to a rapid drop in sea surface temperature during the Hato crossing, but not during the Mangkhut crossing. Impacts of oceanic mixing on forming a storm-produced cold wake were associated with the pre-storm condition of water stratification. In addition to oceanic mixing produced through the diffusion process by shear and buoyancy turbulence productions, the short-time scale of mixing suggested convection/overturning may play a critical role in the rapid cooling at the sea surface. The importance of convection/overturning to mixing depended on the duration of atmospheric cooling above the sea surface-the longer the atmospheric cooling, the more significant effect on mixing. Including the oceanic mixed layer (OML) in the WRF model was capable of reproducing the observed storm-induced variations of wind and air pressure, but not the air and sea surface temperatures. Process-oriented numerical experiments with the OML models supported both observational and modeling findings. To simulate the storm-induced mixing in a coupled atmospheric and oceanic model, we need to improve the physics of vertical mixing with non-hydrostatic convection/overturning. Warming over the shelf is projected to have a more energetic influence on future typhoon intensities and trajectories.
Coastal flooding in Scituate (MA) : A FVCOM study of the 27 December 2010 nor'easter

(John Wiley & Sons, 2013-11-13) Beardsley, Robert C. ; Chen, Changsheng ; Xu, Qichun

A nested Finite-Volume Coastal Ocean Model (FVCOM) inundation forecast model has been developed for Scituate (MA) as part of the Northeast Coastal Ocean Forecast System (NECOFS). Scituate Harbor is a small coastal lagoon oriented north-south with a narrow entrance (with opposing breakwaters) opening eastward onto Massachusetts Bay and the Gulf of Maine. On 27 December 2010, a classic nor'easter produced a ∼0.9 m high surge, which when added to the ∼1.5 m high tide and seasonal higher mean water level, produced significant inundation in Scituate. The Scituate FVCOM inundation model includes flooding/drying, seawall/breakwater, and wave-current interaction capabilities, and was driven by one-way nesting with NECOFS. Hindcasts of the 27 December nor'easter event were made with two different resolution Scituate FVCOM grids with and without inclusion of wave-current interaction to examine the influence of spatial resolution and model dynamics on the predicted flooding. In all simulations, a wind-driven coastal current flowed southward across the harbor entrance, with an attached separation eddy forming downstream of the northern breakwater and rapid decrease in wave energy entering the harbor. With wave-current interaction, the southward coastal current was strongly enhanced and currents within the separation eddy increased to more than 1 m/s, making it highly nonlinear with large lateral shears. Comparisons of the model water elevation time series with harbor tide station measurements showed that inclusion of wave-current interaction increased the peak model surge by ∼8 cm, in closer agreement with the observed peak.
Current separation and upwelling over the southeast shelf of Vietnam in the South China Sea

(American Geophysical Union, 2012-03-21) Chen, Changsheng ; Lai, Zhigang ; Beardsley, Robert C. ; Xu, Qichun ; Lin, Huichan ; Viet, Nguyen Trung

The high-resolution, unstructured grid Finite-Volume Community Ocean Model (FVCOM) was used to examine the physical mechanisms that cause current separation and upwelling over the southeast shelf of Vietnam in the South China Sea (SCS). Process-oriented experiments suggest that the southwesterly monsoon wind is a key physical mechanism for upwelling in that area but not a prerequisite to cause current separation. With no wind forcing, current separation in summer can occur as a result of the encounter of a southward along-shelf coastal current from the north and northeastward buoyancy-driven and stratified tidal-rectified currents from the southwest. The southward current can be traced upstream to the Hong River in the Gulf of Tonkin. This current is dominated by semigeostrophic dynamics and is mostly confined to the narrow shelf along the northern Vietnamese coast. The northeastward currents are generated by tidal rectification and are intensified by the Mekong River discharge and southwesterly monsoon wind forcing. The dynamics controlling this current are fully nonlinear, with significant contributions from advection and vertical turbulent mixing. Upwelling in the current separation zone can be produced by a spatially uniform constant wind field and can be explained using simple wind-induced Ekman transport theory. This finding differs from previous theory in which the regional dipole wind stress curl is claimed as a key mechanism for current separation and upwelling in this coastal region. Our SCS FVCOM, driven by the wind stress, river discharge, and tides, is capable of reproducing the location and tongue-like offshore distribution of temperature as those seen in satellite-derived sea surface temperature imagery.
Physical mechanisms for the offshore detachment of the Changjiang Diluted Water in the East China Sea

(American Geophysical Union, 2008-02-02) Chen, Changsheng ; Xue, Pengfei ; Ding, Pingxing ; Beardsley, Robert C. ; Xu, Qichun ; Mao, Xianmou ; Gao, Guoping ; Qi, Jianhua ; Li, Chunyan ; Lin, Huichan ; Cowles, Geoffrey W. ; Shi, Maochong

Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.
Application and comparison of Kalman filters for coastal ocean problems : an experiment with FVCOM

(American Geophysical Union, 2009-05-13) Chen, Changsheng ; Malanotte-Rizzoli, Paola ; Wei, Jun ; Beardsley, Robert C. ; Lai, Zhigang ; Xue, Pengfei ; Lyu, Sangjun ; Xu, Qichun ; Qi, Jianhua ; Cowles, Geoffrey W.

Twin experiments were made to compare the reduced rank Kalman filter (RRKF), ensemble Kalman filter (EnKF), and ensemble square-root Kalman filter (EnSKF) for coastal ocean problems in three idealized regimes: a flat bottom circular shelf driven by tidal forcing at the open boundary; an linear slope continental shelf with river discharge; and a rectangular estuary with tidal flushing intertidal zones and freshwater discharge. The hydrodynamics model used in this study is the unstructured grid Finite-Volume Coastal Ocean Model (FVCOM). Comparison results show that the success of the data assimilation method depends on sampling location, assimilation methods (univariate or multivariate covariance approaches), and the nature of the dynamical system. In general, for these applications, EnKF and EnSKF work better than RRKF, especially for time-dependent cases with large perturbations. In EnKF and EnSKF, multivariate covariance approaches should be used in assimilation to avoid the appearance of unrealistic numerical oscillations. Because the coastal ocean features multiscale dynamics in time and space, a case-by-case approach should be used to determine the most effective and most reliable data assimilation method for different dynamical systems.
Impact of current-wave interaction on storm surge simulation : a case study for Hurricane Bob

(John Wiley & Sons, 2013-05-30) Sun, Yunfang ; Chen, Changsheng ; Beardsley, Robert C. ; Xu, Qichun ; Qi, Jianhua ; Lin, Huichan

Hurricane Bob moved up the U.S. east coast and crossed over southern New England and the Gulf of Maine [with peak marine winds up to 54 m/s (100 mph)] on 19–20 August 1991, causing significant damage along the coast and shelf. A 3-D fully wave-current-coupled finite-volume community ocean model system was developed and applied to simulate and examine the coastal ocean responses to Hurricane Bob. Results from process study-oriented experiments showed that the impact of wave-current interaction on surge elevation varied in space and time, more significant over the shelf than inside the inner bays. While sea level change along the coast was mainly driven by the water flux controlled by barotropic dynamics and the vertically integrated highest water transports were essentially the same for cases with and without water stratification, the hurricane-induced wave-current interaction could generate strong vertical current shear in the stratified areas, leading to a strong offshore transport near the bottom and vertical turbulent mixing over the continental shelf. Stratification could also result in a significant difference of water currents around islands where the water is not vertically well mixed.
Observed wintertime tidal and subtidal currents over the continental shelf in the northern South China Sea

(John Wiley & Sons, 2014-08-19) Li, Ruixiang ; Chen, Changsheng ; Xia, Huayong ; Beardsley, Robert C. ; Shi, Maochong ; Lai, Zhigang ; Lin, Huichan ; Feng, Yanqing ; Liu, Changjian ; Xu, Qichun ; Ding, Yang ; Zhang, Yu

Synthesis analyses were performed to examine characteristics of tidal and subtidal currents at eight mooring sites deployed over the northern South China Sea (NSCS) continental shelf in the 2006–2007 and 2009–2010 winters. Rotary spectra and harmonic analysis results showed that observed tidal currents in the NSCS were dominated by baroclinic diurnal tides with phases varying both vertically and horizontally. This feature was supported by the CC-FVCOM results, which demonstrated that the diurnal tidal flow over this shelf was characterized by baroclinic Kelvin waves with vertical phase differences varying in different flow zones. The northeasterly wind-induced southwestward flow prevailed over the NSCS shelf during winter, with episodic appearances of mesoscale eddies and a bottom-intensified buoyancy-driven slope water intrusion. The moored current records captured a warm-core anticyclonic eddy, which originated from the southwestern coast of Taiwan and propagated southwestward along the slope consistent with a combination of β-plane and topographic Rossby waves. The eddy was surface-intensified with a swirl speed of >50 cm/s and a vertical scale of ∼400 m. In absence of eddies and onshore deep slope water intrusion, the observed southwestward flow was highly coherent with the northeasterly wind stress. Observations did not support the existence of the permanent wintertime South China Sea Warm Current (SCSWC). The definition of SCSWC, which was based mainly on thermal wind calculations with assumed level of no motion at the bottom, needs to be interpreted with caution since the observed circulation over the NSCS shelf in winter included both barotropic and baroclinic components.
A finite volume numerical approach for coastal ocean circulation studies : comparisons with finite difference models

(American Geophysical Union, 2007-03-28) Chen, Changsheng ; Huang, Haosheng ; Beardsley, Robert C. ; Liu, Hedong ; Xu, Qichun ; Cowles, Geoffrey W.

An unstructured grid, finite volume, three-dimensional (3-D) primitive equation coastal ocean model (FVCOM) has been developed for the study of coastal ocean and estuarine circulation by Chen et al. (2003a). The finite volume method used in this model combines the advantage of finite element methods for geometric flexibility and finite difference methods for simple discrete computation. Currents, temperature, and salinity are computed using an integral form of the equations, which provides a better representation of the conservative laws for mass, momentum, and heat. Detailed comparisons are presented here of FVCOM simulations with analytical solutions and numerical simulations made with two popular finite difference models (the Princeton Ocean Model and Estuarine and Coastal Ocean Model (ECOM-si)) for the following idealized cases: wind-induced long-surface gravity waves in a circular lake, tidal resonance in rectangular and sector channels, freshwater discharge onto the continental shelf with curved and straight coastlines, and the thermal bottom boundary layer over the slope with steep bottom topography. With a better fit to the curvature of the coastline using unstructured nonoverlapping triangle grid cells, FVCOM provides improved numerical accuracy and correctly captures the physics of tide-, wind-, and buoyancy-induced waves and flows in the coastal ocean. This model is suitable for applications to estuaries, continental shelves, and regional basins that feature complex coastlines and bathymetry.

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