Xue Pengfei

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Xue
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Pengfei
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  • Article
    Saltwater intrusion into the Changjiang River : a model-guided mechanism study
    (American Geophysical Union, 2009-02-12) Xue, Pengfei ; Chen, Changsheng ; Ding, Pingxing ; Beardsley, Robert C. ; Lin, Huichan ; Ge, Jianzhong ; Kong, Yazhen
    The Changjiang River (CR) is divided into a southern branch (SB) and a northern branche (NB) by Chongming Island as the river enters the East China Sea. Observations reveal that during the dry season the saltwater in the inner shelf of the East China Sea flows into the CR through the NB and forms an isolated mass of saltwater in the upstream area of the SB. The physical mechanism causing this saltwater intrusion has been studied using the high-resolution unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM). The results suggest that the intrusion is caused by a complex nonlinear interaction process in relation to the freshwater flux upstream, tidal currents, mixing, wind, and the salt distribution in the inner shelf of the East China Sea. The tidal rectification, resulting from the interaction of the convergence or divergence of tidal momentum flux and bottom friction over abrupt topography, produces a net upstreamward volume flux from NB to SB. With river discharge the net water transport in the NB is driven through a momentum balance of surface elevation gradient forcing, horizontal advection, and vertical diffusion. In the dry season, reducing the surface elevation gradient forcing makes tidal rectification a key process favorable for the saltwater intrusion. A northerly wind tends to enhance the saltwater intrusion by reducing the seaward surface elevation gradient forcing rather than either the baroclinic pressure gradient forcing or the wind-driven Ekman transport. A convergence experiment suggests that high grid resolution (∼100 m or less) is required to correctly resolve the net water transport through the NB, particularly in the narrow channel on the northern coast of Chongming Island.
  • Article
    Observing system simulation experiments of dissolved oxygen monitoring in Massachusetts Bay
    (American Geophysical Union, 2012-05-08) Xue, Pengfei ; Chen, Changsheng ; Beardsley, Robert C.
    Observing system simulation experiments (OSSEs) were performed in Massachusetts Bay for the design of optimal monitoring sites for dissolved oxygen (DO) measurements. Experiments were carried out using the Ensemble Kalman Filter (EnKF) for data assimilation with focus on initial and boundary perturbations. Running a well-validated water quality model with a perturbed initial field of DO but “true” boundary forcing conditions, the model is capable of restoring DO back to the true state without data assimilation over a recovery time scale of about a month. Since DO in Massachusetts Bay has a bay-wide correlation scale, placing a monitoring site of DO near the northern boundary or at a location that has maximum correlation to the entire domain can shorten the restoring time to a week. Running the model with perturbed boundary forcing without data assimilation, the results show that the errors propagate into Massachusetts Bay following the inflow from the northern boundary and spread southward to Cape Cod Bay over a time scale of about a month. Using a DO monitoring site located near the northern entrance, the data assimilation can efficiently control the error propagation and prevent the model field from deviating from the true state. The model shows that the inflow from the northern entrance, which is connected to the upstream Western Maine Coastal Current, plays an important role in controlling the DO variation in Massachusetts Bay, and the residence time of the bay controlled by this flow is about one month. Understanding the upstream boundary-control nature of this system is critical for optimal design of sampling strategies of water quality variables in this region.
  • Article
    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.
  • Article
    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.
  • Article
    Observing system simulation experiments with ensemble Kalman filters in Nantucket Sound, Massachusetts
    (American Geophysical Union, 2011-01-20) Xue, Pengfei ; Chen, Changsheng ; Beardsley, Robert C. ; Limeburner, Richard
    Observing system simulation experiments (OSSEs) were performed for Nantucket Sound, Massachusetts, as a pilot study for the design of optimal monitoring networks in the coastal ocean. Experiments were carried out using the ensemble Kalman filter (EnKF) for data assimilation with ensemble transform Kalman filter (EnTKF) and proper orthogonal decomposition (POD) for selecting the optimal monitoring sites. The singular evolutive interpolated Kalman filter (SEIK) was compared with EnKF for the data assimilation efficiency. Running the unstructured grid Finite-Volume Community Ocean Model (FVCOM) with perturbed initial fields of currents, water temperature, and salinity show that in this shallow coastal system, the velocity and surface elevation are able to restore themselves back to the true state over an inertial time scale after perturbation without data assimilation, while the water temperature and salinity are not. This suggests that in this vertically well mixed region with strong tidal influence, monitoring should be targeted at water properties rather than velocities. By placing measurement sites at an entrance or exit or a location with the maximum signal variance (EnTKF) or at extrema of the dominant EOF spatial modes (POD), we evaluated the capability of EnTKF and POD in designing the optimal monitoring site for the forecast model system in this region. The results suggest that understanding the multiscale dynamical nature of the system is essential in designing an optimal monitoring network since “optimal” sites suggested by an assimilation method may only represent a local-scale feature that has little influence on a region-wide system. Comparing EnKF and SEIK simulations shows that SEIK can significantly improve the data assimilation efficiency by reducing the ensemble number and increasing the convergence rate.