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George L. Mellor
and
Alan F. Blumberg

Abstract

The use of diffusive terms in numerical ocean models is examined relative to different coordinate systems. The conventional model for horizontal diffusion is found to be incorrect when bottom topographical slopes are large. A new formulation is suggested which is simpler than the conventional formulation when transformed to a sigma coordinate system and makes it possible to model realistically both surface Ekman and bottom boundary layers.

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Julie Pullen
,
Teddy Holt
,
Alan F. Blumberg
, and
Robert D. Bornstein

Abstract

Multiply nested urbanized mesoscale model [Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)] simulations of the New York–New Jersey metropolitan region are conducted for 4–11 July 2004. The simulations differ only in their specification of sea surface temperatures (SSTs) on nest 4 (1.33-km resolution) and nest 5 (0.44-km resolution). The “control SST” simulation (CONTROL-SST) uses an analyzed SST product, whereas the “New York Harbor Observing and Prediction System (NYHOPS) SST” simulation (NYHOPS-SST) uses hourly SSTs from the NYHOPS model hindcast. Upwelling-favorable (southerly) winds preceding the simulation time period and continuing for much of the first 5 days of the simulation generate cold water adjacent to the New Jersey coast and a cold eddy immediately outside of the harbor in the New York Bight. Both features are prominent in NYHOPS-SST but are not pronounced in CONTROL-SST. The upwelled water has a discernible influence on the overlying atmosphere by cooling near-surface air temperatures by approximately 1°–2°C, slowing the near-surface winds by 15%–20%, and reducing the nocturnal urban heat island effect by up to 1.3°C. At two coastal land-based sites and one overwater station, the wind speed mean bias is systematically reduced in NYHOPS-SST. During a wind shift to northwesterly on day 6 (9 July 2004) the comparatively cooler NYHOPS-SSTs impact the atmosphere over an even broader offshore area than was affected in the mean during the previous 5 days. Hence, air temperature evolution measured at the overwater site is better reproduced in NYHOPS-SST. Interaction of the offshore flow with the cool SSTs in NYHOPS-SST induces internal boundary layer (IBL) formation, sustained and deepened by turbulent kinetic energy advected from adjacent land areas; IBL formation did not occur in CONTROL-SST.

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Lakshmi H. Kantha
,
George L. Mellor
, and
Alan F. Blumberg

Abstract

A diagnostic calculation that involves integration of the geostrophic equations for total transport along contours of constant planetary potential vorticity (f/H) is described and applied to the South Atlantic Bight. The total transport in the entire region is determined by specifying transport or bottom velocity on one transect intersecting the f/H contours. A new method of computing the joint effect of baroclinicity and bottom topography (JeBar) in the vorticity equation permits the application of the model to oceanic regions with large bottom-topographic variations. Local wind-stress curl and bottom frictional torque have been ignored in the current version of the model; their effect is estimated to be small seaward of the shelf break. JeBar terms are the dominant factor in the vorticity balance. The results indicate realistic climatological Gulf Stream behavior in the Bight and are not overly sensitive to the conditions prescribed on the southern transect cast of the Bahamas. The southward flow from the, Middle Atlantic Bight is a substantial contribution to the Gulf Stream transport off Cape Hatteras.

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Leiv H. Slørdal
,
Eivind A. Martinsen
, and
Alan F. Blumberg

Abstract

To validate a three-dimensional hydrodynamic model for use in coastal waters, two test cases with idealized geometry and forcing functions were performed. The tests involve the barotropic and baroclinic response of a coastal ocean with a uniform alongshore shelf to the passage of a storm and the circulation induced by flow over a topographic feature. The model used in this study is the estuarine, coastal, and ocean model developed by Blumberg and Mellor. The model is three dimensional and solves for three components of the current field, temperature, and salinity. The model has a terrain-following sigma (σ) coordinate system, a coastal-following curvilinear grid in the horizontal, and an embedded second-order turbulence closure submodel to provide vertical mixing coefficients and uses the free surface as a prognostic variable. At the open boundaries, a flow relaxation scheme (FRS) has been implemented to pass out internally generated disturbances with minimum reflection. The results from the first test case demonstrate that the model successfully reproduces the expected theoretical response to a traveling storm. Both standing and propagating shelf waves with the proper spatial structure are found. In the upper part of the ocean, wind-generated oscillations are the dominant response. In the second test case, a stationary anticyclone forms over the topographic feature and a cyclonic eddy is shed downstream in a time of the order of the advective timescale, in agreement with theory and previous studies. When investigating the long-term evolution, the model simulations reproduce the baroclinic instability mechanisms expected from analytical considerations.

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Amin Salighehdar
,
Ziwen Ye
,
Mingzhe Liu
,
Ionut Florescu
, and
Alan F. Blumberg

Abstract

Accurate prediction of storm surge is a difficult problem. Most forecast systems produce multiple possible forecasts depending on the variability in weather conditions, possible temperature levels, winds, etc. Ensemble modeling techniques have been developed with the stated purpose of obtaining the best forecast (in some specific sense) from the individual forecasts. In this work a statistical methodology of evaluating the performance of multiple ensemble forecasting models is developed. The methodology is applied to predicting storm surge in the New York Harbor area. Data from three hurricane events collected from multiple locations in the New York Bay area are used. The methodology produces three key findings for the particular test data used. First, it is found that even the simplest possible way of creating an ensemble produces results superior to those of any single forecast. Second, for the data used and the events under study the methodology did not interact with any event at any location studied. Third, based on the methodology results for the data studied selecting the best-performing ensemble models for each specific location may be possible.

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Reza Marsooli
,
Philip M. Orton
,
George Mellor
,
Nickitas Georgas
, and
Alan F. Blumberg

Abstract

The Stevens Institute of Technology Estuarine and Coastal Ocean Model (sECOM) is coupled here with the Mellor–Donelan–Oey (MDO) wave model to simulate coastal flooding due to storm tides and waves. sECOM is the three-dimensional (3D) circulation model used in the New York Harbor Observing and Prediction System (NYHOPS). The MDO wave model is a computationally cost-effective spectral wave model suitable for coupling with 3D circulation models. The coupled sECOM–MDO model takes into account wave–current interactions through wave-enhanced water surface roughness and wind stress, wave–current bottom stress, and depth-dependent wave radiation stress. The model results are compared with existing laboratory measurements and the field data collected in New York–New Jersey (NY–NJ) harbor during Hurricane Sandy. Comparisons between the model results and laboratory measurements demonstrate the capabilities of the model to accurately simulate wave characteristics, wave-induced water elevation, and undertow current. The model results for Hurricane Sandy reveal the successful performance of sECOM–MDO in situations where high waves and storm tides coexist. The results indicate that the temporal maximum wave setup in NY–NJ harbor was 0.26 m. On the other hand, the contribution of wave setup to the peak storm tide was 0.13 m, a contribution of only 3.8%. It is found that the inclusion of wave radiation stress and wave-enhanced bottom friction in the circulation model can reduce the errors in the calculated storm tides. At the Battery (New York), for example, the root-mean-square error reduced from 0.17 to 0.12 m.

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Alan F. Blumberg
,
Nickitas Georgas
,
Larry Yin
,
Thomas O. Herrington
, and
Philip M. Orton

Abstract

A new, high-resolution, hydrodynamic model that encompasses the urban coastal waters of New Jersey along the Hudson River Waterfront opposite New York City, New York, has been developed and validated for simulating inundation during Hurricane Sandy. A 3.1-m-resolution square model grid combined with a high-resolution lidar elevation dataset permits a street-by-street focus to inundation modeling. The waterfront inundation model is a triple-nested Stevens Institute Estuarine and Coastal Ocean Hydrodynamic Model (sECOM) application; sECOM is a successor model to the Princeton Ocean Model family of models. Robust flooding and drying of land in the model physics provides for the dynamic prediction of flood elevations and velocities across land features during inundation events. The inundation model was forced by water levels from the extensively validated New York Harbor Observing and Prediction System (NYHOPS) hindcast of that hurricane.

Validation against 56 watermarks and 16 edgemarks provided via the USGS and through an extensive crowdsourcing effort consisting of photographs, videos, and personal stories shows that the model is capable of computing overland water elevations quite accurately throughout the entire surge event. The correlation coefficient (R 2) between the watermark observations and the model results is 0.92. The standard deviation of the residual error is 0.07 m. Comparisons to the 16 flood edgemarks suggest that the model was able to reproduce flood extent to within 20 m. Because the model was able to capture the spatial and temporal variation of water levels in the region observed during Hurricane Sandy, it was used to identify the flood pathways and suggest where flood-preventing interventions could be built.

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Igor Shulman
,
James K. Lewis
,
Alan F. Blumberg
, and
B. Nicholas Kim

Abstract

An optimization approach is derived for assimilating tidal height information along the open boundaries of a numerical model. The approach is then extended so that similar data along transects inside a model domain can also be optimally assimilated. To test the application of such an optimized methodology, M 2 tidal simulations were conducted with a numerical ocean model of the Yellow Sea, an area with a strong tidal influence. The use of the optimized open boundary conditions and internal data assimilation leads to a significant improvement of the predictive skill of the model. Average errors can be reduced by up to 75% when compared to nonoptimized boundary conditions.

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Tom Di Liberto
,
Brian A. Colle
,
Nickitas Georgas
,
Alan F. Blumberg
, and
Arthur A. Taylor

Abstract

Three real-time storm surge forecasting systems [the eight-member Stony Brook ensemble (SBSS), the Stevens Institute of Technology’s New York Harbor Observing and Prediction System (SIT-NYHOPS), and the NOAA Extratropical Storm Surge (NOAA-ET) model] are verified for 74 available days during the 2007–08 and 2008–09 cool seasons for five stations around the New York City–Long Island region. For the raw storm surge forecasts, the SIT-NYHOPS model has the lowest root-mean-square errors (RMSEs) on average, while the NOAA-ET has the largest RMSEs after hour 24 as a result of a relatively large negative surge bias. The SIT-NYHOPS and SBSS also have a slight negative surge bias after hour 24. Many of the underpredicted surges in the SBSS ensemble are associated with large waves at an offshore buoy, thus illustrating the potential importance of nearshore wave breaking (radiation stresses) on the surge predictions. A bias correction using the last 5 days of predictions (BC) removes most of the surge bias in the NOAA-ET model, with the NOAA-ET-BC having a similar level of accuracy as the SIT-NYHOPS-BC for positive surges. A multimodel surge ensemble (ENS-3) comprising the SBSS control member, SIT-NYHOPS, and NOAA-ET models has a better degree of deterministic accuracy than any individual member. Probabilistically, the ALL ensemble (eight SBSS members, SIT-NYHOPS, and NOAA-ET) is underdispersed and does not improve after applying a bias correction. The ENS-3 improves the Brier skill score (BSS) relative to the best deterministic member (SIT-NYHOPS), and the ENS-3 has a larger BSS and better reliability than the SBSS and ALL ensembles, thus illustrating the benefits of a multimodel storm surge ensemble.

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Talmor Meir
,
Julie Pullen
,
Alan F. Blumberg
,
Teddy R. Holt
,
Paul E. Bieringer
, and
George Bieberbach Jr.

Abstract

Results are presented from a tracer-release modeling study designed to examine atmospheric transport and dispersion (“T&D”) behavior surrounding the complex coastal–urban region of New York City, New York, where air–sea interaction and urban influences are prominent. The puff-based Hazard Prediction Assessment Capability (HPAC, version 5) model is run for idealized conditions, and it is also linked with the urbanized COAMPS (1 km) meteorological model and the NAM (12 km) meteorological model. Results are compared with “control” plumes utilizing surface meteorological input from 22 weather stations. In all configurations, nighttime conditions result in plume predictions that are more sensitive to small changes in wind direction. Plume overlap is reduced by up to 70% when plumes are transported during the night. An analysis of vertical plume cross sections and the nature of the underlying transport and the dispersion equations both suggest that heat flux gradients and boundary layer height gradients determine vertical transport of pollutants across land–sea boundaries in the T&D model. As a consequence, in both idealized and realistic meteorological configurations, waterfront releases generate greater plume discrepancies relative to plumes transported over land/urban surfaces. For transport over water (northwest winds), the higher-fidelity meteorological model (COAMPS) generated plumes with overlap reduced by about one-half when compared with that of the coarser-resolution NAM model (13% vs 24% during the daytime and 11% vs 18% during the nighttime). This study highlights the need for more sophisticated treatment of land–sea transition zones in T&D calculations covering waterside releases.

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