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V. J. Cardone, R. E. Jensen, D. T. Resio, V. R. Swail, and A. T. Cox

Abstract

Two recent severe extratropical storms, the “Halloween storm” of October 26–November 2 1991 (HOS) and the “storm of the century” (SOC) of March 12–15 1993, are characterized by measurements of sea states of unprecedented magnitude off the east coast of North America. A Canadian buoy moored in deep water south of Nova Scotia recorded peak significant wave heights (HS) exceeding 16 m in both storms. In SOC, a NOAA buoy moored southeast of Cape Hatteras recorded a peak HS of 15.7 m, a record high for NOAA buoys. These extreme storm seas (ESS) exceed existing estimates of the 100-yr estimated design wave in these regions by about 50%. The extensive wave measurements made in both storms from buoys moored in deep water provide a rare opportunity to validate modern ocean wave models in wave regimes far more severe than those used for model tuning. In this study, four widely applied spectral wave models (OWI1G, Resio2G, WAM4, and OWI3G) are adapted to the western North Atlantic basin on fine mesh grids and are driven by common wind fields developed for each storm using careful manual kinematic reanalysis. The alternative wave hindcasts are evaluated against time series of measured HS and dominant wave period obtained at nine U.S. and Canadian buoys moored in deep water between offshore Georgia and Newfoundland. In general, it was found that despite the large differences in model formulation, the hindcasts were almost uniformly skillful in specification of the evolution of wave height and period in these two storms. The skill was much greater than achieved routinely in real time wave analyses provided by some of these same models operating at U.S., Canadian, and European centers, confirming that at least for these particular models, typically large errors in operational surface marine wind field analyses are the dominant source of errors in operational wave analyses and forecasts. However, all models were found to systematically underpredict the magnitude of the peak sea states in both storms at buoys that recorded peak HS in excess of about 12 m (ESS). This bias in ESS wave heights was 3.2 m for OWI1G, 1.9 m for Resio2G, 2.2 m for OWI3G, and 1.5 m for WAM4. These results provide an interesting assessment of the Progress made in the past decade in ocean wave modeling, both in terms of improvements of 1G and 2G models, and the introduction of 3G models. The 2G and 3G models show a slight advantage over the 1G model in simulating the most extreme wave regimes. These results suggest strongly that, for applications where supercomputers are not available, and especially for most operational applications where only integrated properties of the spectrum (e.g., HS) are required or where errors in forcing wind fields are typical of real time objective analyses and forecasts, highly developed and validated 1G and 2G wave models may continue to be used. However, accurate specification of ESS is especially critical for application of wave models to determine the extreme wave climate for ship, offshore, and coastal structure design. Therefore, further study is required to isolate the contribution of remaining wind field errors and model physics and numerics to the underprediction of ESS in extreme storms. The common phenomenological link between these two storms in the regions of ESS appears to be wave generation along a dynamic fetch associated with intense surface wind maxima or jet streaks (JS), which maintain high spatial coherency over at least 24 h and propagate at speeds of 15–20 m s−1. ESS were observed only at those buoys directly in the path of the core of such features. This finding suggests that high-resolution wave models are required to model ESS, but these are justified only if the small-scale JS phenomena can be resolved in operational analysis and forecast systems.

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G. Z. Forristall, E. G. Ward, V. J. Cardone, and L. E. Borgmann

Abstract

Knowledge of the kinematics of the flow beneath surface waves is vital for the design of offshore structures. Due to the technical of making pertinent measurement in storm conditions, knowledge of the kinematics of storm has been based almost entirely on theoretical considerations. Now measurements made with electromagnetic current meters during Tropical Storm Delia have permitted verification of the theories.

There was considerable scatter between the measured velocities and the predictions of unidirectional wave theories, with a clear bias toward overprediction. Use of higher order and irregular unidirectional theories did not substantially improve the comparison. A good fit with the data could, however, be obtained by using the concept of a directional wave spectrum based on linear wave theory.

The simultaneous wave and particle velocity measurements were used to estimate the directional spectrum through an analysis procedure which took into account the presence of a strong current. The directional spectrum was also hindcast using a numerical model and the comparison of the hindcast with data was good.

The fact that velocity spectra in confused storm seas can be accurately calculated will be directly important in some design problems. In other cases, it is necessary to know the probability distribution of the extreme events. Using the assumption of a Gaussian sea surface, it was possible to satisfactorily predict the distribution of the magnitudes of velocity. All of the comparisons lead to the conclusion that a proper description of storm wave kinematics is dependent on correctly accounting for the directional spreading of the wave energy.

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V. Cardone, H. Carlson, J. A. Ewing, K. Hasselmann, S. Lazanoff, W. McLeish, and D. Ross

Abstract

The surface wave environment in the GATE B/C scale is described from wave measurements made from buoys and aircraft during Phase III (September 1974). Particular emphasis is given to the wave measurements made from the pitch-roll buoy deployed in the B-scale array from the ship Gilliss and a similar buoy deployed in the C-scale array from Quadra. Reduction of the pitch-roll buoy measurements provided estimates of the one-dimensional wave spectrum as well as of the mean direction and spread of wave energy as a function of frequency. The data clearly revealed the importance of external forcing on the wave climate in GATE. Most of the wave energy present in the GATE areas was found to be swell imported from the trade wind circulations of both hemispheres and from an intense extratropical cyclone which crossed the North Atlantic at high latitudes early in Phase III. Locally generated waves were clearly evident in the wave spectra, but their energy level way have been modulated significantly by the low-frequency swell. The GATE wave data set can provide a powerful test of contemporary numerical wave-prediction models. The present study defines the, attributes which are required of such models for meaningful application to the GATE needs.

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Y. Kushnir, V. J. Cardone, J. G. Greenwood, and M. A. Cane

Abstract

The nature and causes of the recent increase in North Atlantic wave heights are explored by combining a numerical hindcast with a statistical analysis. The numerical hindcast incorporates a 10-yr history (1980–89) of North Atlantic, twice daily wind analyses to generate a monthly averaged significant wave height (SWH) history. The hindcast compares favorably with published monthly averaged SWH observations. The link between model-generated wintertime monthly SWH and monthly averaged sea level pressure (SLP) data is determined by means of a canonical correlation analysis (CCA). Within the analysis domain, most of the variance in SWH and SLP is captured by two pairs of joint patterns. The leading pair consists of a SLP dipole resembling the North Atlantic Oscillation (NAO) and a SWH dipole in spatial quadrature relation to it. Using the CCA results, an extended statistical hindcast of monthly wave fields is generated from sea level pressure data and used to quantitatively estimate the systematic increase in wave heights since the 1960s. It is shown that an increasing trend in SWH at several northeast Atlantic locations since 1960 or so is related to the systematic deepening of the Icelandic low and intensification of the Azores high over the last three decades. The analysis suggests that wave height south of 40°N has decreased during the same period.

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J. C. Dietrich, J. J. Westerink, A. B. Kennedy, J. M. Smith, R. E. Jensen, M. Zijlema, L. H. Holthuijsen, C. Dawson, R. A. Luettich Jr., M. D. Powell, V. J. Cardone, A. T. Cox, G. W. Stone, H. Pourtaheri, M. E. Hope, S. Tanaka, L. G. Westerink, H. J. Westerink, and Z. Cobell

Abstract

Hurricane Gustav (2008) made landfall in southern Louisiana on 1 September 2008 with its eye never closer than 75 km to New Orleans, but its waves and storm surge threatened to flood the city. Easterly tropical-storm-strength winds impacted the region east of the Mississippi River for 12–15 h, allowing for early surge to develop up to 3.5 m there and enter the river and the city’s navigation canals. During landfall, winds shifted from easterly to southerly, resulting in late surge development and propagation over more than 70 km of marshes on the river’s west bank, over more than 40 km of Caernarvon marsh on the east bank, and into Lake Pontchartrain to the north. Wind waves with estimated significant heights of 15 m developed in the deep Gulf of Mexico but were reduced in size once they reached the continental shelf. The barrier islands further dissipated the waves, and locally generated seas existed behind these effective breaking zones.

The hardening and innovative deployment of gauges since Hurricane Katrina (2005) resulted in a wealth of measured data for Gustav. A total of 39 wind wave time histories, 362 water level time histories, and 82 high water marks were available to describe the event. Computational models—including a structured-mesh deepwater wave model (WAM) and a nearshore steady-state wave (STWAVE) model, as well as an unstructured-mesh “simulating waves nearshore” (SWAN) wave model and an advanced circulation (ADCIRC) model—resolve the region with unprecedented levels of detail, with an unstructured mesh spacing of 100–200 m in the wave-breaking zones and 20–50 m in the small-scale channels. Data-assimilated winds were applied using NOAA’s Hurricane Research Division Wind Analysis System (H*Wind) and Interactive Objective Kinematic Analysis (IOKA) procedures. Wave and surge computations from these models are validated comprehensively at the measurement locations ranging from the deep Gulf of Mexico and along the coast to the rivers and floodplains of southern Louisiana and are described and quantified within the context of the evolution of the storm.

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J. C. Dietrich, S. Bunya, J. J. Westerink, B. A. Ebersole, J. M. Smith, J. H. Atkinson, R. Jensen, D. T. Resio, R. A. Luettich, C. Dawson, V. J. Cardone, A. T. Cox, M. D. Powell, H. J. Westerink, and H. J. Roberts

Abstract

Hurricanes Katrina and Rita were powerful storms that impacted southern Louisiana and Mississippi during the 2005 hurricane season. In , the authors describe and validate a high-resolution coupled riverine flow, tide, wind, wave, and storm surge model for this region. Herein, the model is used to examine the evolution of these hurricanes in more detail. Synoptic histories show how storm tracks, winds, and waves interacted with the topography, the protruding Mississippi River delta, east–west shorelines, manmade structures, and low-lying marshes to develop and propagate storm surge. Perturbations of the model, in which the waves are not included, show the proportional importance of the wave radiation stress gradient induced setup.

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S. Bunya, J. C. Dietrich, J. J. Westerink, B. A. Ebersole, J. M. Smith, J. H. Atkinson, R. Jensen, D. T. Resio, R. A. Luettich, C. Dawson, V. J. Cardone, A. T. Cox, M. D. Powell, H. J. Westerink, and H. J. Roberts

Abstract

A coupled system of wind, wind wave, and coastal circulation models has been implemented for southern Louisiana and Mississippi to simulate riverine flows, tides, wind waves, and hurricane storm surge in the region. The system combines the NOAA Hurricane Research Division Wind Analysis System (H*WIND) and the Interactive Objective Kinematic Analysis (IOKA) kinematic wind analyses, the Wave Model (WAM) offshore and Steady-State Irregular Wave (STWAVE) nearshore wind wave models, and the Advanced Circulation (ADCIRC) basin to channel-scale unstructured grid circulation model. The system emphasizes a high-resolution (down to 50 m) representation of the geometry, bathymetry, and topography; nonlinear coupling of all processes including wind wave radiation stress-induced set up; and objective specification of frictional parameters based on land-cover databases and commonly used parameters. Riverine flows and tides are validated for no storm conditions, while winds, wind waves, hydrographs, and high water marks are validated for Hurricanes Katrina and Rita.

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