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Il-Ju Moon
,
Isaac Ginis
,
Tetsu Hara
,
Hendrik L. Tolman
,
C. W. Wright
, and
Edward J. Walsh

Abstract

Numerical simulation of sea surface directional wave spectra under hurricane wind forcing was carried out using a high-resolution wave model. The simulation was run for four days as Hurricane Bonnie (1998) approached the U.S. East Coast. The results are compared with buoy observations and NASA Scanning Radar Altimeter (SRA) data, which were obtained on 24 August 1998 in the open ocean and on 26 August when the storm was approaching the shore. The simulated significant wave height in the open ocean reached 14 m, agreeing well with the SRA and buoy observations. It gradually decreased as the hurricane approached the shore. In the open ocean, the dominant wavelength and wave direction in all four quadrants relative to the storm center were simulated very accurately. For the landfall case, however, the simulated dominant wavelength displays noticeable overestimation because the wave model cannot properly simulate shoaling processes. Direct comparison of the model and SRA directional spectra in all four quadrants of the hurricane shows excellent agreement in general. In some cases, the model produces smoother spectra with narrower directional spreading than do the observations. The spatial characteristics of the spectra depend on the relative position from the hurricane center, the hurricane translation speed, and bathymetry. Attempts are made to provide simple explanations for the misalignment between local wind and wave directions and for the effect of hurricane translation speed on wave spectra.

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Wei Chen
,
Michael L. Banner
,
Edward J. Walsh
,
Jorgen B. Jensen
, and
Sunhee Lee

Abstract

The Southern Ocean Waves Experiment (SOWEX) was an international collaborative air–sea interaction experiment in which a specially instrumented meteorological research aircraft simultaneously gathered marine boundary-layer atmospheric turbulence data and sea surface roughness data over the Southern Ocean, particularly for gale-force wind conditions. In this paper analysis and findings are presented on key aspects of the coupled variability of the wind field, the wind stress, and the underlying sea surface roughness. This study complements the overview, methodology, and mean results published in Part I.

Weakly unstable atmospheric stratification conditions prevailed during SOWEX, with wind speeds ranging from gale force to light and variable. Throughout the SOWEX observational period, the wind field was dominated by large-scale atmospheric roll-cell structures, whose height scale was comparable with the thickness of the marine atmospheric boundary layer (MABL). Well above the sea surface, these coherent structures provide the dominant contribution to the downward momentum flux toward the sea surface. Closer to the sea surface, these organized large-scale structures continued to make significant contributions to the downward momentum flux, even within a few tens of meters of the sea surface.

At the minimum aircraft height, typical cumulative stress cospectra indicated that 10-km averages along crosswind tracks appeared adequate to close the stress cospectrum. Nevertheless, a large-scale spatial inhomogeneity in the wind stress vector was observed using 10- and 20-km spatial averaging intervals on one of the strongest wind days when the mean wind field was close to being spatially uniform. This indicates a departure from the familiar drag coefficient relationship and implies large-scale transverse modulations in the MABL with an effective horizontal to vertical aspect ratio of around 20.

A high visual correlation was found between mean wind speed variations and collocated sea-surface mean square slope (mss) variations, averaged over 1.9 km. A comparable plot of the 10-km running average of the downward momentum flux, observed at heights from 30 to 90 m, showed appreciably lower visual correlation with the wind speed variations and mss variations. The 10–20 km averaging distance needed to determine the wind stress was larger than the local scale of variation of the mss roughness variations. It also exceeded the scale of the striations often observed in synthetic aperture radar imagery under unstable atmospheric conditions and strong wind forcing. This highlights an overlooked intrinsic difficulty in using the friction velocity as the wind parameter in models of the wind wave spectrum, especially for the short wind wave scales.

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Il-Ju Moon
,
Isaac Ginis
,
Tetsu Hara
,
Hendrik L. Tolman
,
C. W. Wright
, and
Edward J. Walsh

Abstract

Numerical simulation of sea surface directional wave spectra under hurricane wind forcing was carried out using a high-resolution wave model. The simulation was run for four days as Hurricane Bonnie (1998) approached the U.S. East Coast. The results are compared with buoy observations and NASA Scanning Radar Altimeter (SRA) data, which were obtained on 24 August 1998 in the open ocean and on 26 August when the storm was approaching the shore. The simulated significant wave height in the open ocean reached 14 m, agreeing well with the SRA and buoy observations. It gradually decreased as the hurricane approached the shore. In the open ocean, the dominant wavelength and wave direction in all four quadrants relative to the storm center were simulated very accurately. For the landfall case, however, the simulated dominant wavelength displays noticeable overestimation because the wave model cannot properly simulate shoaling processes. Direct comparison of the model and SRA directional spectra in all four quadrants of the hurricane shows excellent agreement in general. In some cases, the model produces smoother spectra with narrower directional spreading than do the observations. The spatial characteristics of the spectra depend on the relative position from the hurricane center, the hurricane translation speed, and bathymetry. Attempts are made to provide simple explanations for the misalignment between local wind and wave directions and for the effect of hurricane translation speed on wave spectra.

Full access
Shuyi S. Chen
,
James F. Price
,
Wei Zhao
,
Mark A. Donelan
, and
Edward J. Walsh
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Yalin Fan
,
Isaac Ginis
,
Tetsu Hara
,
C. Wayne Wright
, and
Edward J. Walsh

Abstract

The performance of the wave model WAVEWATCH III under a very strong, category 5, tropical cyclone wind forcing is investigated with different drag coefficient parameterizations and ocean current inputs. The model results are compared with field observations of the surface wave spectra from an airborne scanning radar altimeter, National Data Buoy Center (NDBC) time series, and satellite altimeter measurements in Hurricane Ivan (2004). The results suggest that the model with the original drag coefficient parameterization tends to overestimate the significant wave height and the dominant wavelength and produces a wave spectrum with narrower directional spreading. When an improved drag parameterization is introduced and the wave–current interaction is included, the model yields an improved forecast of significant wave height, but underestimates the dominant wavelength. When the hurricane moves over a preexisting mesoscale ocean feature, such as the Loop Current in the Gulf of Mexico or a warm- and cold-core ring, the current associated with the feature can accelerate or decelerate the wave propagation and significantly modulate the wave spectrum.

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Paul A. Hwang
,
David W. Wang
,
Edward J. Walsh
,
William B. Krabill
, and
Robert N. Swift

Abstract

An airborne scanning lidar system acquires three-dimensional (3D) spatial topography of ocean surface waves. From the spatial data, wavenumber spectra are computed directly. The spectral properties in terms of the spectral slope and dimensionless spectral coefficient have been verified to be in very good agreement with existing data. One of the unique features of the 3D spatial data is its exceptional directional resolution. Directional properties such as the wavenumber dependence of the directional spreading function and the evolution of bimodal development are investigated with these high-resolution, phase-resolving spatial measurements. Equations for the spreading parameters, the lobe angle, and the lobe ratio are established from the airborne scanning lidar datasets. Fourier decomposition of the measured directional distribution is presented. The directional parameters can be represented by a small number (4) of the Fourier components. The measured directional distributions are compared with numerical experiments of nonlinear wave simulations to explore the functional form of the dissipation source term.

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Edward J. Walsh
,
David W. Hancock III
,
Donald E. Hines
,
Robert N. Swift
, and
John F. Scott

Abstract

The Surface Contour Radar (SCR) is a 36-GHz computer-controlled airborne system, which produces ocean directional wave spectra with much higher angular resolution than pitch-and-roll buoys. SCR observations of the evolution of the fetch-limited directional wave spectrum are presented which indicate the existence of a fully-developed sea state. The JONSWAP wave growth model for wave energy and frequency was in best agreement with the SCR measurements. The model of Donelan et al. correctly predicted the propagation direction of waves in the asymmetrical fetch situation nearshore. The Donelan et al. parameterization is generalized to permit other growth algorithms to predict the correct direction of propagation in asymmetrical fetch situations.

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Paul A. Hwang
,
David W. Wang
,
Edward J. Walsh
,
William B. Krabill
, and
Robert N. Swift

Abstract

An airborne scanning lidar system acquires 3D spatial topography of ocean surface waves. From the spatial data, wavenumber spectra are computed directly. The spectral analyses of two distinctively different wave fields are presented. The first one is a quasi-steady wave field under active wind generation, and the second one is a decaying wave field following a slackening of the wind field. Subtle differences in different representations of the one-dimensional spectrum (omnidirectional, marginal, and traverse) are illustrated. The spectral properties in terms of the dimensionless spectral coefficient and spectral slope in the equilibrium range are investigated using the wavenumber spectra directly computed from the 3D topography of the ocean surface. The results are in excellent agreement with existing data. The rapid data acquisition afforded by an airborne system provides an enhanced capability for studying the spatial variation of a wave field with minimal temporal changes in the environmental forcing conditions. The data of the 3D surface topography are also ideal for the quantitative investigation of the directional properties of a random wave field.

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Michael L. Banner
,
Wei Chen
,
Edward J. Walsh
,
Jorgen B. Jensen
,
Sunhee Lee
, and
Chris Fandry

Abstract

The Southern Ocean Waves Experiment (SOWEX) was an international collaborative air–sea interaction experiment in which a specially instrumented meteorological research aircraft simultaneously gathered atmospheric turbulence data in the marine boundary layer and sea surface topography data over the Southern Ocean for a wide range of wind speeds. The aim was to increase present knowledge of severe sea state air–sea interaction. This first paper presents an overview of the experiment and a detailed discussion of the methodology and mean results. A companion paper describes the findings on variability of the wind speed and wind stress and their relationship to variations in the underlying sea surface roughness.

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Peter G. Black
,
Eric A. D'Asaro
,
William M. Drennan
,
Jeffrey R. French
,
Pearn P. Niiler
,
Thomas B. Sanford
,
Eric J. Terrill
,
Edward J. Walsh
, and
Jun A. Zhang

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (C D ) values derived from CBLAST momentum flux measurements show C D becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements.

The Dalton number estimates (C E ) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a C E /C D ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery.

As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

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