Tetsu Hara

Effect of Surface Waves on Air–Sea Momentum Exchange. Part II: Behavior of Drag Coefficient under Tropical Cyclones

Abstract Present parameterizations of air–sea momentum flux at high wind speed, including hurricane wind forcing, are based on extrapolation from field measurements in much weaker wind regimes. They predict monotonic increase of drag coefficient (Cd) with wind speed. Under hurricane wind forcing, the present numerical experiments using a coupled ocean wave and wave boundary layer model show that Cd at extreme wind speeds strongly depends on the wave field. Higher, longer, and more developed waves in the right-front quadrant of the storm produce higher sea drag; lower, shorter, and younger waves in the rear-left quadrant produce lower sea drag. Hurricane intensity, translation speed, as well as the asymmetry of wind forcing are major factors that determine the spatial distribution of Cd. At high winds above 30 m s−1, the present model predicts a significant reduction of Cd and an overall tendency to level off and even decrease with wind speed. This tendency is consistent with recent observational, experime...

A Physics-Based Parameterization of Air–Sea Momentum Flux at High Wind Speeds and Its Impact on Hurricane Intensity Predictions

Abstract A new bulk parameterization of the air–sea momentum flux at high wind speeds is proposed based on coupled wave–wind model simulations for 10 tropical cyclones that occurred in the Atlantic Ocean during 1998–2003. The new parameterization describes how the roughness length increases linearly with wind speed and the neutral drag coefficient tends to level off at high wind speeds. The proposed parameterization is then tested on real hurricanes using the operational Geophysical Fluid Dynamics Laboratory (GFDL) coupled hurricane–ocean prediction model. The impact of the new parameterization on the hurricane prediction is mainly found in increased maximum surface wind speeds, while it does not appreciably affect the hurricane central pressure prediction. This helps to improve the GFDL model–predicted wind–pressure relationship in strong hurricanes. Attempts are made to provide physical explanations as to why the reduced drag coefficient affects surface wind speeds but not the central pressure in hurric...

Numerical Simulation of Sea Surface Directional Wave Spectra under Hurricane Wind Forcing

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.

Relationship between air-sea gas transfer and short wind waves

Laboratory studies have been conducted in two circular wind wave flumes to investigate the relationship between air-sea transfer velocities of weakly soluble, nonreactive gases and wind-generated surface waves over clean water surfaces and in the presence of surface films. Detailed surface wave measurements have been made using a scanning laser slope gauge. In the circular tanks, longer gravity waves (wavenumber below 12 rad/m) are hardly affected by surfactant, while shorter waves (above 100 rad/m) are significantly reduced. With higher surfactant concentrations, waves above 200–300 rad/m may be completely eliminated. Because of the absence of narrow-banded fetch-limited gravity waves, the wave fields in the circular tanks are significantly different from those in linear wind wave flumes. At a given wind friction velocity, the transfer velocity may decrease by as much as 60% because of surface films. Regardless of the surfactant concentrations, the transfer velocity shows a reasonable correlation with the total mean square slope and with the mean square slope of shorter wind waves (wavenumber above 200 rad/m). However, it shows a poor correlation with the mean square slope of longer wind waves (wavenumber below 50 rad/m). These observations suggest that short wind waves play an important role in air-sea gas exchange.

Hydrodynamic modulation of short wind‐wave spectra by long waves and its measurement using microwave backscatter

In this paper we use results of microwave backscattering experiments over the past decade to attempt to present a coherent picture of the ocean wave-radar modulation transfer function (MTF) based on composite surface theory, short-wave modulation, and modulated wind stress. A simplified relaxation model is proposed for the modulation of the gravity-capillary wavenumber spectrum by long waves. The model is based on the relaxation rate and the equilibrium gravity-capillary wavenumber spectrum. It differs from previous models by including all airflow modulation effects in the response of the equilibrium spectrum to changes in the airflow. Thus the explicit modulation of individual source functions such as wind input, short-wave dissipation, and nonlinear interactions need not be known in order to calculate the hydrodynainic MTF. By combining this new model of the hydrodynamic MTF with microwave measurements, we attempt to determine wind shear stress modulation caused by the long waves. In order to extract the hydrodynamic MTF from the microwave data, we remove tilt and range change effects from the measured MTFs using the published analytical forms for these effects. Our results show that the inferred hydrodynamic MTF is higher for II polarization scattering than for V polarization. Since this is impossible if we have obtained the true hydrodynamic MTF, these results strongly indicate a problem with composite scattering theory as it has been traditionally applied. One explanation for this result may be the effects of intermediate-scale waves suggested by Romeiser et al. (1993). Since these effects are much stronger for H polarization than for V polarization, they may explain our observed discrepancy and, if so, imply that V polarization return should yield an acceptable upper limit for the true hydrodynamic MTF. Thus we incorporate our V polarization results into the proposed model to estimate an upper limit for the wind shear stress modulation along the long-wave profile. We infer that the primary source of modulation of Bragg resonant waves depends strongly on Bragg wavenumber and windspeed. For low values of these quantities, straining by long-wave orbital velocities dominates the modulation process, while for higher values modulated wind stress becomes increasingly important. Our calculations indicate that wind stress modulation dominates the process for 3 cm Bragg waves at moderate to high wind speeds.

The Effect of Wind-Wave-Current Interaction on Air-Sea Momentum Fluxes and Ocean Response in Tropical Cyclones

Abstract In this paper, the wind–wave–current interaction mechanisms in tropical cyclones and their effect on the surface wave and ocean responses are investigated through a set of numerical experiments. The key element of the authors’ modeling approach is the air–sea interface model, which consists of a wave boundary layer model and an air–sea momentum flux budget model. The results show that the time and spatial variations in the surface wave field, as well as the wave–current interaction, significantly reduce momentum flux into the currents in the right rear quadrant of the hurricane. The reduction of the momentum flux into the ocean consequently reduces the magnitude of the subsurface current and sea surface temperature cooling to the right of the hurricane track and the rate of upwelling/downwelling in the thermocline. During wind–wave–current interaction, the momentum flux into the ocean is mainly affected by reducing the wind speed relative to currents, whereas the wave field is mostly affected by ...

Effect of Surface Waves on Air–Sea Momentum Exchange. Part I: Effect of Mature and Growing Seas

The effect of surface waves on air‐sea momentum exchange over mature and growing seas is investigated by combining ocean wave models and a wave boundary layer model. The combined model estimates the wind stress by explicitly calculating the wave-induced stress. In the frequency range near the spectral peak, the NOAA/ NCEP surface wave model WAVEWATCH-III is used to estimate the spectra, while the spectra in the equilibrium range are determined by an analytical model. This approach allows for the estimation of the drag coefficient and the equivalent surface roughness for any surface wave fields. Numerical experiments are performed for constant winds from 10 to 45 m s21 to investigate the effect of mature and growing seas on air‐sea momentum exchange. For mature seas, the Charnock coefficient is estimated to be about 0.01; 0.02 and the drag coefficient increases as wind speed increases, both of which are within the range of previous observational data. With growing seas, results for winds less than 30 m s 21 show that the drag coefficient is larger for younger seas, which is consistent with earlier studies. For winds higher than 30 m s 21, however, results show a different trend; that is, very young waves yield less drag. This is because the wave-induced stress due to very young waves makes a small contribution to the total wind stress in very high wind conditions.

In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars

Capillary-gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary-gravity waves is in reasonable agreement with measurements by Jahne and Riemer (1990) obtained from measurements in a large wind-wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope-frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary-gravity waves. Close examination of combined wavenumber-frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary-gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra.

Wind Profile and Drag Coefficient over Mature Ocean Surface Wave Spectra

The mean wind profile and the Charnock coefficient, or drag coefficient, over mature seas are investigated. A model of the wave boundary layer, which consists of the lowest part of the atmospheric boundary layer that is influenced by surface waves, is developed based on the conservation of momentum and energy. Energy conservation is cast as a bulk constraint, integrated across the depth of the wave boundary layer, and the turbulence closure is achieved by parameterizing the dissipation rate of turbulent kinetic energy. Momentum conservation is accounted for by using the analytical model of the equilibrium surface wave spectra developed by Hara and Belcher. This approach allows analytical expressions of the Charnock coefficient to be obtained and the results to be examined in terms of key nondimensional parameters. In particular, simple expressions are obtained in the asymptotic limit at which effects of viscosity and surface tension are small and the majority of the stress is supported by wave drag. This analytical model allows us to identify the conditions necessary for the Charnock coefficient to be a true constant, an assumption routinely made in existing bulk parameterizations.

Bound waves and Bragg scattering in a wind-wave tank

We present optical and microwave measurements that show the presence of bound waves traveling at the speed of the dominant wave in a wind-wave tank. We suggest that when these bound waves are much shorter than the dominant waves, they are preferentially located on the leeward face of the dominant wave and hence have a mean tilt. We hypothesize that the turbulence associated with these bound waves suppresses freely propagating, wind-generated waves where bound waves are present so that we may divide the rough water surface into patches containing free and patches containing bound waves. This model is shown to account for the observed histograms of slope measured in the tank and, at least qualitatively, for the observed decrease in the probability of finding bound waves with increasing wind speed. Furthermore, if we add these bound, tilted waves to the free waves of the standard Bragg/composite-surface scattering model for microwave scattering from rough water surfaces, then the model can account for many otherwise unexplained features of the scattering. Principal among these features are the rapid decrease in polarization ratio and rapid increase in the first moment of the microwave Doppler spectrum with increasing wind speed when the antenna is directed upwind, features that occur to a much lesser extent when the antenna looks downwind.