Showing papers on "Swell published in 1996"

Bar/trough generation on a natural beach

Abstract: Mechanisms for bar/trough generation are examined using velocities measured in the field applied to the Bowen [1980]/Bailard [1981] energetics-based sediment transport model. Measurements consist of a cross-shore array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a 10-day period during which two storms occurred, when the bathymetry evolved from a three-dimensional terrace to a well-developed linear bar. The model predicts bed and suspended load transport separately based on various velocity moments. The velocities are partitioned into mean currents, low-frequency infragravity and shear instabilities ( 0.05 Hz) to determine the relative importance of various mechanisms to the total transport. Velocity moments are computed over 90-min intervals to resolve tidal fluctuations. Tidal signatures were apparent in all modes of transport. Predicted transport rates are integrated and compared with daily cross-shore bathymetric profiles (averaged over a 400-m length of beach). The suspended load terms were an order of magnitude greater than bed load terms owing to the low fall velocity of the fine-grain sand within the surf zone. Model results for this experiment indicate the dominant mechanism for bar development was sediments mobilized by the strong longshore current and incident short waves within the surf zone and transported offshore by the mean undertow and shoreward transport onshore due to short wave velocity skewness. Using standard coefficients, the model correctly predicted the first-order movement of the bar during storms, but underpredicted trough development, and did not always perform well during mild wave conditions.

Wave transformation across the inner surf zone

Abstract: Sea and swell wave heights observed on transects crossing the mid and inner surf zone on three beaches (a steep concave-up beach, a gently sloped approximately planar beach, and a beach with an approximately flat terrace adjacent to a steep foreshore) were depth limited (i.e., approximately independent of the offshore wave height), consistent with previous observations. The wave evolution is well predicted by a numerical model based on the one-dimensional nonlinear shallow water equations with bore dissipation. The model is initialized with the time series of sea surface elevation and cross-shore current observed at the most offshore sensors (located about 50 to 120 m from the mean shoreline in mean water depths 0.80 to 2.10 m). The model accurately predicts the cross-shore variation of energy at both infragravity (nominally 0.004 < f < 0.05 Hz) and sea swell (here 0.05 < f ≤ 0.18 Hz) frequencies. In models of surf zone hydrodynamics, wave energy dissipation is frequently parameterized in terms of γs, the ratio of the sea swell significant wave height to the local mean water depth. The observed and predicted values of γs increase with increasing beach slope β and decreasing normalized (by a characteristic wavenumber k) water depth kh and are well correlated with β/kh, a measure of the fractional change in water depth over a wavelength. Errors in the predicted individual values of γs, are typically less than 20%. It has been suggested that infragravity motions affect waves in the sea swell band and hence γs, but this speculation is difficult to test with field observations. Numerical simulations suggest that for the range of conditions considered here, γs is insensitive to infragravity energy levels.

Observations and predictions of run‐up

Abstract: For a significant range of offshore wave conditions and foreshore slopes, run-up observations are compared to semiempirical formulations and predictions of an existing numerical model based on the depth-averaged one-dimensional nonlinear shallow water equations with bore-like breaking wave dissipation and quadratic bottom friction. The numerical model is initialized with time series of sea surface elevation and cross-shore velocity observed in 80 cm mean water depth (approximately 50 m offshore of the mean shoreline) on a gently sloping beach and in 175 cm water depth (100 m offshore of the shoreline) on a steep concave beach. Run-up was measured with a stack of resistance wires at elevations 5, 10, 15, 20, and 25 cm above and parallel to the beach face. At sea swell frequencies (nominally 0.05 < f ≤ 0.18 Hz), run-up energy is limited by surf zone dissipation of shoreward propagating waves so that increasing the offshore wave height above a threshold value does not substantially increase the predicted or observed sea swell run-up excursions (e.g., run-up is “saturated”). Existing semiempirical saturation formulations are most consistent with the observations and numerical model predictions of run-up excursions nearest the bed. In contrast, at infragravity frequencies (0.004 < f ≤ 0.05 Hz) where surf zone dissipation is relatively weak and reflection from the beach face is strong (e.g., saturation formulas are not applicable), the run-up excursions increase approximately linearly with increasing offshore wave height. The numerical model also accurately predicts that the tongue-like shape of the run-up results in sensitivity of run-up measurements to wire elevation. For instance, run-up excursions and mean vertical superelevation (above the offshore still water level) increase with decreasing wire elevation, and continuous thinning of the run-up tongue during the wave uprush can result in large phase differences between run-up excursions measured at different wire elevations. Numerical model simulations suggest that run-up measured more than a few centimeters above the bed cannot be used to infer even the sign of the fluid velocities in the run-up tongue.

Atlas of the Oceans: Wind and Wave Climate

Abstract: Global meteorology - major surface wind patterns what drives the wind? the global meridional circulation zonal circulations storms tropical cyclones subtropical cyclones hybrid systems. Ocean wind waves -basic ocean wave properties the random sea surface dependence on wind speed and fetch wave propagation and swell. The GEOSAT satellite mission - the GEOSAT coverage wave measurement using the GEOSAT radar altimeter wave height measurement validation wind measurement using the GEOSAT radar altimeter wind speed measurement validation and choice of algorithm Brown algorithm Chelton and McCabe algorithm Goldhirsh and Dobson algorithm Chelton and Wentz algorithm Witter and Chelton algorithm comparison of exiting algorithms Young's algorithm for high wind speed wind speed validation. Data preparation - introduction data quality control partitioning of data mean monthly statistics probability of exceedence. Data validation - mean monthly significant wave heights probability of exceedence - significant wave height mean monthly wind speed probability of exceedence - wind speed conclusions as to the validity of results. Global wind and wave statistics - background global monthly means and exceedence probabilities detail of each ocean basin the Indian Ocean the North Pacific Ocean the South Pacific Ocean the North Atlantic Ocean the South Atlantic Ocean latitude, longitude and great-circle sections selected output at points. References glossary - common terms unit conversion length time speed conversion C grades to F grades conversion K grades to C grades the Beaufort Scale.

A Comparison of Directional Buoy and Fixed Platform Measurements Of Pacific Swell

Abstract: The performance of the Datawell Directional Waverider and the National Data Buoy Center (NDBC) 3-m discus buoy, widely used to measure the directional properties of surface gravity waves, are evaluated through comparisons to an array of six pressure transducers mounted 14 m below the sea surface on a platform in 200-m depth. Each buoy was deployed for several months within a few kilometers of the platform. The accuracy of the platform ground-truth array was verified by close agreement of wavenumber estimates with the theoretical linear dispersion relation for surface gravity waves. Buoy and array estimates of wave energy and directional parameters, based on integration of the directional moments across the frequency band of energetic swell (0.06–0.14 Hz), are compared for a wide range of wave conditions. Wave energy and mean propagation direction estimates from both buoys agree well with the platform results. However, the Datawell buoy provides significantly better estimates of directional spread...

Suspended-sediment transport in the surf zone: response to breaking waves

Abstract: A field experiment designed to investigate the influence of wave breaking on suspended-sediment transport was conducted at Duck, NC, from 6 to 9 September 1985. Arrays of optical backscatter sensors, electromagnetic current meters and pressure sensors were deployed at five positions on a shore-normal transect that spanned the surf zone. At each position measurements were made of cross-shore and longshore velocity, sea-surface fluctuations, and suspended sediment at five levels above the bed. Experimental data runs were conducted when incident swell waves ( Hs = 0.5m, T= 10–12s) broke (primarily plunging) within the experimental transect. This paper describes the spatial characteristics of the plunge-to-bore tranformation region and describes (1) the cross-shore variability of sediment resuspension, including the mean concentrations and mean suspended load; (2) the net longshore and cross-shore flux across the surf zone; (3) mean suspended-sediment profiles as a function of wave type, e.g. plunging, spilling and bore, and unbroken at four positions across the surf zone; and (4) discusses the relative contribution of each wave type to the net longshore and cross-shore sediment flux.

Study of the air-sea interactions at the mesoscale: the SEMAPHORE experiment

Abstract: The SEMAPHORE (Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddles), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 degrees C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m(-2). This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.

Observations of seiche forcing and amplification in three small harbors

Abstract: Extensive field observations are used to characterize seiches (periods 0.5–30 min) in three small harbors with similar surface areas (∼1 km²), water depths (5–12 m), and swell wave climates. On the continental shelf just offshore of each harbor mouth, the energy levels of waves in the infragravity frequency band 0.002–0.03 Hz (periods 0.5–10 min) vary by more than a factor of 200 in response to comparably large variations in swell energy levels. Energy levels in this swell-driven frequency band also vary (less dramatically) in response to changes in the swell frequency and with tidal stage. Motions at longer seiche periods (10–30 min) are primarily driven by meteorological and other processes (a tsunami-generated seiche is described). As has often been observed, the amplification of seiche energy within each harbor basin (relative to energy in the same frequency band outside the harbor) varies as a function of seiche frequency, and is largest at the frequency of the lowest resonant harbor mode (i.e., the Helmholtz or grave mode). At all three harbors, the average amplification of the grave mode decreases (by at least a factor of 2) with increasing seiche energy, a trend consistent with a nonlinear dissipation mechanism such as flow separation in the harbor mouth or sidewall and bottom friction.

Rayleigh-wave dispersion along the Hawaiian Swell: a test of lithospheric thinning by thermal rejuvenation at a hotspot

Abstract: SUMMARY In this paper we present a seismological test of the 'thermal rejuvenation' model for the formation of the Hawaiian Swell, the archetype of a midplate hotspot swell. Twostation measurements of Rayleigh-wave group and phase velocities between Midway Atoll and O'ahu provide the basis for the test: comparison of the observed dispersion curves with age-dependent, regionalized dispersion curves suggests that the swell has the seismic velocity structure of 50-110 Myr old lithosphere rather than that of 20-50 Myr old lithosphere as suggested by the rejuvenation model. Joint inversion of the group- and phase-velocity curves for isotropic velocity models yields an estimate of - 100 km for the seismic lithosphere's thickness along the swell. This value is difficult to reconcile with the 40-50 km thickness demanded by the rejuvenation model. Preliminary measurements of the coefficients of anelastic attenuation are also presented. Although the accuracy of these values is difficult to assess, they too appear to be consistent with thick lithosphere.

A gravity current model of cooling mantle plume heads with temperature-dependent buoyancy and viscosity

Abstract: Gravity currents are a ubiquitous fluid dynamical phenomenon which involve the horizontal spreading of fluid masses under their own weight or buoyancy. A theoretical model is developed to account for the effects of bulk cooling on the dynamics and morphology of geological gravity currents, with particular focus on mantle plume heads spreading beneath the lithosphere. As many geological gravity currents (e.g., plume heads and lava flows) spread, they cool and thereby become more viscous and dense. All gravity currents initially spread at the same rate as the isothermal currents predicted by Huppert [1982]. However, currents with temperature-dependent viscosity and/or buoyancy eventually go to a much slower spreading rate than the isothermal currents. Moreover, unlike the isothermal gravity currents, cooling variable-viscosity and/or variable-buoyancy currents do not conserve shape as they spread. Both constant volume and constant volume flux currents with strongly temperature dependent viscosity develop steep-sided flat-topped, plateau shapes which become more rounded once the currents lose most of their heat. Currents with temperature-dependent buoyancy develop inflections or even extensive swelling at their flow fronts. The surface expression of the edge-steepening effect in mantle plume heads is likely to be filtered by lithospheric flexure but may contribute to the flattened plateau shape inferred by Wessel [1993] for the Hawaiian swell. The frontal inflation effect due to variable buoyancy may contribute to the dual-lobe structure of the Hawaiian swell gravity anomaly and suggests an alternate physical mechanism for forming the torus- or horseshoe-shaped geochemical patterns in the Galapagos and the Marquesas hotspots. Perhaps most significantly, the gravity current model also predicts the thermal (i.e., degree-of-melting) pattern for Galapagos and Marquesas hotspots more readily than the traditional entrainment models.

Radar measurements of critical-layer absorption in mountain waves

Abstract: The conditions controlling the vertical propagation of mountain waves in the troposphere and lower stratosphere have been examined using radar observations at 46.5 MHz of vertical-velocity perturbations at Aberystwyth (52.4°N, 4.1°W) and radiosonde observations from Aberporth, some 50 km to the south-west. Attention has been paid to the influence of the variation with height of the mean wind vector (U), the Brunt-Vaisala frequency and the Scorer parameter. Four case-studies show the absorption of mountain waves at heights where the mean wind component normal to the wave front vanishes; this corresponds to U = 0 if the wind direction does not change with increasing height, and to the wind vector being orthogonal to the wave vector if the wind rotates with increasing height. For two cases, enhanced turbulence is found in the upper few hundred metres of the mountain-wave field, directly below the height at which the waves disappear; for a third case, the enhancement extends over a broader range of heights, and for the fourth no clear identification of the enhancement is possible. The frequency power-spectra of the vertical-velocity oscillations show a ω n dependence, with n approaching -5/3 for regions of mountain-wave activity below the critical layers, and n = 0 for heights above the critical layers. Statistically, mountain waves generated by low-level easterly winds are often confined to tropospheric heights. An analysis of a large sample of radiosonde data indicates that critical-layer absorption associated with rotation of the background wind in addition to wave reflection and trapping, plays an important role in this confinement. The results are discussed in terms of the momentum flux associated with gravity waves, their turbulent breakdown near the critical layer, and the implications of the frequency spectra below and above such layers.

Ocean wave dispersion surface measured with airborne IR imaging system

Abstract: Image sequences of the ocean have been collected at long range and low grazing angle with an airborne infrared system. The images are geographically registered, and 3D frequency-wavenumber spectra are calculated and shown to have a strong 2D dispersion surface that is characteristic of wind waves and swell. Wave directions compare well with in situ measurements, and their speeds are consistent with the water depth. The authors conclude that temporal sequences of IR images acquired from long distance can provide information for extracting surface wave parameters.

Modulation of ocean skin temperature by swell waves

Abstract: Infrared measurements of sea surface temperature from R/P Flip in the deep ocean show that there is significant modulation of ocean skin temperature by swell waves and that the wind plays a dominant role in the process. The squared coherence and the magnitude of the transfer function between the skin temperature and surface displacement respond to the wind speed, while its phase is determined by the direction of the wind relative to the swell. When the swell and wind are in the same direction, the transfer function phase indicates that the maximum skin temperature occurs on the forward face, which, in this case, is also the downwind side. Remarkably, the phase changes by roughly 180° when the wind direction reverses from going with the swell to going against it, so that the maximum switches to the rear face, which is again downwind. The peak-to-peak modulation T 0 is found to be correlated with the bulk-skin temperature difference ΔT. Furthermore, T 0 is of the same order as ΔT, suggesting that small-scale wave breaking due to longwave/shortwave interaction may dominate the phenomenon.

Wind-Wave Coupled Downward-Bursting Boundary Layer (DBBL) beneath the Sea Surface

Abstract: By synthesizing data of the turbulent structure beneath wind waves in laboratory tanks, with some re-analyses, we propose the existence of a particular turbulent boundary layer which is directly coupled with wind waves, a “downward-bursting boundary layer (DBBL)” in water beneath wind waves. The data set indicates that the depth of this layer is from 3 to 7, or about 5 times the significant wave height of wind waves. The data observed in laboratory tanks agree with data of acoustic observations of bubble clouds under breaking wind waves in the sea made by Thorpe (1986, 1992). It is inferred that DBBL is formed in equilibrium with the local wind waves, as a common feature from initially generated wind waves, young laboratory wind waves to mature wind waves in the sea.

Validation of Numerical Model for Wind Waves and Swell in Harbors

Abstract: The numerical wave model HARBD has been used extensively within and outside the U.S. Army Corps of Engineers for estimating waves in harbors. The model was originally developed for long waves (harbor oscillations). It has been extended for use with short waves (wind waves and swell) and validated by several investigations, most notably Chen's (1986) fundamental investigation of long waves in a rectangular harbor. This study focuses on short-wave validation. The model is tested with several simple domains, in most cases with uniform depth. It is validated with laboratory data for irregular wave diffraction around a semi-infinite breakwater. Both broad and narrow directional spreads are considered. The model is also compared to widely accepted curves for regular and irregular wave diffraction through a breakwater gap. Field validation of the model for Kaumalapau Harbor, Lanai, Hawaii, is discussed. Tests demonstrate the effect of input parameters on model results and provide guidelines about the choice of p...

Numerical Analysis of the Sea State Bias for Satellite Altimetry

Abstract: Theoretical understanding of the dependence of sea state bias (SSB) on wind wave conditions has been achieved only for the case of a unidirectional wind-driven sea. Recent analysis of Geosat and TOPEX altimeter data showed that additional factors, such as swell, ocean currents, and complex directional properties of realistic wave fields, may influence SSB behavior. Here we investigate effects of two-dimensional multimodal wave spectra using a numerical model of radar reflection from a random, non-Gaussian surface. A recently proposed ocean wave spectrum is employed to describe sea surface statistics. The following findings appear to be of particular interest: (1) Sea swell has an appreciable effect in reducing the SSB coefficient compared with the pure wind sea case but has less effect on the actual SSB owing to the corresponding increase in significant wave height. (2) Hidden multimodal structure (the two-dimensional wavenumber spectrum contains separate peaks, for swell and wind seas, while the frequency spectrum looks unimodal) results in an appreciable change of SSB. (3) For unimodal, purely wind-driven seas, the influence of the angular spectral width is relatively unimportant; that is, a unidirectional sea provides a good qualitative model for SSB if the swell is absent. (4) The pseudo wave age is generally much better fo parametrization the SSB coefficient than the actual wave age (which is ill-defined for a multimodal sea) or wind speed. (5) SSB can be as high as 5% of the significant wave height, which is significantly greater than predicted by present empirical model functions tuned on global data sets. (6) Parameterization of SSB in terms of wind speed is likely to lead to errors due to the dependence on the (in practice, unknown) fetch.

Swell and distortions of high-density polyethylene extruded through capillary dies

Abstract: The effect of varying the die entrance angle and the die length on extrudate swell and on the onset of extrudate distortion in capillary extrusion has been studied. Using theory from the literature, we have analyzed the contribution to the total pressure drop from the elongational and shear deformation in the entrance region, and from the capillary pressure drop in the land region of the die. From the contribution of the elongational deformation, we obtained an estimate for the elongational viscosity of the polymer. The same analysis was used to study the influence of the die geometry on the stick-slip instability. It is found that the elongational component at the inlet region mainly influences the extrudate distortions. The onset of the stick-slip instability occurs within 10% at a wall stress τ w of 0.3MPa, where τ w is calculated from expressions assuming fully developed flow. The variation around this average value is systematic with changes in die geometry, and the observed variations are probably due to the non-homogeneous pressure field in the die. We also propose a model for predicting extrudate swell. Input to the model are material parameters obtainable from oscillatoric measurements of the loss and storage modulus and residence times calculated from the geometry of the die. The swell model includes a fitting parameter that sets the overall scale of the swell.

Observations of surface wave-current interaction during swade

Abstract: On 5 March 1991, during the Surface Wave Dynamics Experiment (SWADE), airborne expendable current profilers were deployed from the NASA P-3 aircraft to document the Gulf Stream and a warm core ring, while the GSFC Scanning Radar Altimeter measured the effect of these features on the directional wave spectrum. North propagating swell of about 10 s wave period appeared to have experienced a double reflection by the Gulf Stream, which turned it toward the east.

Changes to wave parameters in the surf zone due to wind effects

Abstract: Wind/wave interactions in the surf zone were studied using a wave tank and environmental wind tunnel. Waves were modelled at approximately 1/50 scale and broke on a model scale beach of 1/20 slope. The wind was simulated using accelerated growth techniques over a relatively short fetch, and a model scale of roughly 1/100 was achieved. Thus there was some scaling mis-match between wind and wave simulations. Results show that the wind has a significant effect on the breaking of the waves. Both breaker location and breaker type are affected, and the results agree with those of Douglass [10 and 11], The main findings, common to both studies, were that offshore winds promote plunging waves and delay the moment of breaking (reducing surf zone width), whereas onshore winds encourage early wave breaking, generally by spilling. Wave heights at breaking were not generally affected, but due to changes in breaker depth, the height-to-depth ratio is shown to be affected by the wind. Additionally, studies of the motion...

Numerical simulation of blow molding—prediction of parison diameter and thickness distributions in the parison formation process

Abstract: In our previous study, we calculated the time course of parison length in the parison formation stage, but it could predict only the parison area swell. The next target in our study is to calculate the parison diameter and thickness swell. Annular extrudate swell simulation is necessary for the understanding of various kinds of swelling ratios in blow molding. We have examined three kinds of swells (outer diameter, thickness, and area swells) obtained from simulation results of annular extrudate swell, using the Giesekus model, and have developed a method of predicting parison outer diameter and thickness swell values. The predicted values of parison outer diameters are discussed in comparison with experimental data, and reasonable results are obtained by the proposed method. This prediction method could also be applied to the parison formation process using a parison controller. As a result, it is possible to predict approximately the whole process of parison formation by numerical simulation.

Gravity wave exclusion circles in background flows modulated by the semidiurnal tide

Abstract: In this short paper the exclusion circles and vertical phase locities for gravity waves launched from the ground into a time-varying wind are studied using a ray-tracing technique. It is shown that waves with initial observed phase speeds that should place them within the local temporally varying exclusion circle, are often Doppler shifted outside of the circle. This, and the finite lifetime of some critical levels, allow waves to survive the critical layer and reach higher altitudes. Also, for slower phase-speed waves, the temporally varying wind can shift the observed frequency to negative values, so that the observed phase motions will be opposite (i.e. horizontally reversed and vertically upward), even though the energy still propagates upward. This effect can also cause the phase velocity to move inside the local exclusion circle. Due to the directional filtering of wave sources by the stratospheric wind, the percentage of such reverse-propagating waves will change systematically with local time and height in our simplified but realistic model. These results are related to ground-based systems, optical and radar, which sample the wind field and gravity waves in the middle atmosphere.

Ocean waves producing means

Abstract: An ocean wave producing structure (10, 50, 60, 70, 80, 90, 100) having a support structure (14) and a web (12). The support structure (14) is fixed to the ocean floor (32) and the web (12) is arranged with its upswell end (12a) lower than its downswell end (12b) so that ocean swells (34a) travelling over the web (12) are forced upwardly to form into waves (34b). The shape and characteristics of the waves (34b) vary depending upon the gradients (24) of the sections (W1 to W8, 52 to 56) of the web (12). The support structure (14) allows ocean currents to flow through it and hence has a low impact upon the ocean environment.

Ocean bottom microseisms from a distant supertyphoon

Abstract: In this paper we describe microseisms and swell produced by a supertyphoon in the western Pacific, near the Philippine Islands, recorded from an ocean-bottom seismic ‘station’ (OBSS) located about 200 km west of San Francisco, California at a depth of about 4 km and from a pressure sensor installed in shallow water near Pt. Arena, California. The microseisms recorded on OBSS are generated by the swell in the vicinity of the station and narrow-band spectral peaks observed with frequency increasing slowly with time from the ocean-wave dispersion correlate well, at twice the frequency, with swell near Pt. Arena. Similar, previously reported, observations at OBSS from storms in the northeastern Pacific are consistent with fundamental Rayleigh wave propagation, toward the coast in that case and away in the present, possibly related to a large difference in the estimated width of the zone of interaction between incident and reflected swell along the California coast.

Some Evidence of Colinear Wind Stress and Wave Breaking

Abstract: Data collected during the Surface Waves and Processes Program are employed to investigate a possible interrelation between wind stress and surface wave breaking. From comparison of data from 15 half-hour long time segments, the directions of the wind stress and the whitecap motion are observed to be generally colinear, with both lying between the mean wind and the swell. As well, a nondimensionalized whitecap speed is found to correlate with the drag coefficient. These results suggest that the magnitude and direction of the wind stress might be estimated from wave breaking information.

Resolving small scale turbulence with acoustic Doppler and acoustic travel time difference current meters from an underwater tower

Abstract: Turbulence is an important factor for planktonic predator-prey interrelationships, as it increases the contact rate between the relatively immobile planktonic organisms. Actual turbulent scales in planktonic predator-prey relationship are those less than the typical planktonic separation distance. For fish larvae feeding on copepod nauplii of concentrations 1-50 nauplii/l the effective turbulence scale is smaller than 10 cm. In regions where the wind-induced turbulence dominates, empirical relationships between wind mixing and wind speed may be applied to calculate rough estimates of the turbulent energy dissipation rate /spl epsi/. In regions where other sources of turbulent energy (tidal mixing, convection, swell in shallow regions and internal breaking waves) are important, the available empirical data are poor, and direct measurements of the turbulence and the derived turbulent frequency spectra are needed. During spring 1995 and 1996 direct turbulence measurements were carried out in Lofoten, Northern Norway, above a shallow plateau of 20-35 m depth where turbulent mixing is mainly generated by tidally and swell-induced mixing. Lofoten is an important feeding area for cod larvae. The turbulence measurements were conducted from a fixed point at the top of 65 m high submarine tower sitting on the sea bed. Three types of ultrasonic current meters, one Doppler current meter and two travel time current meters with different sampling volume were used in the experiment. The results indicate that both instrument types can be used to resolve small scale turbulence. However, Doppler noise limits the performance of the Doppler current meter, while the acoustic pathlength limits the performance of travel time current meters.

Predictions of primary normal stress difference from extrudate swell of rubber compound

Abstract: The first normal stress difference N 1 in capillary flow of viscoelastic fluids has been evaluated via measurements of the extrudate swell and other rheological parameters. Advances in this field are summarised briefly. Two equations describing the relationship between N 1 and rheological parameters such as the die/swell ratio are presented and predictions of N 1 from a rubber compound in die extrusion are analysed and compared with several well known literature equations.