Showing papers on "Wind stress published in 1971"

Measurements of atmospheric pressure on wind-generated sea waves

Abstract: Simultaneous measurements of wave elevation and atmospheric pressure on wind-driven sea waves were made using a vertical wave-sensing rod and a small (23 cm diameter) pancake-shaped styrofoam buoy in which was embedded a sensitive pressure transducer; the wave probe constrained the buoy to move with the waves only in the vertical direction. Care was taken to avoid contamination of the pressure signal with dynamic pressures caused by flow distortion around the buoy.Results are presented as power and cross-spectra of wave elevation and pressure, spectra of the fluxes of energy and momentum from the wind to the waves, and spectra of ζ the fractional increase in wave energy per radian.The phase shifts of the pressure signal are compared with the laboratory and field results of other investigators, and with the theoretical predictions of Miles's (1957) inviscid laminar model of wave growth. Agreement is reasonably good among the experimental results, but observed phase shifts are an order of magnitude larger than the theoretically predicted values.Integrals under the momentum flux spectra are compared in all runs with the predictions of the standard empirical formula, and in two cases are compared with the values of the total wind stress as measured with a sonic anemometer; the indication is that a large fraction of the total flux of momentum from the air to the sea goes initially into the wave field.The ζ spectra are compared with the field results of Snyder & Cox (1966) and with the theoretical predictions of Miles's (1957) model; agreement is again good between the field results while the theory underpredicts ζ by factors of between 5 and 8.A simple dimensionless relation is found between ζ and the ratio of wind speed to wave phase speed.

The variations of the statistics of wind, temperature and humidity fluctuations with stability

Abstract: Measurements of the turbulent fluctuations of wind, temperature and humidity were made in the atmospheric surface layer. The statistics of the fluctuations were investigated in both the time and frequency domains. The vertical wind, temperature and smaller-scale horizontal wind fluctuations appear to obey the Monin-Obukhov similarity hypothesis. The humidity fluctuations were found to be governed by a humidity flux stability parameter rather than the normal Monin-Obukhov length.

A Theoretical Study of the Land and Sea Breeze Circulation

Abstract: Estoque's model for the sea breeze is modified with respect to some essential and other, less essential, details. It is shown among other things that the accelerational terms in the equation for the vertical component of motion are important and should therefore be retained. In addition, the equation of continuity is retained in its original form in order to prevent violation of the mass conservation law. The new model is integrated numerically and the results presented. The results include horizontal and vertical winds, mass conservation, vertical components of vorticity, time hodographs, temperatures (including vertical profiles), vertical transfer of sensible heat, and wind stress at the surface. Particular attention is paid to the land breeze phase of the circulation which so far has received little attention in the published literature. Further, the sea breeze front is discussed in some detail. The integration is carried out for three daily cycles setting out from an atmosphere at rest. The ...

Surface Stresses Produced by Rainfall

Abstract: The previous solution to the problem of determining the effect of rainfall on the sea-surface stress is found to be inadequate A correct treatment confirms that rainfall may contribute significantly to the surface stress and may under some conditions produce stresses comparable in value to the wind stress Also the drag of the drops in the lower air layer may induce measurable distortions from the logarithmic wind profile

Wind stress criteria in eolian sand transport

Abstract: The linear relationship between shear velocities and wind velocities was verified from field data collected on Ecuadorian beaches during periods of sand transport. From these data a Froude number was established that expresses the relationship among wind stress, gravity acceleration, and mean grain size of sand particles. The rate of eolian sand transport by wind can be scaled by using this number. The concept was verified by available field and wind tunnel data on the subject. The equation is expressed as follows: q=K (Fr)3 where q is the rate of eolian sand transport, K is the eolian sand transport coefficient with the same dimension as q, and Fr is the Froude number.

Vertical Diffusion in Shallow Waters

Abstract: From tracer measurements in stratified, shallow waters with a vertical current shear the vertical diffusion coefficient is determined. Two different models are used, treating separately the cases of strong and weak vertical stratification. The diffusion coefficient is related to the stratification, the frictional stress, and the shear. With a persisting wind of strength above 4–5 m/s, the stress in the upper layer is assumed to be proportional to the wind stress. In this special case the theoretical and experimental results can be compared, and a good agreement is found. DOI: 10.1111/j.2153-3490.1971.tb00555.x

On the relationship between geostrophic and surface wind at sea

Abstract: The ratio between observed surface and geostrophic wind speed has been investigated from observations at the German Bight, taking geostrophic wind and the air-sea temperature difference as parameters. The ratio decreases with increasing geostrophic wind and increasing stability. While stability is an important parameter for light to moderate winds, variation of the ratio with geostrophic wind speed cannot be neglected, taking the full range of geostrophic wind speeds into consideration. From the Navier-Stokes equations, such a variation is to be expected. For light winds, the (local) surface wind may exceed the (mesoscale) geostrophic wind. Both effects together can be described approximately by a linear relation between the surface wind and geostrophic wind, with a slope of 0.56 and a constant term b>0 varying with stability. The residual error was 2 m/s. Variation with latitude is inferred from the Navier-Stokes equations.

Slope and Curvature Distributions of Wind-Disturbed Water Surface

Abstract: The microstructure of the wind-disturbed water surface, characterized by surface-slope and surface-curvature distributions, is studied in a laboratory tank with a newly developed optical instrument. Some features of these distributions under various wind and wave conditions are systematically presented. It is also shown that wind waves arise at about the time when the airflow boundary layer becomes turbulent. At lower wind velocities, the formation of parasitic waves causes a skewed slope distribution; at high wind velocities, the wave breaking causes a peaked slope distribution. With a turbulent wind, similar variation of the mean-square slope with the shear velocity is shown in the present laboratory results and in older oceanic data, suggesting that the laboratory facility may be able to simulate the microstructure of the air–sea interface.

Structure of Wind-Driven Equatorial Currents in Homogeneous Oceans

Abstract: The properties of wind-driven equatorial currents in homogeneous oceans are investigated for variable wind stresses and variable eddy viscosities. In the case of a uniform westward stress, the calculated cross-stream and depth variations of the undercurrent are found to be in qualitative agreement with observations for a suitable constant value of the eddy viscosity. When the stress has a cross-equatorial component, the undercurrent is displaced slightly upwind of the equator but preserves its boundary-layer character, including the vorticity discontinuity due to the juxtaposition of water particles with different histories. The currents produced by pure north-south wind stresses are in qualitative agreement with Indian Ocean observations in the southwest and northeast monsoon regimes. An eastward wind stress produces an inertial equatorial current rising to a maximum at the surface. The current becomes extremely strong for moderate viscosities and probably unstable. This may account for the abse...

A Simple Model of Coastal Upwelling Dynamics

Abstract: As a step in understanding the complicated dynamics of coastal upwelling areas, a simple theoretical model is examined. The motion is driven by surface wind stress acting on homogeneous water of constant depth adjacent to a long straight coastline. Order-of-magnitude analysis is used to argue that the upwelling is induced by the horizontal divergence of a lateral, frictional boundary layer. A vertical integration of the equations of motion shows the necessity of retaining the pressure gradient term in the longshore direction even though the velocity field is two-dimensional. The motion in the lower return layer and upper Ekman layer is analyzed. It is found that the surface layer motion may he deduced independently of the return flow layer. The mass flux pumped into it should not be affected by bathymetry or stratification, provided that the depth is much greater than the Ekman layer depth. Streamlines are shown for different surface wind stress orientations. The results show that some upwelling ...

The movement of oil spills

Abstract: The effects of winds, waves, and currents, and the physical properties of oil and water on the drift rates of oil spills were studied in tests carried out in a combined water basin wind tunnel On calm water, oil drifted at a fairly constant percentage of the wind speed regardless of the nature and spreading tendencies of the oil, the spill size, and water temperature, depth, and salinity. Percent drift varied with wind tunnel height. Extrapolation to infinite height indicated that on calm open water wind drift should be 3.7%. Shallow water waves, which produced no significant drift themselves, reduced wind drift. Analysis indicated that deep water waves produced by the wind should produce significant drift, complicating wind drift prediction, but the magnitude of the wind wave interaction effects is not yet known. Test wind drifts and current drifts were found not to be directly additive.

Wind structure in the atmospheric boundary layer

Abstract: The semi-empirical laws for the variation of mean wind speed with height and for the statistical properties of the turbulent fluctuations are briefly outlined. Similarity considerations provide some useful ordering of the mean wind profile characteristics in relation to surface roughness and thermal stratification. Appreciable uncertainties prevail, however, especially as a consequence of the effect of thermal stratification and of variable terrain roughness. Some generalization on similarity grounds can also be made regarding the fluctuations of horizontal wind speed as a function of roughness and stability, but there are wide variations of spectral density and scale which are not immediately explicable and which at present preclude anything more than a relatively coarse specification of the spectrum. Features which are of special relevance to architectural aerodynamics and which are discussed briefly are: ( a ) the difficulty of generalizing about the wind profile and turbulence above an urban complex; ( b ) the requirement for estimating the magnitudes of extreme gusts as a function of mean wind speed, averaging time and height; ( c ) the problem of generalizing about flow properties below roof level; ( d ) the effect of urban airflow on the travel and dispersion of pollutants

Anemometer Height in Froude Scaling of Wind Stress

Abstract: Extending previously proposed Froude scaling of wind stress coefficients, the paper suggests the appropriate anemometer heights for different wind fetches, and proposes a procedure of obtaining the wind stress by a single wind velocity measurement at this suggested anemometer height. The procedure consists of first estimating the fetch Renynolds number and then obtaining a tentative anemometer height from a prepared figure. Setting the anemometer at this height, an accurate wind velocity is measured; from this measurement and knowledge of the wind fetch, a corrected anemometer height can then be determined. Having the wind velocity measured at this final anemometer height, the wind stress coefficient can be obtained from another prepared figure.

Wind profile and vertical motions above an abrupt change in surface roughness and temperature

Abstract: A review is presented of some Russian-language papers published as early as 1952–1954 that are unknown to most English-speaking readers. In these papers the influence of an abrupt change in the surface friction velocityv*, surface roughnessz0 and surface temperature or heat flux on the wind velocity profile and vertical motions has been investigated analytically and numerically. Most of the theories are based on the exchange-coefficient approximation for momentum and heat. In terms of this approximation, further generalization and development of the problem is discussed.

On the variation of ocean circulation produced by bottom topography

Abstract: The three dimensional circulation produced by a given wind stress in a homogeneous ocean with an arbitrary bottom topography is studied. The solution is illustrated by the use of various bottom shapes and a particular wind stress distribution. It is shown that in general a current deviates southwards in a region of decreasing depth and northwards in a region of increasing depth. However, if the wind stress has a perturbation out of phase with the bottom corrugations this result is considerably modified. Moreover, when there is a deepening of the ocean towards the north there may exist a “critical line” at which ?/?y(f/H) is zero ( f is the Coriolis parameter, H is the depth and y is the northward co-ordinate). The oceanic circulation is then divided into two parts: if the wind stress curl is positive, then for latitudes north of the “critical line” where ?/?y(f/H) is positive the Sverdrup-topographic interior flow is basically northward and there is an eastern boundary current, while for latitudes south of the critical line, where ?/?y(f/H) is negative the interior flow is basically southward and there is a western boundary current. DOI: 10.1111/j.2153-3490.1971.tb00553.x

Structure of the Antarctic Circumpolar Current

Abstract: The paper by Callahan [1971] shows an intriguing feature of the circulation south of Australia The total eastward flux is concentrated compared to the space available between Australia and Antarctica, or compared to the wind stress It is so concentrated that the current is actually westward on both sides, near 5O°S and near 42°S Callahan comments that these features appear to be related to the topography of the region

The effect of time-variable fluxes on mean wind and temperature profile structure

Abstract: Synthetic wind speed and air temperature profiles based on the sensible heat flux density and stress at the surface are averaged for the four possible ways in which the suface stress and heat flux density can vary maintaining the same average values. The analysis of the averaged wind and temperature profiles shows that, when the surface stress and/or heat flux density are time-variable, and wind speed and air temperature are averaged linearly, an erroneous estimate of surface roughness, surface stress, heat flux density and profile structure parameters will result.

Two-Dimensional Wind Setup of Oil on Water

Abstract: The geometry of an oil wedge that floats on water and is retained by a barrier depends on the wind and current forces acting on the oil. An investigation of the two-dimensional setup of floating oil retained by a rigid barrier and subjected to a wind stress was conducted. An analytical expression for the oil wedge thickness as a function of oil properties, wedge length and wind stress or wind velocity was developed. This expression was then experimentally verified in a two-dimensional wind-wave flume. Experiments were conducted for a range of wind speeds using three oils with different density and viscosity. The air-oil interfacial stress for a smooth interface is the same as that for wind over a smooth water surface. However, the development of waves in the oil significantly increases the interfacial stress and resulting setup. A prediction equation is presented for conditions when waves exist but further research on the nature and effects of waves at the air-oil interface is needed. Interfacial stresses evaluated from the setup prediction equation were in agreement with stress values determined from wind velocity profile measurements and the Karman-Prandtl equations.

A note on wind‐stress in open sea

Abstract: In JGR Wu [1969] has computed the wind stress coefficient from oceanic observations on wind speed U10 measured at a height of 10 meters from sea level The calculation assumes a logarithmic wind velocity distribution and Charnock's [1955] relationship relating the origin of the wind profile to the friction velocity An important feature of this analysis is that the averaged values of the wind-stress coefficient indicate a significant and discontinuous rise for wind speeds above 15 m/sec I should like to comment on this point

Directional energy distribution of wind waves

Abstract: Two-dimensional ocean wave spectrum developing under the atmospheric surface pressure fluctuations is linearly correlated with that of wind pressure itself, so that angular distribution of energy of ocean surface waves can be determined by directional properties of surface pressure fluctuations with the same frequency to the surface wave.

Response of a simple two-layer ocean to divergent and rotational wind stress

Abstract: Response of an unbounded two-layer ocean to traveling atmospheric disturbances is analytically investigated. Under the assumption that (1) lower layer is motionless, (2) each layer is homogeneous and in hydrostatic balance, (3) the Coriolis parameter is constant, and (4) the density difference between the two layers is small compared to the density itself, the one-dimensional Klein-Gordon's equation is solved analytically for both the divergent and rotational wind stress. Numerical examples of the wake pattern of the upwelling behind the regional divergent and rotational wind fields are also represented.

The response of a lake with a depth-discontinuity to a time-dependent wind-stress

Abstract: The effect of a travelling atmospheric pressure disturbance on the water level in a narrow lake with a depth-discontinuity has been studied using the method of characteristics. Both semi-infinite stress-bands and finite stress-bands were considered. For semi-infinite bands the water level at the left side is predominantly negative while it is positive on the right side and it becomes both positive and negative for finite band widths. The geometry with a depth-discontinuity differs from that without the discontinuity in that while for the latter case the set-up (defined as the difference in the water level between the right and left boundaries for a disturbance travelling from left to right) becomes periodic some time after the disturbance crosses the lake, this does not happen for the former case.

The Entrainment Influence on the Ocean Surface Layer in Tropical Latitudes

Abstract: A nonlinear, time-dependent, baroclinic model is developed for a zonally uniform, tropical, two-layer ocean on a north-south vertical section. The lower layer is infinitely deep, at rest, and at constant temperature. The dynamics of the well-mixed surface layer are described in terms of the components of horizontal mass transport, the specific mass, and the specific enthalpy. The forcing functions of the model are the zonal wind stress, the vertical entrainment of cold water from the lower layer into the surface layer, and the surface thermal energy input. The concept of entrainment forcing is based on the approach of Kraus and Turner for parameterizing the vertical motion of the seasonal thermocline. Since zonal gradients of all quantities are neglected, the model applies only to the ocean's interior. This is rationalized by oceanographical observations. In particular, the cast-west pressure gradient term is one order of magnitude smaller than the wind stress; it may be considered as an addition...

Doppler Radar Observation of Wind Structure in Snow

Abstract: Sinusoidal variations in the longitudinal speed of the wind in the planetary boundary layer are observed with a C-band multiple-gate Doppler radar using snow as a wind tracer. These undulations in the horizontal wind are believed to be orographically generated wave phenomena. The observed wind structures have average scale lengths of 300 m and their amplitude decreases with altitude. The propagation speeds of the, wind structures have been determined with a spatial correlation technique and have, at times, been found to depart appreciably from the mean ambient wind. Under stable atmospheric conditions the wind structures persist for a considerable period. In one extreme case the structures preserved identifiable characteristics during advection over a distance of 9 km. This remarkable persistence resulted in a Lagrangian-Eulerian time-scale ratio of 45.

Observations of the Mean Wind Profile Over the Open Ocean

Abstract: : The purpose of this study was to investigate the mean wind speed profile over the open ocean. Analysis of wind profile data showed that averaging periods must be considered when the profile form is discussed since 95 percent of the 10 minute averaged profiles were found to be logarithmic while less than 50 percent of the 10 second averaged profiles were logarithmic.