Showing papers on "Wind stress published in 1970"

The Mathematical Representation of Wind Speed and Temperature Profiles in the Unstable Atmospheric Surface Layer

Abstract: Analytical expressions which specify non-dimensionalized wind speed and potential temperature gradients as functions of stability are integrated. The integrated equations are tested against Swinhank's wind and temperature profiles measured at Kerang, Australia. It is found that a representation suggested independently by Businger and by Dyer gives the best fit to temperature profiles and describes the wind profiles equally as well as a relation suggested by Panofsky et al.

Aspects of the Mid-Pacific Transition Zone

Abstract: In the central North Pacific, the transition between subarctic and subtropical water masses takes place between 42° and 32°N. The width of the zone and the sharpness of its boundaries are largely determined by the wind stress distribution at the sea surface. The northern boundary is characterized by numerous temperature inversions, the disappearance of the subarctic halocline, gravitational instabilities, and a complicated sound velocity structure. The southern boundary is marked by sharp thermohaline gradients and instabilities in the upper-most layer. The vertical distribution of properties in the transition zone is characterized by a deep salinity minimum of about 33.95‰ at 500 meters, by a sound velocity minimum of slightly less than 1480 m sec−1 at 800 meters, and by an almost linear temperature gradient in the upper 600 meters. Both the salinity and sound velocity minimum rise northward and lose their identity at the northern boundary. The baroclinic and barotropic modes of motion in the transition zone are very slow with speeds rarely exceeding 5 cm sec−1. A strong westward setting current with surface speeds in excess of 100 cm sec−1 was found 22 km north of Kahuku Point, Oahu, Hawaii. The width of the high-speed core was about 6 km, and the relative vorticity due to horizontal current shear reached values in excess of 2×10−4 sec−1.

Calculation of the steady state wind-driven circulations in Lake Ontario

Abstract: Numerical calculations were made to determine the steady state features of the wind-driven circulations in Lake Ontario. It is assumed that the water in the lake may be represented by an incompressible homogeneous fluid layer, a condition that is typical of a winter situation. The linearized mass transport equations are then solved for an imposed wind stress on the surface of the lake. Effects of bottom friction, bottom topography, lateral boundary configuration are taken into account Effect of rotation is represented by a constant Coriolis parameter. Circulation patterns were calculated for the cases of uniform and spacially variable wind stress. It was shown that bottom topography strongly influences the circulation features. The dominant feature of the circulation is a two-cell pattern with a small counter-clockwise circulation cell in the southern part of the lake and a large clockwise cell in the northern part with an intense west-ward return flow in the interior of the lake.

The Vertical Mean Wind Profile Over the Ocean for Light to Moderate Winds

Abstract: Analysis of 299 wind profile observations collected at the Massachusetts Institute of Technology oceano-graphic research platform provides a detailed picture of the behavior of wind profile parameters. A plot of roughness length vs velocity suggests the existence of classical hydrodynamic phenomena such as Jeffreys’ minimum wind speed and the Kelvin-Helmholtz instability. The friction velocity appears to be, in general, a linear function of the wind velocity, except in regions where discontinuities exist. It is suggested that the air flow regime over the ocean in the wind range of 2-10 m sec−1 (measured at a height of 10 m) is best described as a region of constant drag coefficient (CD = 1.6 × 10−3) modified by the superposition of almost singular departures at a number of discrete wind speeds, apparently caused by the onset of distinct instabilities at the water's surface.

Wind stress and turbulence over ice in the Gulf of St. Lawrence

Abstract: A sonic anemometer has been used in a preliminary study of the three components of wind turbulence over ice floes near a manned drifting station. Reynolds stresses and velocity spectra have been computed. The ice field studied has an average wind drag coefficient C10 = 0.0026 under neutral atmospheric conditions.

Circulation in the Panama Bight

Abstract: A stationary circulation model for the Panama Bight is developed and used with hydrographic data from four cruises. The horizontal circulation is described by geostrophic and wind-driven currents. Two derived contributions to vertical motion are the horizontal geostrophic divergence and the vorticity equation involving the wind stress. The surface geostrophic circulation is cyclonic during each cruise period with the Colombia Current flowing northward at speeds up to 100 cm sec−1; at no time is the width of this current more than 180 km nor the depth in excess of 100 meters. Farther offshore a previously unreported current flows southward between the surface and 250 meters; its transport across a transect along 4°N and within 200 km of the coast accounts for at least 20% of the net meridional geostrophic transport (northward) of 0.6–8.7×106 m3 sec−1 within 400 km of the coast. Meridional Ekman transport across the same transect is about one-tenth the geostrophic transport. Upwelling due to geostrophic divergence is typically 13×10−4 cm sec−1 across the base of the surface layer (25 meters), whereas upwelling produced by the curl of the wind stress across the same surface is about 50×10−4 cm sec−1. Upwelling caused by the meridional Ekman transport is usually 10% or less of the wind stress curl. The resultant upwelling from geostrophic and wind stress divergence and from the wind stress curl is largest in two locations: in the center of the bight and along the eastern half of the gulf entrance, where both rates are about 45×10−4 cm sec−1.

Breaking of wind waves and the sea surface wind stress

Abstract: In the conventional treatment of the coefficient of sea surface wind stress by plotting it against 10-m wind speed, there are inevitable discrepancies among results of various investigators. The reason is considered to lie primarily in the fact that the state of the sea surface or of waves is disregarded, which may have great influence on the sea surface wind stress. Former concepts concerning the conditions which control the sea surface wind stress are discussed, and it is shown that a more universal expression may be obtained by plotting the coefficient against a kind of roughness Reynolds number:Re 2 *=u*H/ν, whereu* is the friction velocity of air, ν the kinematic viscosity of air, andH the characteristic wave height.H is used here to treat some data in wind-wave tunnels, as a tentative variable, one step towards a more rigorous approach to the problem. This variableRe 2 *, orRe 4 *=u *w /L/v w =2πgu *w /v w n 1, where the subscript ω represents values for water,L andn 1 the characteristic wave length and frequency, respectively, is also the condition describing the air entrainment or the breaking of wind waves. In this case, these Reynolds numbers are interpreted as the quantity describing the intensity of turbulence of the water surface itself. It is shown, using data from our wind-wave tunnel experiments, that the breaking commences asRe 2 * reaches 1×103, or asRe 4 * reaches 3×103. Simultaneously, the stress-coefficient begins to increase sharply at this value ofRe 2 *. This phenomenon is understood as an increased momentum transfer from the air to the water through “boundary penetration of turbulence” caused by the breaking of wind waves. Further, it is suggested that there is a possibility that this excess momentum transfer does not increase wave momentum, but reinforces drift current.

Preliminary results from a numerical multilayer model for the circulation in the North Atlantic

Abstract: A 14-layer model of the general circulation in the North Atlantic Ocean is presented. The model is bounded by the coastline and by latitude circles at 57.5°N and 17.5°S. This area is covered by a grid system of 3° horizontal resolution. The actual topography is accounted for to the extent possible with this resolution. Climatological fields of temperature, salinity and wind stress are used as boundary conditions at the sea surface. These fields are fitted to an annual variation approximated by the first harmonic only. At the two open lateral boundaries temperature and salinity are fixed in time according to observations, but a weaker condition is used here on the velocity field to allow implicitly for an interaction with the adjacent ocean. Starting from an initial state at which the ocean is at rest and uniformly stratified, an initial-and boundary-value problem is solved numerically until the circulation and the interior mass field approximately are in equilibrium with the boundary conditions. The ‘ESSA Box Model” is applied in the numerical integration (K. Bryan [1969]). Selected numerical results are compared with observed data. The model reveals a remarkable topographic control of the circulation. At present the integration is being continued on a 1°-grid. A final analysis and discussion of the results will be based on that model and will be published in a further paper.

Wind‐Wave Interactions

Abstract: A criterion proposed earlier for determining airflow separation from wind waves and a previously determined relationship between surface roughness and wind stress are herein correlated. The results indicate that the airflow boundary layer is developed due to the form drag provided by wavelets (surface roughness) superimposed on big waves. Based on the magnitudes of surface roughness, viscous‐sublayer thickness and critical height, the mechanisms of wind‐wave generation are discussed.

Observations of Reynolds Stresses in wind waves

Abstract: Data are presented concerning Reynolds Stresses in wind waves obtained from time series records of horizontal and vertical velocity components of motion beneath the ocean free surface. The stresses, of the order of 25 dyne cm−2, are generally positive indicating horizontal momentum transfer downward through the dynamic wind wave regime. The magnitude of the observed stress increases with wind speed and sea state. The co-spectra show strong negative peaks which appear at the ambient wave frequencies and indicate that the correlations or eddy stresses of the gross wave motions are responsible for the momentum flux. This is a corroboration of results reported previously by the writer in this journal.

Diurnal Variation of Onshore Wind Speed Near a Coastline

Abstract: Surface wind data taken from 19 km offshore to 14 km inland during several days of onshore wind occurrence were reduced and analysed. The mean kinetic enemy per unit mass and its changes normal to the coastline were computed directly from the wind data. Analysis of these data shows two well defined regimes of diurnal variation in wind speed. The marine air has a nighttime speed maximum and a daytime minimum. As the air moves inland, the speed distribution becomes bimodal with the primary maximum occurring in the daytime and the secondary maximum at night. As expected Intuitively and predicted by theory, the speed changes most abruptly near the change in surface roughness (the coastline). Also as predicted by theory, complete dynamic equilibrium with the new lower boundary is not achieved until the air is ∼5–12 km downwind from the coastline.

Structure of Wind and the Shearing Stress in the Planetary Boundary Layer

Abstract: Analysis of the “Leipzig Wind Profile” shows close agreement between the direction of the wind and of the shearing stress throughout the planetary boundary layer. A formulation is developed, based on this quasi-parallelism and a boundary condition at the top of the Ekman layer, to relate the pressure gradient, the wind structure and the shearing stress throughout the layer. The predicted relationships are found to agree well with the Leipzig observations.

Computation for Circulation in Stratified Lakes

Abstract: A mathematical method is developed for calculating the steady-state, three-dimensional, wind-driven circulation in a large stratified lake of arbitrary bottom topography and shape on a rotating earth. The stratification is modeled by a two-layer structure in which the hypolimnion and the epilimnion are considered as two distinct layers with different densities and eddy viscosities. The thermocline location responds sensitively to the applied wind stress and has an important effect on the circulation velocity. An off-shore wind is the primary cause of the cold water up-welling near the shore due to the movement of the thermocline. The surface current velocity generally has a smaller magnitude and greater deflection angle than in the homogeneous case except near the boundaries. The volume transport is strong and in the downwind direction near the shore in the epilimnion and near the deep center in the hypolimnion.

Water Surface Velocities Induced by Wind Shear

Abstract: A theoretical model is developed for calculating the water surface drift velocity which arises when, in the absence of water surface waves, a turbulent shear stress is exerted by wind on the surface of an originally standing body of water. Solutions are given for steady flow in both air and water, for the asymptotic cases of very long fetches (flow along a channel of water) and very short fetches (offshore wind onto deep water). As long as the flow in the water remains laminar, the drift velocity is correctly predicted by the theory and is found to agree with experimental results of Keulegan (4) and Plate, et.al. (10). For turbulent flows, the fetch dependency of the surface velocity is correctly predicted, but the model fails to properly account for the dependency of the drift velocity on the average parameters of the air flow.

Monthly charts of surface wind stress curl over the indian ocean

Abstract: The surface wind stress curl is the forcing function in the equations of vertically integrated water transport of wind-driven ocean currents. Hence, it has become a basic quantity in theoretical oceanography. As the time dependence of all important surface quantities in the Indian Ocean is stronger than in other oceans, it is valuable to look particularly at the time variation in this region. This study presents monthly charts of the wind stress curl at the surface of the Indian Ocean from its land boundaries up to 50° S. and from 20° E. to 116° E. Basic data were the monthly surface maps of the Koninklijk Nederlands Meteorologisch Instituut, derived from ship observations and given as 2° square means of the surface wind. The processing of the data is described in detail. In particular, small-scale fluctuations are objectively filtered out. While earlier compilations are usually on a coarser grid (seasonal and 5° square averages), the present data have a refined time and space resolution. Therefo...

Modelling Of Flows, Sea Level Variations And Bottom Stresses In TheCoastal Zone Of West Estonia

Abstract: Flow features and sea level variations were studied in the nearly tideless subbasins of the Baltic Sea, the Gulf of Riga and the Vainameri, using a highresolution (of order 1 km grid) shallow sea 2D hydrodynamic model. The model is forced by the wind stress calculated from single-point meteorological data, and the tide gauge sea level data applied along the open boundaries. The simulations based both on the realistic and idealistic forcing schemes were carried out. The aim of the study was to analyse circulation patterns during different meteorological conditions and to investigate the influence of currents, waves and sea level fluctuations on the littoral processes. In the Parnu Bay the high flow velocities, participating (along with storm waves) in the coastal erosion events, act 1.5-2 m above the average waterline and about half of the year's summary work is do~e durins the 3-3 nos: stormy days of the year. In the indented West Estonian coast infrequent storm surges flush the eutrophic ends of the narrow and shallow bays, e.g. the Matsalu Bay, but low wave activity and weak currents cannot either remove muddy deposits of the delta region or *(van. import sandy deposits from the nearby Vainameri rc,'

On the Mean Path of a Buoyant Chimney Plume in Non-Uniform Wind

Abstract: Based on the turbulent entrainment hypothesis of Morton et al. and the eddy diffusivity hypothesis, approximate theories have been developed for the rise of a buoyant chimney plume in the atmospheric boundary layer, incorporating wind shear effects by assuming a power law type of wind velocity profile, U∝Zp, for the boundary layer. The two theoretical approaches suggest that the mean path of a hot plume in a neutral atmospheric boundary layer can be represented by a power law Z*∝X*n,where the exponent n is related to the wind velocity profile exponent p by n=⅔(1+p), and is less than the value of ⅔ reported in the case of uniform wind. Wind tunnel test results show fair agreement with the theoretical results.

Vertical Wind Component Estimates up to 1.2 km above Ground

Abstract: Vertical wind components were computed up to 1.2 km from 37 wintertime and 10 summertime balloon observations between 0900 and 1200 local time utilizing the accurate and high-resolution cinetheodolite/Jimsphere system. The mean ascent rate of the Jimsphere was computed from all observations taken on a particular day. The ascent rate was found to be 5.16 m sec−1 for the winter and 5.10 m sec−1 for the summer months. The individual variations of a given observation from the mean ascent rate were assumed to be the vertical component. Variations in balloon ascent caused by variation in drag, anomalous variation in atmospheric density, balloon response to the wind, and aerodynamically induced motions are discussed. Vertical wind components ranged from 10–25 cm sec−1 in a stable atmosphere and 55–100 cm sec−1 under unstable conditions depending on wind speed.

The coastal boundary layers of a lake when horizontal and vertical Ekman numbers are of different orders of magnitudes

Abstract: We investigate the linear hydrostatic boundary layers which bring the horizontal motion to rest at the shore of a lake of horizontal extent L and depth H containing a homogeneous fluid with constant, though vastly differing, horizontal and vertical eddy viscosities, A H and A v . The lake rotates with angular speed f /2 about a vertical axis and the motion is induced by a steady, nonuniform wind stress. Our results are applicable when ( H / L ) 2 ? ( A v / fH 2 ) 1/2 ( A H / fL 2 ) ? ( A v / fH 2 )3 < < 1. It is found that under these conditions the flow near the shore is determined by the local wind stress and the local value of the interior velocity parallel to the coast. It is found that under certain wind stress conditions there is a region of flow, adjacent to the shore, in which the fluid is effectively confined to the coast. DOI: 10.1111/j.2153-3490.1970.tb00526.x

Fluctuations of horizontal wind speed near the ground in convective conditions

Abstract: Simultaneous records of temperature fluctuations and wind speed at several heights have been obtained with instruments suspended from captive balloons flying 225 m above level, uniform land. Variations in the horizontal wind speed near the ground appear to be associated with bursts of temperature fluctuation at heights of more than about 100 m in a way which suggests that convective plumes, or ‘thermals,’ become organized within the lowest few metres of the atmosphere.

Study Of Atmospheric Dispersion Under Low Wind Speed Conditions

Abstract: This paper presents the analysis of atmospheric SF6 tracing experiments carried out under low wind speed conditions associated with strong thermal stability on a flat terrain. Analysis of meteorological data, processed in the form of probability densities, reveals that unsteady character of the current increase the greater the stability. The spectral densities of the three wind components deduced from sonic anemometers show that the horizontal current is dominated by large-scale turbulent structures, and that there is a weakening of energy levels in the neighborhood of 0, 01 Hz. Tracings confirm that the levels of ATC (Atmospheric Transfer Coefficient) are greater the stronger the stability. Release impact is as closely linked to the increase in pollutant residence time as to higher instantaneous concentration levels. Lastly, ATCs deduced from a gaussian model being developed at IPSN gives good agreements with the experimental values.