Fetch

About: Fetch is a research topic. Over the lifetime, 1636 publications have been published within this topic receiving 41794 citations.

Open Ocean Momentum Flux Measurements in Moderate to Strong Winds

Abstract: Measurements of the momentum flux were made by the Reynolds flux and dissipation methods on a deep water stable tower operated by the Bedford Institute of Oceanography, A modified Gill propeller-vane anemometer was used to measure the velocity. Drag coefficients from 196 Reynolds flux measurements agree well with those reported in Smith (1980) based on independent observations at the same site. Based on 192 runs, a comparison of the dissipation and Reynolds flux results shows excellent agreement on average, for wind speeds from 4 to 20 m s−1. The much more extensive dissipation data set (1086 h from the tower and 505 h from the weathership PAPA, CCGS Quadra) was used to investigate the dependence of the drag coefficient on wind speed, fetch and stability. The drag coefficient reduced to 10 m height and neutral conditions (CDN), is independent of stability and fetch (for fetch/height ≳800) but increases with wind speed above 10 m s−1. Some time series of the momentum flux and drag coefficient are ...

Wind stress on a water surface

Abstract: The vertical distribution of horizontal mean wind in the lowest 8 metres over a reservoir (1·6 km × 1 km) has been measured using sensitive anemometers freely exposed from a fixed mast in water 16 m deep, the fetch being more than 1 km. The resulting profiles are closely logarithmic, the small differences being systematic and possibly due to the thermal instability which existed when the measurements were made. The usual law for wind profiles in neutral stability is where u is the wind speed at height z, k is von Karman's constant, log z (0) the intercept on the log z axis, and u* the so-called friction velocity defined by τ0 = pu, τ0 being the surface drag and rH the density of the air. To characterize the profiles u*/k, their slope, was plotted in relation to z (0), their intercept; this allowed a direct comparison with other profiles, in particular those recently measured in a laboratory channel by Sibul. The agreement was better than expected and indicated that z (0) was comparatively independent of fetch and stability but was largely determined by u*. The relation between u* and z (0) agreed roughly with the simplest non-dimensional relation between them, gz (0)/u = constant, so that one is led to a generalized wind profile for flow over a water surface which specifies the drag, given the wind at one known height. An approximate value of the constant is 12·5. This expression can be compared with earlier work. The better wind-profile observations show rough agreement; the experimental scatter is necessarily large since a water surface is aerodynamically much smoother than most land surfaces, precision anemometry in difficult circumstances being required to provide sufficiently precise values. Oceanographic measurements of the tilt of water surfaces are in fair agreement at high wind speeds but at low wind speeds the data are conflicting. The early results which imply that the drag-coefficient (u/u2) increases with decreasing wind speed in light winds are thought to be in error; some support for this belief comes from recent estimates of drag using a modified ageostrophic technique, which agree roughly among themselves and with the general expression.

Review of Drag Coefficients over Oceans and Continents

Abstract: Observations of wind stress and wind profiles over the ocean reported in the literature over the past 10 years are consistent with Charnock's (1955) relation between aerodynamic roughness length (z0) and friction velocity (u*), viz, z0= αu*2/g, with α= 0.0144 and g= 9.81 m s−2. They also imply a von Karman constant = 0.41±0.025. For practical purposes Charnock's relation may he closely approximated in the range 4&

Spectral and statistical properties of the equilibrium range in wind-generated gravity waves

Abstract: Recent measurements of wave spectra and observations by remote sensing of the sea surface indicate that the author's (1958) conception of an upper-limit asymptote to the spectrum, independent of wind stress, is no longer tenable. The nature of the equilibrium range is reexamined, using the dynamical insights into wave–wave interactions, energy input from the wind and wave-breaking that have been developed since 1960. With the assumption that all three of these processes are important in the equilibrium range, the wavenumber spectrum is found to be of the form , where p ∼ ½ and the frequency spectrum is proportional to u*gσ−4. These forms have been found by Kitaigorodskii (1983) on a quite different dynamical basis; the latter is consistent with the form found empirically by Toba (1973) and later workers. Various derived spectra, such as those of the sea-surface slope and of an instantaneous line traverse of the surface, are also given, as well as directional frequency spectra and frequency spectra of slope.The theory also provides expressions for the spectral rates of action, energy and momentum loss from the equilibrium range by wave-breaking and for the spectrally integrated rates across the whole range. These indicate that, as a wave field develops with increasing fetch or duration, the momentum flux to the underlying water by wave-breaking increases asymptotically to a large fraction of the total wind stress and that the energy flux to turbulence in the water, occurring over a wide range of scales, increases logarithmically as the extent of the equilibrium range increases. Interrelationships are pointed out among different sets of measurements such as the various spectral levels, the directional distributions, the total mean-square slope and the ratio of downwind to crosswind mean-square slopes.Finally, some statistical characteristics of the breaking events are deduced, including the expected length of breaking fronts (per unit surface area) with speeds of advance between c and c+dc and the number of such breaking events passing a given point per unit time. These then lead to simple expressions for the density of whitecapping, those breaking events that produce bubbles and trails of foam, the total number of whitecaps passing a given point per unit time and, more tenuously, the whitecap coverage.

Representative roughness parameters for homogeneous terrain

Abstract: Objective requirements are drawn up for experimental determination of the roughness of homogeneous terrain, particularly with respect to matching of the observation array to a terrain situation with given fetch, blending height and displacement length. Some fifty well-documented published experiments over various surfaces, ranging from mobile surfaces (sea, or moving sand or snow) to forests and towns, are shown to meet these criteria and are compiled. It is shown that most presently popular terrain reviews underestimate actual terrain roughness lengths by about a factor two.