Threshold Wind Speed Research Articles

Wind erosion climatic erosivity

A physically based wind-erosion climatic factor has been derived: $$CE = \rho \int {[u^2 } - (u_T^2 + \gamma ^l /\rho a^2 )]^{3/2} f(u)du$$ where ϱ is the air density, a is a constant made up of other constants (von Karman, height of wind speed observation, roughness parameter), u is the horizontal wind speed, u T is threshold wind speed, f(u) u) is a wind speed probability density function, and γ is the cohesive resistance caused by water on the soil particles. Cohesive resistance is proportional to the square of water content relative to water content at −1500 J kg−1. Relative water content is approximated from the Budyko dryness ratio and the Thornthwaite PE index with similar results. CE is calculable from wind speed and other generally available meteorological data, and is usable in the wind erosion equation without some of the limitations of a previously used wind erosion climatic factor.

Diurnal variation of tree pollen in the air in finland

Abstract The pollen was collected with Burkard spore traps in Turku and Jyvaskyla, in southern and central Finland. Selected days of high concentration were analysed for hour to hour concentrations. The diurnal variation of pollen concentration was variable in Alnus, Betula, Juniperus and Pinus, while Picea, Populus, Quercus, Salix and Ulmus had regular daytime maxima och nightly minima. The pollen concentrations were closely correlated with increasing temperatures and decreasing relative humidities. A significant relationship with wind speed was also found, except in Alnuts and Betula. The most significant relationships with wind speed were found in Salix and Quercus, the pollen of which tends to remain in lumps. Generally, the bigger the pollen grains are, the more their concentrations are dependent on wind speed. Pinus and Picea seem to have a threshold wind speed about 4 m/s above which concentrations become higher.

FLIGHT ACTIVITY OF PLUTELLA XYLOSTELLA (L.) (LEPIDOPTERA: YPONOMEUTIDAE)

AbstractThe flight activity of P. xylostella in Victoria was studied with the aid of suction traps during 1971–74.Flights occurred all the year round with the peaks in the aerial populations coinciding with those of crop populations. There were no indications of discrete generations. The aerial populations were dominated by males, but a large influx of migrant moths in September, October 1973 increased the population of female moths in the area.Onset of flight occurred during the 2 h period prior to sunset, with male and female flight activities peaking at 2–4 and 4 h after sunset respectively. Flights ceased several hours after sunrise, but males had a longer flight period (19 h) than females (16 h). The light thresholds for flight occurred at 1 h after sunset and 1 h before sunrise. The lower temperature threshold was 7 °C and the upper threshold appeared to be above the maximum of 26 °C recorded in this study. At wind speeds of 0.8–2.2 ms‐1 flights decreased with increasing wind speed. The lower wind speed threshold was below 0.4 ms‐1 and the upper threshold was determined as 1.7 ms‐1.A lunar periodicity of flight activity was demonstrated with maximum flights occurring at full moon. There were 2 minor flight peaks, a few days before and after the day of new moon.

Ambient airborne solids concentrations including volcanic ash at Hanford, Washington sampling sites subsequent to the Mount St. Helens eruption

A major eruption of Mount St. Helens occurred on May 18, 1980. Subsequently, airborne solid concentrations were measured as a function of time at two sites within the southern edge of the fallout plume about 211 km east of Mount St. Helens. This ash was a source for investigating area‐wide resuspension. Rain had a variable effect on decreasing airborne concentrations from resuspension. From 0.5 to 1.5 cm of rain were required to significantly reduce airborne solid concentrations through July. For a more aged resuspension source in September, a rain of 2.0 cm had a negligible effect. A monthly average threshold‐wind speed for resuspension was defined as 3.6 m/s. For monthly‐average wind speeds less than the threshold wind speed, monthly‐average airborne concentrations tended to decrease with time. A decrease was recorded between September and October. For this 4‐month time period, the half‐life was on the order of 50 days, corresponding to a weathering rate of 5.1 year−1.

Dunes on the skeleton coast, Namibia (South West Africa): Geomorphology and grain size relationships

AbstractSimple, and locally compound, transverse and barchanoid dunes dominate the 2000 km2 Skeleton Coast dunefield in northwestern Namibia/South West Africa. Dune height and spacing are closely correlated (r = 0‐89) and decrease across the dunefield from southwest to northeast, with an accompanying change from transverse to barchanoid ridges and ultimately barchans.The dunes are aligned transverse to the dominant strong south and south southwest onshore winds. Alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune‐forming winds to swing to the right over the dunes, but resume their original direction beyond.Grain size and sorting vary at three scales: the dune, the dune landscape and through the dunefield. Overall the sands, derived from three localities by deflation from beaches supplied by vigorous longshore drift, become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing.A statistically significant relationship (r = −0−65) was established between dune spacing and the phi grain size of the coarser fraction of the dune sands, demonstrating the importance of the protective effects of coarse grains, and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand.

Properties of blowing snow

The size and shape of windblown snow particles determine not only the mass transported by turbulent fluxes but also the rate of phase change from ice to water vapor that occurs in this multiphase flow. These properties and particle densities dictate particle fall velocity and therefore the vertical distribution of mass and surface area, which strongly influence the gradients and fluxes of sensible heat and water vapor within the transport layer. Initial movement at the snow surface depends more on availability and impact forces of loose particles than on aerodynamic drag. Cohesion between surface particles and particle restitution coefficient are important properties that determine threshold wind speeds for snow transport. Threshold speeds for blowing snow vary over such a large range in nature that formulations predicting transport rate as a function of wind speed should include threshold speed as a parameter. The expression derived by Iversen et al. (1975) is compared with low‐level snow transport in the atmospheric boundary layer. Self‐similarity of wind profiles in blowing snow is a property of the flow that has been exploited for scale modeling of snow deposition around obstacles, both outdoors and in wind tunnels. Good quantitative results are obtained by careful attention to similitude requirements.

Local Mars dust storm generation mechanism

On Ls = 340°, first Mars year of Viking on the surface, a local dust storm was observed at the Viking Lander #1 site by Viking Orbiter A. The storm lasted less than one martian day (sol) with the dust raised affecting the site for about three sols. It is concluded that this storm was caused by baroclinic waves and that the threshold wind speed for saltation was 25‐30 m sec−1.

Estimates of threshold windspeed from particle sizes in blowing snow

Particle size distributions measured in the saltation layer of drifting snow are fitted reasonably well by the two-parameter gamma function. Using these data, the mean particle size 0.25 m above the surface is predicted as a function of windspeed, from the theoretical relationship developed by Budd (1966). Average particle diameters, measured with a photoelectric snow particle counter at a site in southeastern Wyoming, U.S.A., during drifting of moderate intensity, are larger than mean sizes predicted by theory. These experiments provide empirical equations that allow threshold windspeeds for snow transport to be estimated by monitoring an anemometer and particle counter.

Threshold windspeeds for sand on Mars: Wind tunnel simulations

Wind friction threshold speeds (u*t) for particle movement (saltation) were determined in a wind tunnel operating at martian surface pressure with a 95 percent CO2 and 5 percent air atmosphere. The relationship between friction speed (u*) and free‐stream velocity (u∞) is extended to the critical case for Mars of momentum thickness Reynolds numbers (Reθ) between 425 and 2000. It is determined that the dynamic pressure required to initiate saltation is nearly constant for pressures between 1 bar (Earth) and 4 mb (Mars) for atmospheres of both air and CO2; however, the threshold friction speed (u*t) is about 10 times higher at low pressures than on Earth. For example, the u*t (Earth) for particles 210 µm in diameter is 0.22 m s−1 and the u*t (Mars, 5 mb, 200 K) is 2.2 m s−1.

Threshold Wind-Speeds and Elastic Impact in Snow Transport

AbstractCohesive forces are added to the analysis of forces on sand and soil particles to show that fluid drag, alone, often cannot initiate movement of a snow surface. The impact force of saltating ice spheres, however, can easily provide the force to break cohesive bonds, according to these calculations. The argument suggests a balance between the distribution of bond strengths of exposed surface particles and the distribution of saltation trajectory heights.

Threshold Wind-Speeds and Elastic Impact in Snow Transport

AbstractCohesive forces are added to the analysis of forces on sand and soil particles to show that fluid drag, alone, often cannot initiate movement of a snow surface. The impact force of saltating ice spheres, however, can easily provide the force to break cohesive bonds, according to these calculations. The argument suggests a balance between the distribution of bond strengths of exposed surface particles and the distribution of saltation trajectory heights.

Tests with a portable wind tunnel for determining wind erosion threshold velocities

A portable open-floored wind tunnel was used to develop threshold wind speeds over two pebble covered desert soils and a sandy agricultural soil.

A Comparison of Aerosol and Momentum Mixing in Dust Storms Using Fast-Response Instruments

Fast-response light scattering measurements at two heights during a Texas dust storm are combined with horizontal and vertical wind data to derive and compare aerosol flux estimates using three techniques. The major result of this study is that a relative equivalence exists between the fine-particle (0.1 μm < radius < 1 μm) exchange coefficient and the eddy viscosity of the wind. The data also shed some light on the complex dependence of wind speed threshold for suspension and aerosol flux in high winds for different surface conditions and soil types. These results show the value of the experimental technique to studies of toxic particulate suspension and deposition by wind.

Estimates of the wind speeds required for particle motion on Mars

We have obtained estimates of the threshold wind speed V gt near the top of the atmospheric boundary layer on Mars and of the rotation angle α between this wind velocity and the direction of the surface stress. this calculation has been accomplished by combining wind tunnel determinations of the friction velocity with semi-empirical theories of the Earth's atmospheric boundary layer. Calculations have been performed for a variety of values of the surface pressure, ground temperature, roughness height, boundary layer height, atmospheric composition atmospheric stability, particle density, particle diameter, and strength of the cohesive force between the particles. The curve of threshold wind speed as a function of particle diameter monotonically decreases with decreasing particle diameter for a cohesionless soil but has the classical U shape for a soil with cohesion. Observational data indicate that the latter condition holds on Mars. Under “favorable” conditions minimum threshold wind speeds between about 50 and 100m/sec are required to cause particle motion. These minimum values lie close to the highest wind speeds predicted by general circulation models. Hence, particle motion should be an infrequent occurence and should be strongly correlated with nearness to small topographic features. The latter prediction is in accord with the correlation found between albedo markings and topographic obstacles such as craters. For equal wind speeds at the midpoint of the boundary layer, particle movement occurs more readily in general at night than during the day, more readily in the winter polar areas than the equatorial areas noon, and more readily for ice particles than for silicate particles. The boundary between saltating and suspendable particles is located at a particle diameter of about 100 μm. This value is close to the diameter at which the V gt curve has its minimum. Hence, the wind can set directly into motion both saltating and larger-sized suspendable particles, but dust-storm-sized particles usually require impact by a saltating particle for motion to be initiated. Albedo changes occur most often in regions containing a mixture of dust-stoorm-sized particles and saltating particles. The threshold wind speed for surfaces containing large, nonerodible roughness elements can either be larger or smaller than the value for surfaces with only erodible material. The former condition for V gt holds when the roughness height z 0 is less than about 1 cm and may be illustrated by craters that have experienced less erosion than their environs. The latter condition for V gt may be partly responsible for albedo changes detected on the elevated shield volcano, Pavonis Mons. Values of the angle α generally lie between 10 and 30°. These figures place a modest limitation on the utility of surface albedo streaks as wind direction indicators.

Height variation of wind speed and wind distributions statistics

For use in various wind engineering applications (e.g. wind energy conversion, wind loads on structures, air pollution transport) it is desirable to have a consistent relationship by which to project height variations of both "instantaneous" (e.g. few minute average) winds and parameters of the wind speed probability distribution. The power law V2/V1 = (Z2/Z1)n is often used for height projection of wind profiles, with the exponent n sometimes taken as depending on surface conditions or on atmospheric stability. The power law profile for wind speed is shown here to be consistent with observed height variation of Weibull wind speed probability distribution functions which have been found to fit observed wind speed distributions (at least above relevant threshold wind speeds). For consistency between the wind speed profiles and the height variation of the Weibull wind speed probability distributions, it is necessary only that the exponent n vary as n = a + b ℓn V1, where a and b are constants whose values depend on the reference height at which wind speed V1 is measured. For a reference height of 10 m, it is found that a = 0.37 and b = −0.0881 (with V1 in m/s) adequately describes both the observed height variation of wind speed and wind speed probability distributions.

Flight thresholds and seasonal variations in flight activity of the light-brown apple moth, Epiphyas postvittana (Walk.) (Tortricidae), in Victoria, Australia.

The flight activity of Epiphyas postvittana was studied at two sites near Melbourne with the aid of suction traps, over a period of 4 years. Maximum numbers were found to fly during the period September to March with peak activity coinciding with the emergence of winter, spring and summer generation moths. E. postivittana is predominantly a nocturnal flier with maximum activity around 20.00-24.00 h. The lower temperature threshold of flight was 8-11°C. The upper temperature threshold varied from 20-21°C, 24-25°C and 27-28°C for the winter, spring and summer generation moths respectively. Flight was highly influenced by the prevailing wind. The lower wind speed threshold was 0.5-0.8 m-s and the upper wind speed threshold was 2.6-2.7 m-s. The relationship between wind speed and the amount of flight was non-linear, with the frequency of flights decreasing sharply with increasing wind speed. No flights occurred at wind speeds greater than 2.8 m-s. Variation in relative humidity had no influence on flight, but lack of rain favoured flight. The amount of flight activity and the amount of rainfall were negatively correlated; flights did not occur when the daily precipitation exceeded 32.5 mm, and with a precipitation exceeding 39 mm no flights could be expected. The value of these findings to pest control programmes is discussed.

Wind noise under winter ice fields

Wind blowing over a snow layer on sea ice produces noise in the water beneath as a result of a multitude of snow grain impacts on the upper boundary of the snow. These impacts are caused by the downwind flow of saltating snow grains, which extract energy from the wind. Saltation of snow grains and the noise commence at a threshold wind speed sufficient to dislodge loose snow grains at the snow-air boundary. The resulting spectrum of the noise in the 400-Hz to 20-kHz band, sensed by a hydrophone in the water below, depends on the acoustic impulse responses of the paths between the snow grain impacts and the hydrophone. As a consequence, above the threshold wind speed the shape of the underice noise spectrum at a given site will remain constant, subject to changes in the snow and ice layers; however, the noise intensity will increase with the wind speed. The application of Bagnold's (1941) theory of blown sand shows that the noise intensity increases as the cube of the wind speed above the threshold speed for saltation. Comparisons with experimental measurements of wind noise intensity versus wind speed support this dependence. The noise intensity is independent of snow grain size and depends primarily on the snow grain flow.