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Ann-Sofi Smedman

Abstract

Some coherence measurements, taken at the Näsudden wind energy test site, for small separation distances at a height of 65–77 m, are analyzed in terms of a theoretical coherence model for isotropic turbulence, and for Davenport similarity.

The decay parameters for the horizontal wind components are found to be about the same for vertical and horizontal separation in the inertial subrange.

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Ann-Sofi Smedman

Abstract

Atmospheric boundary layer measurements during stable and near neutral condition from seven sites in different kinds of terrain have been analyzed in order to find relationships among turbulence parameters.

The shape of the spectral and cospectral distributions turned out to be well represented by the universal expressions found for ideal sites.

For near neutral conditions in the surface layer σ w /u * increases and σ u /u * decreases with height. These results agree with previous result found in the literature for both atmospheric and some wind tunnel data.

The ratio σ w /σ u first increases and then decreases as stability increases in the surface layer. This can be interpreted as a combined effect of stability and the presence of the ground. In a stable layer well above the ground σ w /σ u decreases monotonically with stability, in accordance with laboratory measurements and numerical simulations of free shear flow.

The ratio / was also investigated. For near neutral conditions the ratio scatters around 0.2 with no apparent variation with height or roughness. The stability dependence of this quantity is very similar to that of the ratio of the vertical to the horizontal standard deviation in the surface layer which first increases and then decreases with increasing stability. Its neutral value in a free shear flow layer is 0.8, decreasing rapidly with stability, in agreement with laboratory data.

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Ann-Sofi Smedman
and
Hans Bergström

Abstract

A complex flow situation over a frozen lake during winter in the northern part of Sweden during 1987 and 1989 has been studied. The high mountains on the order of 1100 m surrounding the lake dominate the flow, which can be characterized by three regimes. For very stable stratification (Froude number less than 1) blocking occurs on the upwind slope of the mountains, while in the ice there are calm wind conditions. Second, for less stable stratification the wind blows down the slope of the mountains at all levels. When there is an uplifted inversion with near neutral stratification below, a recirculation flow pattern occurs in the lower part of the boundary layer. Third, for slightly stable stratification and with a pressure gradient oriented along the lake, a mesoscale flow pattern will create strong “gap winds” near the surface, preferably from an easterly direction. One case has near-surface winds of 30 m s−1. At higher levels, a turbulent wake with very low mean wind speed will be present. The wake oscillates vertically and interacts with the flow in the surface layer. The layer near the ground is thus characterized by alternating periods with high wind speed and low turbulence and with low wind speed and high turbulence.

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Ann-Sofi Smedman-Högström
and
Ulf Högström

Abstract

A method is described for determining wind speed frequency distributions at any height up to ∼200 m above ground for a meteorological station where wind speed and direction is measured at a low reference level (usually 10 m) and which reports routine meteorological data at least once every 3 h. The roughness characteristics of the terrain surrounding the station must be known in detail, because the model calculates the rate of growth of internal boundary layers resulting from discontinuities in roughness as well as the shape of the wind profile in the various layers. The rate of growth of the internal boundary layers has been determined from work by Pasquill (1972). The shape characteristics of the profile are determined as a function of roughness length and of stability by the aid of measurements from three Swedish 100 m masts. The method is successfully tested against an independent set of data from a 100 m mast. Also given are some results from application of the method to Swedish data. The method has been developed for comparatively flat rural terrain and is not valid for urban conditions and mountainous areas.

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Ann-Sofi Smedman-Högström
and
Ulf Högström

Abstract

A series of measurements of wind, temperature and humidity from several heights, covering periods of the order of magnitude of one day, have been subjected to spectral analysis. Those spectral parts, which lie in the “spectral gap” region, have been analyzed in terms of the Monin-Obukhov similarity theory and pieced together with high-frequency spectra, treated in the same way. The analysis indicates a definite ordering of the results according to the Monin-Obukhov dimensionless height z/L.

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Ann-Sofi Smedman
and
Knut Lundin

Abstract

Measurements of dry and wet bulb temperature fluctuations in the atmospheric boundary layer are often accomplished with sensors consisting of a relatively long resistance wire mounted on some kind of supporting frame. It is demonstrated that such a frame may product distortion of temperature fluctuations on a scale as large as 103 times the typical dimensions of the supporting structure. The mechanism for this phenomenon is not evident. It is further demonstrated that the distortion error can be entirely removed if the sensor is constructed so that there is no physical obstruction to the flow through the sensor area.

The modified sensors have been used in a recent field experiment. Analysis of the inertial subrange constant β for the temperature spectrum based on these data yield β = 0.80 ± 0.09, in good agreement with other recent observations.

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Ann-Sofi Smedman
,
Michael Tjernström
, and
Ulf Högström

Abstract

Data from a marine coastal experiment over the Baltic Sea, comprising airborne measurements and mast measurements, have been used to highlight the turbulence dynamics of a case with most unusual flow characteristics. The boundary layer had a depth of about 1200 m. The thermal stratification was near neutral, with small positive heat flux below 300 m and equally small negative heat flux above. The entire situation lasted about 6 hours. Turbulence levels were unexpectedly high in view of the fact that momentum flux was negligible (in fact positive) in the layers near the surface, and buoyancy flux was also small. The turbulence was found to scale with the height of the boundary layer, giving rise to velocity spectra having the shape of those characteristic of convectively mixed boundary layers. Analysis of the turbulence budget for the entire planetary boundary layer (PBL) revealed that energy was produced from shear instability in the uppermost parts of the PBL and was distributed to the lower parts of the PBL by pressure transport. Dissipation was found to be evenly distributed throughout the entire PBL. Without data on surface wave characteristics, no firm conclusions concerning air–sea interaction processes can be drawn, but there are clear indications that the dynamical decoupling observed at the surface is due to the effect of decaying sea state conditions (high wave age conditions). In any case, the process of active turbulence production in the layers close to the surface observed in “ordinary” near-neutral boundary layers has been effectively turned off here, leaving only turbulence of the “inactive” kind, imported by pressure transport from layers above. The results strongly support the findings reported in the recent literature on “laboratory turbulence” that the process of strong turbulence and shearing stress production near the wall of boundary layers of very different kinds is virtually independent of forcing from large-scale structures embedded in the flow.

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Ulf Högström
,
Ann-Sofi Smedman
, and
Hans Bergström

Abstract

Data from a period with intensive measurements in the lowest 1000 m of a marine atmosphere over the Baltic Sea with strongly stable stratification and a turbulent boundary layer less than 50 m deep have been analyzed with respect to the mesoscale flow regime.

Analysis of the wind data shows that horizontal wind fluctuations take place in vertically decoupled layers of the order of a few hundred meters deep. In spite of sometimes large shear, turbulence levels are shown to be very low, implying a “dissipation travel distance” of the order 1000 km.

Airborne measurements have been used to produce wavenumber spectra of the zonal and meridional components of the wind and the temperature over the spectral range 10−4κ ⩽ 1 radians m−1 and frequency spectra for the vertical wind component. Extended series of continuous wind measurements on a tower were used to derive frequency spectra of the wind, which were Taylor-transformed to wavenumber spectra, showing good agreement with the spectra derived from the airborne measurements.

The horizontal wavenumber spectra show no significant height variation for the 30–500-m layer studied. For the approximate wavenumber range 10−5κ ⩽ 10−3 radians m−1, these spectra fall off as κ −5/3, with a spectral level only about a factor of 2 lower than the corresponding Global Atmospheric Sampling Program spectra from the upper troposphere and the lower stratosphere. Vertical velocity spectra are very close in shape and spectral amplitude to corresponding spectra derived from Doppler radar measurements at several places around the world during “quiet-time conditions.” However, an important difference of the present spectra is that they show no sign of scaling with the local Brunt–Väisälä frequency. The temperature spectra are closely similar in shape to the horizontal velocity spectra, and their amplitude scale with the square of the local Brunt–Väisälä frequency. Detailed analysis of the spectra for this wavenumber range suggests that the spectra of horizontal motion reflect quasi-two-dimensional stratified turbulence, but that the vertical velocity spectra reflect wave activity.

In the wavenumber range 10−3κ ⩽ 10−2 radians m−1 the horizontal wind spectra fall off approximately as κ −9/4, which suggests that internal waves may make up this spectral region, an assertion that is also supported by application of a polarization relation valid for waves. Temperature–vertical velocity cross spectra show no signs of linear, monochromatic waves. Instead it is possible that the waves in this region are breaking, a process that has previously been suggested as a possible source for the quasi-two-dimensional turbulence.

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Ulf Högström
,
Ann-Sofi Smedman
,
Alvaro Semedo
, and
Anna Rutgersson
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Erik Sahlée
,
Ann-Sofi Smedman
,
Ulf Högström
, and
Anna Rutgersson

Abstract

Processes influencing the air–sea exchange of humidity during unstable and neutral stratification were studied using tower measurements from the island of Östergarnsholm in the Baltic Sea. For small air–sea temperature differences, the neutral exchange coefficient for humidity C EN was found to increase with increasing wind speed, attaining a value of approximately 1.8 × 10−3 at 13 m s−1. The high C EN values were observed during situations when the characteristics of the turbulence structure differed from what would be expected from traditional theory. Results from spectral analysis point to a situation in which the vertical transport of humidity is dominated by smaller-scale eddies. Quadrant analysis showed that these eddies enhance the humidity flux by bringing down drier air from layers aloft. These findings are consistent with recent analyses of the neutral boundary layer in which a change of turbulence regime has been observed. The conclusion is made that this dynamic effect accounts for the observed increase in C EN. Here, C EN was calculated using a wave-dependent normalized wind gradient, which had the effect of reducing the value by about 10% during swell relative to calculations using a non-wave-dependent normalized wind gradient.

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