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| Titel |
Daily variation of the fractal dimension of the velocity components in the turbulent surface layer |
| VerfasserIn |
M. Tijera, G. Maqueda, C. Yagüe, J. L. Cano |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063924
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| Zusammenfassung |
| The turbulence is a dominant property within the Planetary Boundary Layer (PBL). It is the
main characteristic of the mixing in the lower atmosphere since the atmospheric turbulent
fluxes are more efficient than the molecular diffusion. Turbulence can be observed in time
series of meteorological variables (wind velocity for example). The sampling rate of
observation in that time series has to be high in order to detect the turbulent regime. The
analysis of these series presents a self-similarity structure, so the wind velocity can be
considered as a fractal magnitude. This work shows a study of the fractal dimension
of the wind perturbation series u′and w′components of the wind speed. Fractal
dimension of velocity components can be related to others turbulent characteristics of
the fluxes close to the ground. Fluctuation of longitudinal and, specially, vertical
components depend on stability and, therefore, on the solar cycle. In consequence, the
behaviour of fractal dimension should be in agreement with that cycle also. These series
have been obtained once it has carried out the necessary transformation to get the
mean wind series in short intervals, namely 5 minutes, to ensure the consistent
properties of turbulence. The original records available were taken every thirty minutes
by sonic anemometers (20 Hz sampling rate) during a week of a field campaign.
The data analysed was recorded in the experimental campaign SABLES-98 at the
Research Centre for the Lower Atmosphere (CIBA), located in Valladolid province
(Spain).
It has been calculated the fractal dimension (Komolgorov capacity or box- counting
dimension) of the time series of fluctuations of the velocity component along of the mean
wind direction and the vertical component (u′ = u-U, w′ = w -W), both in the physical
spaces (velocity-time). It has been studied the time evolution of the fractal dimension during
several days and at three levels above the ground (5.8 m, 13.5 m, 32 m). The fractal
dimension of theu′ and w′ components of wind velocity series have been studied, as
well as the influence of different turbulent parameters depending on daily cycle:
turbulent kinetic energy, friction velocity, difference of temperature between the
extreme of the layer studied close of the surface (ΔT50-0.22m),etc. It has been
observed that there is a possible correlation between the fractal dimension and some of
these turbulent parameters. Finally, it has been analysed the variation of the fractal
dimension versus stability obtained from the Richardson number along of the day. |
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