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| Titel |
Scaling properties of velocity and temperature spectra above the surface friction layer in a convective atmospheric boundary layer |
| VerfasserIn |
K. G. McNaughton, R. J. Clement, J. B. Moncrieff |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 14, no. 3 ; Nr. 14, no. 3 (2007-06-11), S.257-271 |
| Datensatznummer |
250012199
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| Publikation (Nr.) |
 copernicus.org/npg-14-257-2007.pdf |
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| Zusammenfassung |
We report velocity and temperature spectra measured at nine levels from 1.42
meters up to 25.7 m over a smooth playa in Western Utah. Data are from
highly convective conditions when the magnitude of the Obukhov length (our
proxy for the depth of the surface friction layer) was less than 2 m. Our
results are somewhat similar to the results reported from the Minnesota
experiment of Kaimal et al. (1976), but show significant differences in
detail. Our velocity spectra show no evidence of buoyant production of
kinetic energy at at the scale of the thermal structures. We interpret our
velocity spectra to be the result of outer eddies interacting with the
ground, not "local free convection".
We observe that velocity spectra represent the spectral distribution of the
kinetic energy of the turbulence, so we use energy scales based on total
turbulence energy in the convective boundary layer (CBL) to collapse our
spectra. For the horizontal velocity spectra this scale is (zi εo)2/3, where zi is inversion height and εo is the
dissipation rate in the bulk CBL. This scale functionally replaces the
Deardorff convective velocity scale. Vertical motions are blocked by the
ground, so the outer eddies most effective in creating vertical motions come
from the inertial subrange of the outer turbulence. We deduce that the
appropriate scale for the peak region of the vertical velocity spectra is
(z εo)2/3 where z is height above ground. Deviations from
perfect spectral collapse under these scalings at large and small wavenumbers
are explained in terms of the energy transport and the eddy structures of the
flow.
We find that the peaks of the temperature spectra collapse when wavenumbers
are scaled using (z1/2 zi1/2). That is, the lengths of the thermal
structures depend on both the lengths of the transporting eddies, ~9z,
and the progressive aggregation of the plumes with height into the
larger-scale structures of the CBL. This aggregation depends, in top-down
fashion, on zi. The whole system is therefore highly organized, with even
the smallest structures conforming to the overall requirements of the whole
flow. |
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