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| Titel |
Analysis of second order moments in the Surface Layer turbulence in an Alpine valley |
| VerfasserIn |
Dino Zardi, Massimiliano de Franceschi, Mauro Tagliazucca, Francesco Tampieri |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045384
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| Zusammenfassung |
| Results from the analysis of field measurements in the atmospheric surface layer in the Adige
Valley, south to the city of Bolzano/Bozen in the Alps, are presented. Turbulence
measurements were performed in July 1999, under various weather conditions, with a sonic
anemometer in the middle of the valley floor, in a locally plain and almost horizontal area
(down-valley slope everywhere less than 0.2%), rather regularly surrounded by uniformly
high apple tree orchards. Special care was devoted to the proper choice of design
parameters of the digital recursive filter used to separate turbulent fluctuations from the
mean flow, as well as to evaluation of the rotation angles required for streamwise
alignment.
The resulting values of various turbulence quantities, such as drag coefficient,
displacement height and roughness length, appear similar to those reported in the
literature about surface layer turbulence over plain uniform terrain, provided specific
features, amenable to the complexity of the valley environment, are identified as
such.
As a main result, the analysis of the nondimensional standard deviations (Ïău, Ïăv and Ïăw)
legitimates the adoption, for all the wind components, of the same Monin-Obukhov similarity
relationship in the form Ïăi-u*=αi(1+βi|ζ|)1-3(i = u,v and w), which was originally
proposed only for plain uniform terrain under steady state conditions. This allows to include
also the case of winds over a valley floor under slowly varying situations, provided suitable
coefficients αi and βi are adopted.
It is also shown how the use of more appropriate values of the design parameters for the
digital filter, with respect to other choices proposed in the literature, significantly reduce the
scatter around the best fit curve.
On the other hand the analysis of temperature fluctuations shows two distinct behaviours
of Ïăθ/θ* in the stable and unstable regimes respectively, which are properly reproduced by
suitable expressions already proposed in the literature, although with lower values of the
coefficients than generally reported for near neutral conditions.
In particular under the latter case a large scatter in the data is observed. However the most
scattered data turns out to occur under transition periods (i.e. sunrise and sunset), so that
after a specific data selection the best-fit parameters are more accurately estimated. |
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