Naslov
Evaluating local similarity scaling in the stable, wintertime boundary layer influenced by complex topography

Autori
Babić, Karmen ; Rotach, Mathias W. ; Bencetić Klaić, Zvjezdana

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
33rd International Conference on Alpine Meteorology

Mjesto i datum
Innsbruck, Austrija, 31.08.2015. - 04.09.2015

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Stable boundary layer; Local scaling; Flux-gradient similarity; Heterogeneous terrain

Sažetak
Due to the development (or presence) of internal boundary layers the turbulent characteristics in the lowest meters over heterogeneous surfaces are more complex than above horizontally homogeneous and flat terrain. In this study we investigate the applicability of Nieuwstadt's (1984) local scaling formulation for the stable boundary layer. The data for this study stem from a tall tower within a small forest patch situated in heterogeneous terrain with agricultural land, forested hills and urban surfaces in different upwind sectors. The 62 m tower (levels 20, 32, 40, 55 and 62 m above ground) was situated in the middle of some 120 m x 480 m area of 18 m high trees. Here we only analyze periods with stable stratification from winter 2008/2009 focusing on the influence of the upwind hilly terrain. As turbulent fluxes showed a substantial variation with height, we adopt local scaling approach for which similarity functions and the local stability parameter are based on local fluxes at measurement height. In the data analysis the role of self-correlation is examined. Also an in-depth error analysis applying the Filtering Method allows to estimate the percentage of random errors in the turbulence variables. Using this method it is possible to inspect if the scatter in the measurements is caused by random errors, or by other dynamical factors of the stable boundary layer. Values of scaled standard deviation for wind components in near-neutral conditions are found to be lower at the lowest measurement level and higher at upper levels in comparison to canonical Kansas values. The non-dimensional gradient of wind velocity is also investigated. We compare our results with traditional linear equations for the stable case and also to the empirical non-linear expression proposed by Beljaars and Holtslag (1991). It is found that the stability function for momentum supports the linear equation only up to values of the non-dimensional stability parameter ζ ≈ 0.5. Moreover, we find good agreement between our results and the Beljaars and Holtslag function, which increases more slowly with increasing stability. As we observed more strongly stable stationary cases, local z-less scaling is also addressed. Our analysis supports the validity of z-less stratification for very stable conditions (ζ > 1) for scaled wind velocity variances and correlation coefficients for momentum and heat flux. As a preliminary conclusion, based on scaled velocity variances and non-dimensional mean wind shear, it is found that local scaling is promising even over highly non-homogeneous terrain as in our case.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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