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Evaluation of Local Similarity Theory for Stable Conditions near Kutina, Croatia

In this study we investigate the applicability of Nieuwstadt's (1984) local scaling formulation for the stable boundary layer using the data from a 62 m tall tower (levels 20, 32, 40, 55 and 62 m above ground) within a small forest patch situated in the vicinity of town Kutina in heterogeneous terrain with agricultural land, forested hills, industrial and urban surfaces in different upwind sectors. We only analyze periods with stable stratification from winter 2008/2009 focusing on the influence of different wind directions, accounting for different surface characteristics, and different wind regimes. The large-scale forcing was classified into three wind regimes, namely, weak, intermediate and strong winds. 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. 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. 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. 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 it is found that local scaling is promising even over highly non-homogeneous terrain as in our case.

stable boundary layer; local scaling; roughness sublayer; flux-variance similarity; flux-gradient similarity

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