Analysis of second order moments in Surface Layer turbulence in an Alpine valley

The paper presents the analysis of field measurements in the atmospheric surface layer over the floor of the Adige Valley, near the city of Bolzano in the Alps. Turbulence quantities, such as drag coefficient, displacement height and roughness length, appear similar to those reported in the literature concerning surface-layer turbulence over flat uniform terrain. The analysis of the non-dimensional standard deviations (sigma(u), sigma(v), sigma(w)) legitimates the adoption, for all the wind components, of the same Monin-Obukhov similarity relationship in the form sigma(i)/u(*) = alpha(i) (1 + beta(i) vertical bar zeta vertical bar)(1/3), originally proposed only for flat uniform terrain under steady state conditions, and the extension of this expression to the case of winds over a valley floor in slowly varying situations. The coefficients alpha(i) are very similar for along-valley and cross-valley winds, as are the beta(i) ones. Moreover the alpha(u) values are only slightly larger than the alpha(v), contrary to what is reported in the literature. Conversely, sigma(theta)/theta(*) shows distinctly different behaviour in the stable and unstable regimes. In particular, in the latter case a large scatter in the data is observed. However, since the most scattered data turn out to occur in transition periods (i.e. sunrise and sunset), after a specific data selection the best-fit parameters are more accurately estimated. In addition, emphasis is laid on the relevance of an appropriate formulation and use of a suitable recursive filter to separate the low-frequency unsteadiness of the mean flow from the turbulence signal. Copyright (C) 2009 Royal Meteorological Society.