A method to determine the characteristic time‐scales of quasi‐isotropic surface‐layer turbulence over complex terrain: A case‐study in the Adige Valley (Italian Alps)

The present paper provides a method to identify the appropriate time‐scales at which turbulence components behave quasi‐isotropically. In particular, the scales are identified on the basis of an analysis of anisotropic turbulence in the spectral domain. The definition of the spectral anisotropic tensor in terms of ogive functions, rather than of (co)spectra, allows for the evaluation of the overall degree of isotropy associated with time‐scales smaller than a given one. In this way, the time‐scale separating isotropic from anisotropic turbulence is related to the frequency at which the degree of isotropy meets a threshold value, representative of quasi‐isotropic turbulence. The procedure is tested on a dataset of wind speed components and sonic temperature collected on the floor of the Adige Valley (northeastern Italian Alps), which is adopted as a case‐study. Finally, the suitability of the estimated time‐scales is assessed in the framework of the Monin–Obukhov Similarity Theory by evaluating the agreement of the dimensionless standard deviations with the similarity functions.