Spectral gap characteristics in a daytime valley boundary layer

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 143; no. 707; pp. 2509 - 2523
• Main Authors: Babić, Nevio, Večenaj, Željko, De Wekker, Stephan F. J.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01-07-2017; Wiley Subscription Services, Inc
• Subjects: Anemometers; Atmospheric boundary layer; complex terrain; convective boundary layer; Data processing; Daytime; Fourier transforms; gap scales; Lee waves; Mountain waves; multiresolution flux decomposition; Periodicity; Perturbations; Rotors; Slope; Slope winds; Spatial variability; Spatial variations; spectral analysis; upslope flows; Valleys; Wind; Wind speed; Winds
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.3103

A correct estimation of turbulent variances and covariances in the atmospheric boundary layer relies on the determination of turbulent perturbations of wind speed components and scalar quantities, which requires the presence of a so‐called spectral gap. The goal of this work is to determine the range of gap scales necessary to define turbulent perturbations in a daytime valley boundary layer. To accomplish this, we analyze data from a large number of propeller‐vane and sonic anemometers using the fast Fourier transformation and the multiresolution flux decomposition. Daytime gap scales are found to range from 17 to 29 min and show large spatial variability across the valley floor and the adjacent slopes. Synoptically driven conditions that favour the occurrence of mesoscale phenomena, such as rotors and mountain waves, shift daytime gap scales toward longer periods. The low‐frequency end of the gap is also affected by the presence of slope winds that are characterized by a periodicity ranging from 80 to 200 min. Finally, we present a conceptual model of the daytime valley spectral gap which summarizes the findings of this study.
As contributions to the field of boundary‐layer meteorology and mountain meteorology over complex terrain, we have: gained insight into the spatial sensitivity of gap scales in a valley; presented an innovative use of the multiresolution flux decomposition method, by applying it to propeller‐vane data; developed a genuine way of looking at and detecting upslope flows in a valley; and proposed an expanded conceptual view of the daytime spectral gap in a valley.