Spectral gap characteristics in a daytime valley boundary layer

Abstract

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.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…Typically, the determination of the appropriate TAS is based on detecting the time scale that matches the mesoscale energy gap, i.e., the low-energy intermediate range in the kinetic energy spectrum of atmospheric motions [69]. Often, no obvious mesoscale gap can be found in the Fourier spectra of measurements from complex terrain [70][71][72]. Therefore, the determination of the TAS has to rely on more complex techniques, such as the ogive method [73], multi-resolution flux decomposition [74], or wavelet analysis [75].…”

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)

“…Babić et al. , adopted 8 h 32 min). Ogive functions, instead, are the cumulative distribution of the spectral energy from the high‐frequency range to the lower frequencies.…”

“…Finally, the time‐scale of the spectral gap was recently estimated via the multi‐resolution decomposition method by Stiperski and Calaf () for horizontally homogeneous and flat terrain, by Babić and Rotach () over a heterogeneous surface under stable stratification, and by Babić et al. () along the cross‐section of a valley. In particular, Babić et al.…”

“…In particular, Babić et al. () found that the time‐scale of the spectral gap in a valley boundary layer falls in the range 17–29 min, although it is very sensitive to the wind regime and the ground elevation.…”

Thermally-driven Mesoscale Flows and their Interaction with Atmospheric Boundary Layer Turbulence

“…Roughly in between the mesoscale and microscale, the presence of a minimum of energy or spectral gap was postulated. Subsequent observed spectra of the horizontal wind speed, temperature and turbulent kinetic energy at different emplacements close to the ground and under different synoptic and geographical conditions, have corroborated the presence of such a spectral gap (Panofsky, 1969;Hess and Clarke, 1973;Smedman-Högström and Högström, 1975;Babić et al, 2017). Not only near the surface, but Vinnichenko and Dutton (1969) observed the spectral gap away from the ground in the free atmosphere, and differing from the spectra close to the ground, measured a -5/3 slope.…”