Abstract
The presence of waves is proven to be ubiquitous within nocturnal stable boundary layers over complex terrain, where turbulence is in a continuous, although weak, state of activity. The typical approach based on Reynolds decomposition is unable to disaggregate waves from turbulence contributions, thus hiding any information about the production/destruction of turbulence energy injected/subtracted by the wave motion. We adopt a triple-decomposition approach to disaggregate the mean, wave, and turbulence contributions within near-surface boundary-layer flows, with the aim of unveiling the role of wave motion as a source and/or sink of turbulence kinetic and potential energies in the respective explicit budgets. By exploring the balance between buoyancy (driving waves) and shear (driving turbulence), a simple interpretation paradigm is introduced to distinguish two layers, namely the near-ground and far-ground sublayer, estimating where the turbulence kinetic energy can significantly feed or be fed by the wave. To prove this paradigm, a nocturnal valley flow is used as a case study to detail the role of wave motions on the kinetic and potential energy budgets within the two sublayers. From this dataset, the explicit kinetic and potential energy budgets are calculated, relying on a variance–covariance analysis to further comprehend the balance of energy production/destruction in each sublayer. With this investigation, we propose a simple interpretation scheme to capture and interpret the extent of the complex interaction between waves and turbulence in nocturnal stable boundary layers.
Highlights
Nocturnal boundary layers are typically characterized by a stable thermal stratification and by terrain-following flows
4.1 Near-Ground and Far-Ground Sublayers In Sect. 2.4 we hypothesize the existence of a near-ground sublayer (NGS), where the shear-produced turbulence kinetic energy overcomes the wave kinetic energy, and a far-ground sublayer (FGS), where wave and small-scale turbulence both contribute to the energy budget with the first being a dominant factor
A similar behaviour is observed for the potential energy in Fig. 8b, where the wave is the dominant fluctuation within the FGS, while it resembles the smallscale turbulence in the NGS
Summary
Nocturnal boundary layers are typically characterized by a stable thermal stratification and by terrain-following flows. In addition to non-turbulent motions, the atmospheric flows over complex terrain are known to be in a weak but continuous state of turbulence due to the breakdown of critical internal waves. In the presence of turbulence and submesoscale motions, the power spectra are typically subdivided into two ranges of frequencies by a spectral gap ranging between 60 s and 450 s depending on the atmospheric stability, the geographical location, and the terrain complexity. We adopt a practical approach based on different averaging time intervals to disaggregate and evaluate the low-frequency wave activity and high-frequency turbulence within a nocturnal SBL flow, proposing an interpretation scheme to capture the extents of the wave–turbulence interaction.
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