<p>Low-level jets (LLJs) are a peculiar feature of the nocturnal Planetary Boundary Layer (PBL) and they have been extensively observed both in flat and complex terrain configurations. On the contrary, double-nosed LLJs have been rarely investigated. They essentially consist in the simultaneous occurrence of two noses (i.e. two wind-speed maxima) within the PBL vertical profile of wind speed, but their origin and mechanisms remain rather unclear.</p><p>Data collected in an open valley during the MATERHORN field experiment are used here first to demonstrate that double-nosed LLJs are frequently observed at the site during stable nocturnal conditions, and second to describe the mechanisms that drive their formation. Structural characteristics of these double-nosed LLJs are originally described using refined criteria proposed in the literature.</p><p>Two driving mechanisms for double-nosed LLJs are newly proposed in the current study. The first mechanism is wind-driven, in which the two noses are associated with different air masses flowing one on top of the other. The second mechanism is wave-driven, in which a flow perturbation generates an inertial-gravity wave. This wave vertically transports momentum causing the occurrence of a secondary nose, leading to the formation of a double-nosed LLJ. Careful examination of the temporal evolution of these events also revealed the short-lived and transitional nature of the secondary nose in both the mechanisms, as opposite to the primary nose whose evolution appeared instead driven by inertial oscillations. Application of two analytical inertial-oscillation models retrieved from the literature confirms this hypothesis. Indeed, both models satisfactorily reproduce the observed single-nosed LLJs but fail to capture the temporary formation of the secondary nose.</p>

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.

In this paper, an analytical model based on inertial oscillations (Van15 De Wiel et al, 2010) is used to replicate the evolution on a LLJ in complex terrain and then modified to diagnose the occurrence of a double-nosed LLJ driven by surface wave activity. Using an already-analysed case study from the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) dataset, we test the performance of the model in capturing (i) the role of the inertial oscillations in the evolution of a nocturnal LLJ in a wide and gentle-sloping valley, and (ii) the shape modification of the wind profile at the occurrence of the double-nosed LLJ. The modifications applied to the model are framed on the wave-driven mechanism theorized by Brogno et al (2021), without losing the simplicity of the original model framework. A unified model integrating the original with the modified versions is proposed, revealing accurate in replicating the nocturnal LLJ and appropriate to approximate the double-nosed shape of the LLJ. Further observational studies will be needed to corroborate the unified model and explore its application potential in different wind and energy sectors.

Observations acquired in the stable surface layer during two field experiments (The The Mountain Terrain Atmospheric Modeling and Observations Program and the Climate Change Tower Integrated Project) are considered to test different parametrizations of the dissipation rate of mean turbulence kinetic energy (TKE). Particular attention is dedicated to the effect of the submeso motions on these parametrizations. The analysis shows that TKE-based formulations are particularly prone to the submeso effect, whilst better results are obtained if the vertical velocity variance is considered. In the latter case, stability must be taken into account explicitly in Mellor-Yamada type parametrizations but not in shear-based formulations.

<p>Current climate change projections show that the probability of occurrence and the magnitude of heat-wave events are increasing worldwide. These events have to be considered as higher risks for territories and ecosystems, especially where vulnerability is high. The occurrence of heat waves translates into several potential damages such as an increase in fatalities and production losses, degradation of natural and cultural heritages, or the triggering of other hazards such as wildfires. The overlap of all these consequences may lead to both relevant economic losses and additional CO<sub>2</sub> emissions affecting our resilience and exacerbating in turn climate change.<br />In this context, we propose a novel framework for the assessment of risks resulting from heat waves with the aim of quantifying the main contributions to economic losses and CO<sub>2</sub> emissions. This framework follows the conceptual definition of risk provided by the Intergovernmental Panel on Climate Change (IPCC) as the product of hazard, exposure, and vulnerability components. The newly-proposed formulation of these components includes the concept of Nature-Based Solutions (NBS) as strategies carried out to enhance our adaptive capacity in a sustainable and cost-effective way. Since NBS consist of natural features that are also exposed to heat waves, the entire life cycle of NBS is considered (i.e., the implementation, maintenance, and possible restorations). The proposed framework stands as a tool for assessing the local impacts of already-implemented or designed NBS in the current and future climate scenarios.</p>