the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2024
Revealing the dynamics of a local Alpine windstorm using large-eddy simulations
Abstract. The local atmospheric flow in mountainous terrain can be highly complex and deviate considerably from the ambient conditions. One example is a notorious local windstorm in a narrow and deep valley in north-eastern Switzerland, known as the Laseyer, that had previously even caused a train derailment. This windstorm is characterized by strong south-easterly winds blowing perpendicular to the valley axis during strong north-westerly ambient flow conditions. We investigate the mechanism of this local windstorm and its sensitivity to changes in the prescribed ambient wind using large-eddy simulation (LES). The LES are performed using the Portable Model for Multi-Scale Atmospheric Prediction (PMAP) at a horizontal grid spacing of 30 m and applying a terrain-following vertical coordinate with steep slopes of the real topography reaching nearly 80°. The simulations, driven by strong north-westerly ambient winds, successfully capture the flow reversal in the valley with quasi-periodically occurring short episodes of wind bursts regularly exceeding 20 m s-1 and in exceptional cases exceeding 35 m s-1. The flow reversal is explained by an amplifying interplay of (1) a recirculation region formed by flow separation in the lee of the upstream ridge, and (2) a vortex caused by a positive pressure anomaly formed by the north-westerly winds impinging on the downstream mountain. This formation mechanism is supported by a simulation in which the height of the downstream mountain is reduced, resulting in a decrease in the strength of the reversed in-valley flow. In agreement with previous observational studies, a series of simulations with modified ambient wind conditions reveal that the intense gusts (> 20 m s-1) only occur in a narrow window of ambient wind directions and if its speed is at least 16 m s-1. Smoothing the topography in the LES reduces the maximum wind speeds in the target region by 10–30 %. Overall, our semi-idealized LES in complex and steep terrain reveal the three-dimensional structure and the mechanism of the local windstorm. Moreover, they point to the importance of the local topography and its complex interplay with the three-dimensional and transient flow leading to the in-valley flow reversal and strong winds that characterize the Laseyer. The study further highlights the importance of the topographic details for the quantitatively correct simulation of atmospheric flows in complex terrain.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Weather and Climate Dynamics. The authors also have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2024-3461', Anonymous Referee #1, 06 Dec 2024
Summary
This paper investigates the dynamics of the Laseyer, a potentially damaging local windstorm occurring in a deep, narrow valley in northeastern Switzerland, using semi-idealized, dry Large-Eddy Simulations. Previous observations from measurement stations at the upstream ridge and in the valley resulted in a 10-year climatology, suggesting that the extreme wind gusts in the valley are because of flow separation and a flow reversal during strong ambient flow. Indeed, the simulations validate this hypothesis by successfully reproducing the quasi-periodic flow reversal along with extreme wind gusts. Through the careful selection of Laseyer-characteristic episodes for composite analysis, the study reveals the spatio-temporal structure and the key mechanism driving this potentially damaging storm: a mutually reinforcing interaction between a recirculation region caused by lee-side flow separation at the upstream ridge, and a vortex induced by a positive pressure anomaly from the impinging flow at the downstream ridge. Furthermore, through sensitivity analyses of these mechanisms with respect to changes of ambient flow properties and topographical features, the authors demonstrate that the intensity of local windstorms, such as the Laseyer, is critically influenced by atmospheric conditions and that reproducing the highest wind speeds requires simulations with detailed representations of the topography.
General comments
This work is a unique and valuable contribution to understanding terrain-induced circulations in complex orography, particularly their temporal variability and potential to damaging wind gusts. The manuscript is well-written, clearly structured, and supported by carefully prepared high-quality plots. The methods are thoroughly described, ensuring reproducibility. Despite the comprehensive analysis, the authors effectively highlight the key findings. Given the high quality of the manuscript, I only have a few minor comments and suggestions for further refinement.
Specific and technical comments
- Line 53: Add Prestel and Wirth (2016) to the listed references. They showed similar results for idealized simulations with a three-dimensional mountain.
- Line 57: Add Prestel and Wirth (2016) to the listed references. They showed that for rather shallow mountains without surface friction, lee-side flow separation was only possible with strong stratification. In contrast, with neutral stratification, lee-side flow separation was only possible with surface friction.
- Line 120: In addition to the influence of ambient flow conditions, the description of the sensitivity analysis is missing the influence of topographical conditions including vegetation (roughness length), mountain height and smoothing of the topography.
- Line 140: The year of publication is missing for “Kühnlein et al.”.
- Line 172: I am wondering how variations of the potential temperature perturbations, e.g. its vertical depth or its decay with height, might affect the simulation outcomes. What criteria were used to parameterize these perturbations? Were they based on observational data in this region? Additionally, without employing these perturbations to accelerate the development of turbulence, how much longer would the spin-up period have needed to reach the statistical steady state you deemed suitable for your analysis?
- Line 268: Can you clarify what is meant by the term "streamlined component"? As I understand it, this would correspond to the wind direction specified in line 209 (300 degrees). However, the time series near the measurement station at Wasserauen, as shown in Figure 6, indicates that this wind direction rarely occurs. Are you instead referring to the wind direction sector mentioned?
- Line 469 & 470: Add “(Fig. 14c)” and “(Fig. 14 d)” at the end of both sentences.
- Line 475 ff: Change unspecific “at a point location in the valley” to more specific “at the location of the measurement station Wasserauen in the valley”.
Citation: https://doi.org/10.5194/egusphere-2024-3461-RC1 - RC2: 'Comment on egusphere-2024-3461', Anonymous Referee #2, 13 Dec 2024
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AC1: 'Final author comments on egusphere-2024-3461', Nicolai Krieger, 23 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3461/egusphere-2024-3461-AC1-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2024-3461', Anonymous Referee #1, 06 Dec 2024
Summary
This paper investigates the dynamics of the Laseyer, a potentially damaging local windstorm occurring in a deep, narrow valley in northeastern Switzerland, using semi-idealized, dry Large-Eddy Simulations. Previous observations from measurement stations at the upstream ridge and in the valley resulted in a 10-year climatology, suggesting that the extreme wind gusts in the valley are because of flow separation and a flow reversal during strong ambient flow. Indeed, the simulations validate this hypothesis by successfully reproducing the quasi-periodic flow reversal along with extreme wind gusts. Through the careful selection of Laseyer-characteristic episodes for composite analysis, the study reveals the spatio-temporal structure and the key mechanism driving this potentially damaging storm: a mutually reinforcing interaction between a recirculation region caused by lee-side flow separation at the upstream ridge, and a vortex induced by a positive pressure anomaly from the impinging flow at the downstream ridge. Furthermore, through sensitivity analyses of these mechanisms with respect to changes of ambient flow properties and topographical features, the authors demonstrate that the intensity of local windstorms, such as the Laseyer, is critically influenced by atmospheric conditions and that reproducing the highest wind speeds requires simulations with detailed representations of the topography.
General comments
This work is a unique and valuable contribution to understanding terrain-induced circulations in complex orography, particularly their temporal variability and potential to damaging wind gusts. The manuscript is well-written, clearly structured, and supported by carefully prepared high-quality plots. The methods are thoroughly described, ensuring reproducibility. Despite the comprehensive analysis, the authors effectively highlight the key findings. Given the high quality of the manuscript, I only have a few minor comments and suggestions for further refinement.
Specific and technical comments
- Line 53: Add Prestel and Wirth (2016) to the listed references. They showed similar results for idealized simulations with a three-dimensional mountain.
- Line 57: Add Prestel and Wirth (2016) to the listed references. They showed that for rather shallow mountains without surface friction, lee-side flow separation was only possible with strong stratification. In contrast, with neutral stratification, lee-side flow separation was only possible with surface friction.
- Line 120: In addition to the influence of ambient flow conditions, the description of the sensitivity analysis is missing the influence of topographical conditions including vegetation (roughness length), mountain height and smoothing of the topography.
- Line 140: The year of publication is missing for “Kühnlein et al.”.
- Line 172: I am wondering how variations of the potential temperature perturbations, e.g. its vertical depth or its decay with height, might affect the simulation outcomes. What criteria were used to parameterize these perturbations? Were they based on observational data in this region? Additionally, without employing these perturbations to accelerate the development of turbulence, how much longer would the spin-up period have needed to reach the statistical steady state you deemed suitable for your analysis?
- Line 268: Can you clarify what is meant by the term "streamlined component"? As I understand it, this would correspond to the wind direction specified in line 209 (300 degrees). However, the time series near the measurement station at Wasserauen, as shown in Figure 6, indicates that this wind direction rarely occurs. Are you instead referring to the wind direction sector mentioned?
- Line 469 & 470: Add “(Fig. 14c)” and “(Fig. 14 d)” at the end of both sentences.
- Line 475 ff: Change unspecific “at a point location in the valley” to more specific “at the location of the measurement station Wasserauen in the valley”.
Citation: https://doi.org/10.5194/egusphere-2024-3461-RC1 - RC2: 'Comment on egusphere-2024-3461', Anonymous Referee #2, 13 Dec 2024
-
AC1: 'Final author comments on egusphere-2024-3461', Nicolai Krieger, 23 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3461/egusphere-2024-3461-AC1-supplement.pdf
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