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”.

See uploaded pdf