Windstorms in a Warming Climate: Urban Flow Dynamics, Climate Change, and Future Extremes

Abstract. This study explores how intensified large-scale storm forcing translates into urban-scale wind exposure in a dense historical urban environment using high-resolution large-eddy simulations (LES) with the PALM-4U model. A realistic representation of central Copenhagen is subjected to a series of idealised inflow wind conditions with progressively stronger storm scenarios, ranging from weak to severe wind forcing.

To estimate the influence of mesoscale weather variations due to a change in climatic boundary conditions on the wind field of the built environment, the study introduces and demonstrates a novel statistical wind flow analysis across horizontal layers of the computational domain.

As expected from urban canopy theory the results show that the spatial organisation of the flow field remains largely invariant across all simulations, indicating that urban morphology governs the structure of the wind field, while inflow speed primarily controls the magnitude of the mean response, but here confirmed qualitatively across a wide range of forcing conditions.

In contrast gust characteristics exhibit strongly wind speed-dependent behaviour. At low inflow wind speeds, gustiness is strongly enhanced by intermittent and geometry-driven recirculation processes within the urban canopy, resulting in elevated turbulence intensity and gust factors. At higher inflow wind speeds, both turbulence intensity and gust factors converge toward quasi-stationary values, indicating a transition to a shear-dominated canopy flow in which gust amplification decreases and extreme events scale more proportionally with the mean flow.

These findings demonstrate that urban wind hazards under intensified storm conditions cannot be assessed from mean wind scaling alone, as gust amplification is governed by non-linear interactions between turbulent structures and urban geometry that are most pronounced at low-to-moderate inflow wind speeds. The study provides a physically based framework for understanding how changes in inflow wind intensity map to different wind hazard metrics in complex urban environments, explicitly distinguishing between mean-flow scaling and turbulence-driven amplification of gusts.