PhD defense by Johanne Kristine Haandbæk Øelund

Title: Windstorms: Methods for climate adaptation in the built environment
From synoptic storm dynamics to urban wind risk: a multi-scale modelling framework.

Abstract:
Extreme wind events are a major natural hazard in Northern Europe, and climate change is expected to alter the intensity and structure of extratropical storms. However, it remains poorly quantified how this large-scale intensification translates into wind hazard at the urban scale, where site-specific effects can substantially modify or offset changes in the broader wind climate.
This thesis addresses this scale gap through two case studies. HARMONIE-AROME pseudo-global warming simulations of Storm Anatol (1999), with temperature perturbations from −1 °C to +3 °C, quantify how warming intensifies near-surface wind and gust speeds, introducing a new Cumulative Wind Exposure Index (CWEI) to combine intensity, duration, and spatial extent into one severity measure. PALM-4U large-eddy simulation (LES) captures the wind response of a dense historical district of central Copenhagen (Greyfriar square) to inflow speeds from weak breeze to hurricane force, informed by these storm intensities.
The results show that warming systematically intensifies Storm Anatol: mean and gust wind speeds increase by approximately 0.6 and 0.7 m/s per °C, and the CWEI by roughly 800 km²·h per °C, reflecting a substantial expansion in the area and duration of storm-force winds over Denmark. Within the urban canopy, mean wind speed scales near-linearly with inflow speed (R² = 0.99) and is governed primarily by urban morphology, whereas gust factors and turbulence intensity are strongly non-linear and regime-dependent, peaking under weak-to-moderate inflow (gust factor ≈ 2.6 falling to ≈ 1.7) and plateauing under storm-force forcing. Pedestrian-level exposure follows the same pattern. The fraction of the domain exceeding comfortable wind speeds grows non-linearly with forcing, from negligible at weak-to-moderate inflow to over 10 % under storm-force conditions.
Together, these findings show that while absolute urban wind and gust exposure will increase as storms intensify under climate change, the disproportionate amplification of gusts relative to the mean flow is largest for moderate wind events and saturates under the most extreme forcing. Urban gustrisk therefore cannot be extrapolated linearly from large-scale storm metrics, providing a concrete, quantified basis for incorporating regime-dependent urban amplification into climate risk assessment and adaptation planning for the built environment.

Committee: Associate Professor Helle Astrid Kjær (Chair), Professor Erik Erik Kjellstöm (MISU, Stockholm Universitet and SMHI), and Professor Rediger Höffer (University in Bochum).

Supervisor: Jens Hesselbjerg Christensen