Low-wind events in Europe: the role of vertical structure and ABL coupling

Persistent low-wind events (LWEs) are a key driver of wind-power variability in Europe, yet their vertical structure is rarely considered explicitly in related studies. Most existing studies quantify LWEs using single-level wind speeds, failing to distinguish between vertically coupled weak-flow regimes and near-surface decoupling within the atmospheric boundary layer (ABL). Because wind turbines operate within the ABL, understanding the coupling between near-surface and hub-height winds is essential for characterizing wind-drought risks.

In this study, we propose a two-level classification of European LWEs using ERA5 reanalysis data for 1979-2024. By comparing 10 m and 100 m wind speeds, we distinguish deep LWEs, in which weak winds extend through both levels, from shallow LWEs, in which near-surface winds are weak while turbine-height winds remain available. Using this framework, we analyze the European climatology of LWEs and their associated boundary-layer structure.

Our results show that shallow and deep LWEs are physically distinct weak-wind regimes. Shallow LWEs are mainly a land phenomenon and are concentrated in the cold season. They are associated with strong vertical shear, weakly mixed boundary-layer conditions, and weak or downward surface sensible heat fluxes, consistent with stable and vertically decoupled flow. Deep LWEs show smaller shear and more vertically coupled weak-flow conditions. Their seasonal behavior depends strongly on surface type: over land they are most common in the cold season, while over the ocean they peak in the warm season.

Sensitivity tests using cut-in thresholds between 3 and 4 m s⁻¹ show that the main spatial and seasonal patterns are robust. The classification provides a useful physical framework for future work on low-wind predictability, atmospheric blocking and wind-energy risk.