Downslope windstorms in southeast Greenland – Part II: Multi-scale dynamics and spatial evolution of piteraq events

Abstract. Severe downslope windstorms in southeast Greenland, known locally as piteraq, dominate extreme weather conditions across the region, including the main settlement Tasiilaq. However, their complex multi-scale dynamics remain poorly resolved in numerical weather prediction and global climate models, leading to inaccurate early warnings and biases in regional heat flux and ocean circulation projections. In this study, we utilize polar adapted, high-resolution reanalysis to trace the spatial, kinematic, and thermodynamic evolution of piteraq events from the synoptic scale down to localized turbulent wave breaking. Validation against regional observational networks shows that the reanalysis captures the broad spatial and temporal variability of the wind speed of these events, but it overestimates the peak intensities with a positive mean bias of 6.5 m s⁻¹ during the winter cases.

We demonstrate that piteraq events are dynamically triggered by persistent upstream westerlies followed by the crossing of a Canadian cold vortex over Greenland. While approaching Greenland the Canadian cold vortex replaces a statistically significant, anomalous near-surface warming over the ice sheet to a rapid surface cooling. This transition establishes a strong cross-barrier surface pressure gradient (on median 7.3 hPa 100 km⁻¹) and a reduction in the Scorer parameter profile that supports wave trapping and resonance. As the stably stratified flow is forced to descend the topography, it accelerates into intense low-level jets that are distinctly localized directly beneath regions of frequent wave breaking and overturning flow aloft. Our analysis further reveals a thermodynamically driven seasonal dichotomy. Winter events are characterized by dense, cold drainage flows that erode low-level coastal stratification and drive strong maritime cold-air outbreaks over the Irminger Sea. In contrast, summer events are characterized by a lack of windward blocking, allowing the upstream westerlies to force moister air masses over the crest. The resulting cloud formation and latent heat release, combined with subsequent adiabatic warming during the leeward descent, manifests as a relatively warm, dry windstorm capable of enhancing local ice melt.

Ultimately, our study defines a distinct, synoptic-scale blueprint for piteraq formation. Identifying these resolvable upstream precursors provides a pathway to train machine learning algorithms for improving the reliability of regional early warning systems, and offers a physical approach to dynamically parameterize sub-grid turbulent heat fluxes for more robust Atlantic Meridional Overturning Circulation (AMOC) projections.