Downslope windstorms in southeast Greenland – part I: Definition, seasonality, and regional characteristics of piteraq events

Abstract. Piteraq events are strong downslope windstorms in southeast Greenland that often damage infrastructure and disrupt transport and subsistence activities of local communities. These wind events are more intense and common in winter than in summer, but with enhanced global warming and declining sea ice in the Arctic, their summer characteristics may become more relevant in the future. Using data from six weather stations and radiosonde data in the region around Tasiilaq in southeast Greenland since 1958 along with local media sources, we explore the evolution, regional variability, and local consequences of piteraq events, focusing on seasonal characteristics. Motivated by operational warning procedures and previous studies, we propose a robust definition of a piteraq based on local wind speed and direction that comprises most reported media reports on piteraq events. Using this definition, we find that the imprint of piteraq events cover regional scales of several 100 km by intensifying the wind speed around eastern slopes of the Greenland Ice Sheet and/or shifting the wind direction to down from the ice sheet. Prior to the piteraq events, the wind around Tasiilaq often turns clockwise from nearly parallel to the coast (from southwest) to down the ice sheet (from northwest), particularly at higher elevations that are less disturbed by topography. In Tasiilaq, the main seasonal differences during the developing and mature phases of a piteraq event are a substantially larger local reduction in relative humidity in summer compared to winter and also a relative warming during summer events as opposed to a relative cooling during winter events. These differences in temperature and relative humidity are likely associated with foehn mechanisms that become more common when more moisture is transported to higher levels in summer, combined with reduced regional sea ice in summer and hence larger moisture contrasts between maritime and continental air masses. Both are factors that are expected to become more dominant in a future climate. Our findings highlight the importance of understanding piteraq events in a spatial and temporal perspective for a better integration in early warning systems in the present and future climate.