Downslope windstorms in the Scandinavian mountains from a kilometer-scale regional climate model

Downslope windstorm (DW) is a mesoscale phenomenon with considerable impact on nature and human activities in mountainous regions. It is characterised not only by the descent of air down the lee slopes but also by wind acceleration on the windward side of a mountain. Climate studies of such complex and local events on larger scale are rare. In this work, a high-resolution climate model is used to study the DW characteristics in the present and future climate in the Scandinavian mountains.

The HARMONIE-Climate (HCLIM) regional climate model is used on two nested grids with horizontal grid spacing of 12 and 3 km. The DW’s are identified using a two-step algorithm. First the favorable terrain for DW’s is marked, followed by the identification of DW events in these areas based on stability, horizontal and vertical wind speed. Synoptic circulation types (CT) are used to distinguish DW’s according to different large-scale environments. The Scandinavian mountains are divided into four subregions, and the analysis is focused on winter, which exhibits the highest frequency of DW’s.

The simulation at 3-km grid spacing agrees considerably better with observations in DW-prone regions, indicating that such high resolution is needed to reproduce strong DW’s. The CT’s with highest frequency of DW occurrence are those with strong cross-mountain pressure gradient. For every CT, DW’s were additionally grouped as either cold or warm winds in the lee of a mountain, distinguishing between bora-like (cold) and foehn-like (warm) DW’s. Due to the considerable terrain complexity of the Scandinavian Mountains, a straightforward link between CT’s and the DWs’ effect on local warming or cooling is not evident. Results show that even in a single grid point DW’s can cause warming and cooling regardless of a CT or wind direction. We focus on one such exemplary grid point to highlight how different synoptic settings can lead to very similar local conditions but with opposite DW character. Additionally, insights into future climate projections of DW’s are presented.