The foehn effect during easterly flow over Svalbard

Abstract. This article presents a comprehensive analysis of the foehn episode which occurred over Svalbard on 30–31 May 2017. This episode is well documented by multiplatform measurements carried out during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) and Physical feedbacks of Arctic PBL, Sea ice, Cloud And AerosoL (PASCAL) campaigns. Both orographic wind modification and foehn warming are considered here. The latter is found to be primarily produced by the isentropic drawdown, which is evident from observations and mesoscale numerical modeling. The structure of the observed foehn warming was in many aspects very similar to that for foehns over the Antarctic Peninsula. In particular, it is found that the warming was proportional to the height of the mountain ridges and propagated far downstream. Also, a strong spatial heterogeneity of the foehn warming was observed with a clear cold footprint associated with gap flows along the mountain valleys and fjords. On the downstream side, a shallow stably stratified boundary layer below a well-mixed layer formed over the snow-covered land and cold open water. The foehn warming downwind of Svalbard strengthened the north–south horizontal temperature gradient across the ice edge near the northern tip of Svalbard. This suggests that the associated baroclinicity might have strengthened the observed northern tip jet. A positive daytime radiative budget on the surface, increased by the foehn clearance, along with the downward sensible heat flux provoked accelerated snowmelt in the mountain valleys in Ny-Ålesund and Adventdalen, which suggests a potentially large effect of the frequently observed Svalbard foehns on the snow cover and the glacier heat and mass balance.