Abstract
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.
Highlights
25 The observed climate change in the Arctic is strong, while the mechanisms are not yet fully understood (Serreze and Barry, 2011; Dethloff et al, 2019)
This is well documented for Antarctica, where foehn has a strong effect on the mass balance of glaciers and sea ice in the Antarctic Peninsula region (Elvidge et al, 2015)
Most cases of strong easterly winds over Svalbard are caused by Atlantic cyclones, when they become stationary to the West of Svalbard or when they are moving from West to East 155 across the Barents Sea (Migala et al, 2008; Shestakova et al, 2020)
Summary
25 The observed climate change in the Arctic is strong, while the mechanisms are not yet fully understood (Serreze and Barry, 2011; Dethloff et al, 2019). A high frequency and intensity of foehns can lead to an increased melting of glaciers This is well documented for Antarctica, where foehn has a strong effect on the mass balance of glaciers and sea ice in the Antarctic Peninsula region (Elvidge et al, 2015). Elvidge and Renfrew (2016) list the following mechanisms which produce the foehn warming: 1) isentropic drawdown; 2) turbulent sensible heating (mechanical downward mixing of warmer air in the stratified flow); 3) radiative heating (due to the cloud-free conditions on the lee side of the mountains); and 4) latent heat release and precipitation mechanism They showed that the prevailing mechanism depends on the Froude number of the flow (i.e. wind speed and stratification) as well as the moisture content of the incoming flow and that several mechanisms can act 90 simultaneously.
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