The Impact of Large-scale Flow Direction on the Formation of a Glacier Boundary Layer: Two LES Case Studies&#160;
Authors: Goger Brigitta , Stiperski Ivana ,   SCHISM Team
Published: March 2021

&lt;p&gt;&lt;span&gt;The mass balance of mountain glaciers needs correct assessment for several applications, e. g. sea level rise estimates, catchment hydrology, and natural hazard warnings. &lt;/span&gt;&lt;span&gt;It results, at any point on a glacier,&lt;/span&gt;&lt;span&gt; from energy, mass, and momentum fluxes at the glacier-atmosphere interface. However, surface fluxes on glaciers are highly &lt;/span&gt;&lt;span&gt;hetero&lt;/span&gt;&lt;span&gt;geneous in space and time. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;To learn more about the processes leading to the &lt;/span&gt;&lt;span&gt;spatial&lt;/span&gt;&lt;span&gt; surface flux structure over a glacier surface, we employ large-eddy simulations with the WRF model at a horizontal grid mesh size of 48 m over the Hintereisferner, a&lt;/span&gt;&lt;span&gt;n&lt;/span&gt; &lt;span&gt;approximately 6 km long&lt;/span&gt;&lt;span&gt; valley glacier in the Austrian Alps. For model evaluation purposes, we use, besides our permanent measurement framework, four turbulence flux towers located on along- and across-glacier transects which were &lt;/span&gt;&lt;span&gt;maintain&lt;/span&gt;&lt;span&gt;ed in August 2018 on the glacier surface. Simulations were conducted for two case studies, namely one day with synoptic flow from the South-West (SW), and a day with synoptic flow from the North-West (NW). Comparison with the observations suggests that the model is able to reproduce the larger-scale flow structure and the local processes over the ice surface. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;On the SW day, thermally-induced f&lt;/span&gt;&lt;span&gt;l&lt;/span&gt;&lt;span&gt;ows dominate &lt;/span&gt;&lt;span&gt;the near-surface wind patterns &lt;/span&gt;&lt;span&gt;and a stable boundary layer form&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; above the ice surface &lt;/span&gt;&lt;span&gt;due to&lt;/span&gt;&lt;span&gt; the alignment of the katabatic glacier wind with the larger-scale flow. Under these conditions, the &lt;/span&gt;&lt;span&gt;glacier&lt;/span&gt;&lt;span&gt; surface is &lt;/span&gt;&lt;span&gt;exposed to&lt;/span&gt;&lt;span&gt; horizontal cold-air advection. However, on the NW day, the local terrain leads to the formation of a gravity wave with severe turbulence. &lt;/span&gt;&lt;span&gt;The resulting&lt;/span&gt;&lt;span&gt; cross-glacier flow erod&lt;/span&gt;&lt;span&gt;es&lt;/span&gt;&lt;span&gt; the glacier boundary layer, and &lt;/span&gt;&lt;span&gt;the glacier ice experiences &lt;/span&gt;&lt;span&gt;horizontal warm-air advection. In both cases, &lt;/span&gt;&lt;span&gt;the model simulates &lt;/span&gt;&lt;span&gt;the complex flow structure on different length scales &lt;/span&gt;&lt;span&gt;that &lt;/span&gt;&lt;span&gt;affec&lt;/span&gt;&lt;span&gt;t &lt;/span&gt;&lt;span&gt;the vertical and horizontal exchange processes over the glacier surface and the local heat advection during the daytime. The &lt;/span&gt;&lt;span&gt;spatial &lt;/span&gt;&lt;span&gt;sensible heat flux &lt;/span&gt;&lt;span&gt;pattern&lt;/span&gt;&lt;span&gt; is strongly connected to the horizontal wind speed, wind direction, and TKE. Th&lt;/span&gt;&lt;span&gt;e experiment&lt;/span&gt;&lt;span&gt; suggests &lt;/span&gt;&lt;span&gt;a major impact of&lt;/span&gt;&lt;span&gt; the large-scale flow structure and the flow modification by the underlying terrain. Our model setup is able to resolve the relevant scales and is therefore a valuable tool to gain insight on the surface fluxes over truly complex, heterogeneous terrain.&lt;/span&gt;&lt;/p&gt;