Lake-land interactions in complex terrain

Complex terrain covers the vast majority of the Earth surface and affects local thermal wind flows by adding a gravitational component to the dynamics responding to local pressure gradients. The Leman Lake region, in Switzerland, thanks to its mountaineous topography and the presence of a large alpine lake, represents an ideal testbed for studying the interaction between regional and local scale flows in complex terrain. However, most of the research investigating local fluid dynamics in the area predominantly focuses on the impact of wind flow on internal lake circulations, and limited attention has so far been given to the role of the local lake breeze circulation in modulating wind flow in the area. Here, we use a set of high-resolution Weather Research and Forecast (WRF) model simulation experiments to investigate the diurnal and seasonal evolution of boundary layer dynamics in the Leman Lake region, with a focus on the environmental impacts associated with the city of Lausanne. In order to isolate urban impacts and explore the role of different land cover types, we compare simulation results from an “Urban” scenario featuring a realistic landscape representation, with simulation results from an hypothetical “Rural” scenario where urban areas are replaced by croplands. Analysis of results shows that the Lausanne urban area, although of limited extent, is able to modify wind flows locally, e.g., by anticipating the diurnal onset of the lake breeze circulation. In turn, regional wind flows interact with the local UHI by advecting cold air from the Leman Lake during the winter, in accordance with what has been observed by similar studies in world regions charaterized by different topographical and climatological conditions. In addition, we compare simulation results with Doppler Lidar vertical wind profiles as part of a recently initiated measurement campaign on site.