Investigating the impact of topography on the stable boundary layer structure over complex terrain using Large eddy simulations

Abstract: Large eddy simulations (LES) are performed to better understand the airflow, structure, and mixing processes in the stable boundary layer (SBL) in the bottom of a mid-range mountain valley, Fitchelgebirge, in Southern Germany. The simulated structure and evolution of the SBL over the complex terrain agreed well in comparison with the remote sensing measurements. The simulations were tested using different vertical grid spacings of 10 m, 5 m and 2 m and a stretched version starting at 1 m assuming flat terrain. The topography of the experimental site is complex with mountain ranges of around 700 m on the north and up to 1km on the south. There is a gap on the western side of the site where channel flows are possible. Additional simulations were conducted with topography from a digital elevation model containing elevational differences up to 400 m. Results showed an increased depth of the cold-air pool by up 30 m and lower near-surface temperatures with differences exceeding 5 K in the valley bottom when comparing topography against flat-terrain simulations. The structure of the cold-air drainage followed terrain contours indicating local slope flows being responsible for the enhanced cooling when topography was included, while flat-terrain runs showed no evidence of a coherent cold-air layer. Finer grid resolutions showed much improvement in the resolved cold-pool vertical and horizontal structure. LES output was also compared with in-situ and remote sensing observations in terms of hourly mean profiles of wind speed, direction, and potential temperature, and turbulence kinetic energy. The results highlight the importance of including the topography in SBL modeling for e.g. frost damage forecast, air-pollution studies, fog analyses, and computing greenhouse gas budgets since both the SBL turbulence and time-averaged flow are governed by the thermal structure which is forced by topography even in relatively gentle mountainous terrain in mid-latitude regions.