Open Boundary Conditions for Atmospheric Large Eddy Simulations and the Implementation in DALES4.4

Abstract. Open boundary conditions were developed for atmospheric large eddy simulation (LES) models and implemented into the Dutch Atmospheric Large Eddy simulation model. The implementation was tested in a "Big Brother"-like setup, in which the simulation with open boundary conditions was forced by an identical control simulation with periodic boundary conditions. The results show that the open boundary implementation has minimal influence on the solution. Both the mean state and the turbulent structures are close to the control simulation and disturbances at the in- and outflow boundaries are negligible. To emulate a setup in which the LES is coupled to a coarser model, the influence of coarse boundary input was tested by smoothing the output of the periodic control simulation both temporally and spatially before feeding it as input to the simulation with open boundary conditions. The results show that when the ratio between input and model resolution increases, disturbances start to form at the inflow boundary and an area exists where turbulence needs to develop. Adding synthetic turbulence to the smoothed input reduces the size of this area and the magnitude of the disturbances.