On the sensitivity of turbulent flux profiles to turbulent inflow generation with the eddy recycling method

Large-eddy simulations (LES) are an important tool in studying meteorological processes on the local scale. However, specific techniques are needed to generate turbulence when initializing these models. While random perturbations in the initial condition might be sufficient for periodic boundaries, their effect will rapidly diminish when performing simulations with open boundaries, which are needed to analyze undisturbed flow over isolated topographic features. Hence, more elaborate techniques are needed to generate and maintain turbulence for such boundaries.

Eddy recycling is one of those turbulent inflow generation techniques and already implemented in atmospheric LES codes such as CM1 and PALM. The eddy recycling method captures the turbulent structures a certain distance downstream of the inflow and injects the captured turbulent field in the inflow region. While eddy recycling successfully generates turbulence in the inflow region, flux profiles are very sensitive to the distance between capture region and injection region as well as the geometry of these regions. In addition, artificial periodicity in the fluxes might be introduced when placing injection region and capture region too close together. The sensitivity of turbulent flux profiles towards the shape of the capture region, the specification of the turbulent field and the effect of artificial periodicity of these fluxes has so far received little attention in atmospheric LES.

To address this issue and investigate the sensitivity of the profile shape of turbulent moments to the specification of the eddy recycling module, a set of idealized LES were conducted for a shear and buoyancy driven atmospheric boundary layer with the Cloud Model 1 (CM1). Each simulation had an identical inflow profile but differed in the shape of the capture region and the distance between the capture and injection region. Preliminary results show that the shape of TKE and variances of wind components as well as temperature is highly sensitive to the shape and location of both capture and injection regions, while average profiles are less affected.