Multiscale simulations of atmospheric boundary-layer flows

Advances in computational resources and algorithms have recently enabled simulations of realistic atmospheric flow across a broader range of scales, ranging from quasi two-dimensional mesoscale structures to three-dimensional boundary-layer eddies. With a focus towards developing multiscale capabilities in numerical weather prediction models, the specific problem of the transition from mesoscales to microscale turbulence in atmospheric models is investigated. For that purpose, idealized one-way nested mesoscale-to-LES experiments are carried out using the Weather Research and Forecasting model framework. It is demonstrated that switching from one-dimensional turbulent diffusion in the mesoscale model to three-dimensional LES mixing does not result in an instantaneous development of microscale turbulence in the LES domain. On the contrary, very large fetches are required for the natural transition to turbulence to occur. The computational burden imposed by these long fetches necessitates the development of methods to accelerate the generation of turbulence on a nested LES domain forced by a smooth mesoscale inflow. To that end, four new methods based upon finite amplitude perturbations of the potential temperature field along the LES inflow boundaries are proposed and investigated under different atmospheric stability conditions. Application of these methods results in an efficient generation of realistic turbulent characteristics within close proximity to the perturbation region and through the whole boundary layer, demonstrating their potential to bridge the transition between mesoscale and LES models.