RAL Seminar Series
Wed 15 May 2013
Multiscale modelling of boundary layer flows using WRF
Environmental and Applied Fluid Dynamics Department, von Karman Institute for Fluid Dynamics, Belgium
The Weather Research and Forecasting (WRF) model provides a framework that allows nesting techniques. This nesting environment can be potentially used to bridge the gap between meso- and micro-scale regimes by progressive refinement of spatial and temporal scales. Nesting techniques are commonly used in NWP models, where turbulence is completely unresolved and the role of such a refinement of scales is well understood. However, our knowledge of nesting techniques applied to turbulence resolving models as large-eddy simulation (LES) is limited.
This talk is divided in two parts. Firstly, we will focus on the problem of nesting turbulence in LES models. Here we will address the following questions: Can fine-scale turbulence be progressively developed from a coarse LES solution? What are the transition and equilibration mechanisms that occur? Is it possible to obtain similar statistics and spectral behavior from the nesting approach as from a periodic stand-alone LES domain?
In the second part of the talk, we will focus on the specific problem of the transition from mesoscales to microscales in numerical models. This requires spanning “terra incognita”, a region of scales from a few hundred meters up to few kilometers where neither mesoscale nor LES models are designed to work. The challenge is to generate turbulence on inner LES domain from smooth mesoscale inflow. Several methods are proposed to trigger the development of turbulent features. The inclusion of adequate temperature perturbations near the inflow boundaries of the LES domain result in a very good agreement of mean profiles and spectra, compared to periodic stand-alone simulations. This perturbation method is tested under a broad range of atmospheric stabilities: convective, neutral and stable conditions, showing successful results in all the regimes.