3.6    Toward Multiscale Simulations of Flows in Heterogeneous    Atmospheric Boundary Layers

Kosovic, Branko, National Center for Atmospheric Research

Ever since large-eddy simulation (LES) was developed (Lilly 1967, Deardorff 1970) it has been extensively used to study canonical atmospheric boundary layers (ABLs). In most cases these studies focused on flat plate boundary layers under the assumption of horizontal homogeneity. Carefully designed LES of canonical atmospheric boundary layers have contributed significantly to development of better understanding of these flows and their parameterizations. LES were often carried out using codes specifically designed for simulations of idealized, horizontally homogeneous flows with periodic lateral boundary conditions.

Multiscale simulations over heterogeneous boundary layers are of great importance for a wide range of applications. Recent developments in numerical weather prediction (NWP) codes enable their nearly seamless use across a wide range of atmospheric scales from synoptic to turbulent scales in atmospheric boundary layers.   However, achieving accurate multiscale simulations requires extensive code validation under a range of atmospheric conditions as well as addressing the question of turbulence modeling in the range of scales between mesoscales and boundary layer scales (Wyngaard, 2004). We therefore start by focusing on validation of nested LES of ABLs using the Weather Research and Forecasting (WRF) model and present results of several test cases including: complex terrain and heterogeneous surface characteristics. We first carry out validation of WRF-LES for simulations of flows over complex terrain using data from Askervein Hill (Taylor and Teunissen, 1985, 1987). WRFÕs nesting capability is employed with a one-way nested inner domain that includes complex terrain representation while the coarser outer nest is used to spin up fully developed atmospheric boundary layer turbulence and thus represent accurately inflow to the inner domain. Our validation study employs two subgrid models included in WRF, each with or without a prognostic equation for turbulent kinetic energy. Toward a goal of developing capability for multiscale simulations for wind energy applications, we have implemented a generalized actuator disk model in WRF. The generalized actuator disk model implemented in WRF-LES is capable of representing the drag and torque effects distributed over the turbine rotor disk. We demonstrated the use of WRF-LES to simulate flow through an array of wind turbines under neutrally stratified conditions. Our simulations show that WRF-LES with a generalized actuator disk model is capable of capturing the characteristics of wind turbine wakes and their interactions with consecutive rows of wind turbines.