Coupled Mesoscale-LES Modeling of a Diurnal Cycle with a Nocturnal Low-level Jet: the Wangara Case Study

The current projection for wind energy in the United States is to generate 20% of the nation's energy by the year 2030, and 35% by the year 2050, a target set by the Department of Energy. As wind energy becomes more prevalent, the size of wind farms and the size of turbines, themselves, also increases. It is imperative that when new turbines are designed, they are modeled and tested to determine the aerodynamic loads induced from various flow fields on the turbines. The state of practice today is to use simplified stochastic models for the inflow generation. Unfortunately, these engineering models generate flow fields that miss important physical characteristics of the “real” atmosphere that the turbines will operate in. For example, they do not account for the shear and globally intermittent turbulence associated with nocturnal low-level jets and breaking Kelvin-Helmholtz waves. As an alternative, one could use a coupled mesoscale-large-eddy simulation (LES) approach to generate realistic inflow conditions. However, only a handful of studies exist on this topic in the literature. The strengths and weaknesses of different coupling approaches are not fully understood and not extensively documented. To partially fill this void, in this study, we compare two competing coupled modeling approaches using the well-known Wangara case study as a testbed.

First, we simulate Day 33 and Night 34 of the Wangara case study utilizing the Weather Research and Forecasting (WRF) model (version 3.6.1). The initial and boundary conditions are provided by the Twentieth Century Reanalysis dataset. From the WRF model-simulated flow fields, we extract initial conditions (e.g., velocity and temperature profiles), boundary conditions (e.g., near-surface temperature and moisture), and time-height-dependent forcings (e.g., geostrophic wind, mesoscale advection) for an LES run in offline coupling mode. A pseudo-spectral LES code with a dynamic subgrid-scale model is used for this purpose. The second approach involves WRF's multiscale modeling capability where an LES is nested within a mesoscale simulation. The performance of both the offline and online coupling approaches are rigorously quantified by using observed mean profiles, estimated surface layer variables (e.g., friction velocity, sensible heat flux), and similarity theory (e.g., spectra). Even though both the coupling approaches are able to generate flow fields similar to those observed during the Wangara field experiment, there is still room for significant improvement.