On Bridging A Modeling Scale Gap: Mesoscale to Microscale Coupling for Wind Energy

Abstract

Accurately representing flow across the mesoscale to the microscale is a persistent roadblock for completing realistic microscale simulations. The science challenges that must be addressed to coupling at these scales include the following: 1) What is necessary to capture the variability of the mesoscale flow, and how do we avoid generating spurious rolls within the terra incognita between the scales? 2) Which methods effectively couple the mesoscale to the microscale and capture the correct nonstationary features at the microscale? 3) What are the best methods to initialize turbulence at the microscale? 4) What is the best way to handle the surface-layer parameterizations consistently at the mesoscale and the microscale? 5) How do we assess the impact of improvements in each of these aspects and quantify the uncertainty in the simulations? The U.S. Department of Energy Mesoscale-to-Microscale-Coupling project seeks to develop, verify, and validate physical models and modeling techniques that bridge the most important atmospheric scales determining wind plant performance and reliability, which impacts many meteorological applications. The approach begins with choosing case days that are interesting for wind energy for which there are observational data for validation. The team has focused on modeling nonstationary conditions for both flat and complex terrain. This paper describes the approaches taken to answer the science challenges, culminating in recommendations for best approaches for coupled modeling.

Validation of uncertainty reduction by using multiple transfer locations for WRF-CFD coupling in numerical wind energy assessments

“…As a consequence, care must be taken to consider any gradients in the velocity field caused by mesoscale effects (as discussed by e.g. Haupt et al 2019). When mesoscale https://doi.org/10.5194/wes-2020-62 Preprint.…”

“…The microscale effects have also been modelled by coupling WRF with a large variety of non-linear CFD models (eg. Gopalan et al 2014, Haupt et al 2019.…”

Simulating Real Atmospheric Boundary Layers at Gray-Zone Resolutions: How Do Currently Available Turbulence Parameterizations Perform?

“…In the last decade, advances in computational power and new modeling capabilities have led several WRF user groups to experiment with simulations at higher spatial resolutions [3][4][5][6]. While increased model resolution has the potential to improve simulation performance, it can also complicate the task of turbulence modeling and lead to inaccurate outcomes, especially at horizontal resolutions between the meso and microscales, a modeling region known as the "terra incognita" or "gray zone" [7,8]. At these resolutions, the subgrid-scale (SGS) model lies between the fully parameterized regime, where the mesoscale approach based on horizontal homogeneity across the grid footprint is applicable and the large-eddy simulation (LES) regime, where small isotropic eddies are parameterized according to Kolmogorov's theory.…”

An urban large-eddy-based dispersion model for marginal grid resolutions: CAIRDIO v1.0

“…Another practical resolution and hitherto accuracy limit of NWP models results from the use of parameterizations itself. Using WRF, Haupt et al (2019) observed that for horizontal grid spacings below the typical height of the PBL numerical results can become spurious. Based on their findings, they recommend not to apply NWP on the sub-kilometer scale without careful replacement of the used parameterizations.…”

“…LES models resolve the turbulent spectra up to the filter cut-off size (often the grid size) and rely on simplistic sub-scale parameterizations only (Maronga et al, 2019). These two different approaches (extensive parameterization vs. explicit representation) are difficult to merge at the bridging scale-range (few tens of meters to one kilometer), for which reason Haupt et al (2019) introduced the key word "terra-incognita". An exemplary study where LES have been attempted at horizontal resolutions up to above 100 m is given by Efstathiou et al (2016).…”