Limitations of Mesoscale-to-LES Grid Nesting in a Convective Atmospheric Boundary Layer

Accurate models of atmospheric phenomena are essential for predicting wind energy production, fire propagation, pollution transport and weather conditions. Such predictions require information about both micro-scale flow patterns, and meso-scale weather conditions. Models with both meso and microscale capabilities make it possible to perform grid-nesting between mesoscale parent domains and microscale nests. However, due to nesting resolution-ratio limitations, the mesoscale domain is usually run at resolutions higher than what the models were designed for, in a range known as the Terra Incognita. In cases containing atmospheric convection, this practice can result in the presence of unrealistic flow structures in the parent domain. In the present study we quantify the effect of these unrealistic structures on a nested, Large-Eddy Simulation (LES).

We use the Weather Research and Forecasting Model to perform two sets of nested simulations containing different convective structures: rolls and cells (See cells in figure). The parent domains are run at resolutions within the terra-incognita, and contain unrealistic structures. We compare the vertical profiles of turbulent momentum flux, turbulent heat flux, energy spectra and turbulent kinetic energy of the nested LES and reference LES domains with periodic boundary conditions. Through this analysis, we are able to evaluate the influence of mesoscale flow structures on the nested domains. By quantifying this biasing effect we address the limitations of the grid-nesting technique for meso-to-microscale models. This new understanding of the limitations of grid-nesting will improve nesting practices and inform developers as to where model improvements are needed.

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