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Mesoscale microscale coupling

High-precision wind resource modeling service via meso-micro-scale coupling

Accurate assessment of the wind resource on large sites


Wind farms are getting larger, and consequently, their siting locations are getting more complicated. To assess and model the wind characteristics on these large sites, it becomes necessary to represent both the local characteristics and the general representation of the wind climate. The mesoscale is then required to consider regional effects.

Meteodyn has developed its own mesoscale-microscale coupling method to overcome this problem. We couple the WRF (Weather Research and Forecasting) mesoscale model to the Meteodyn WT microscale model of CFD-RANS type for wind resource assessment.


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Figure 1 - Elevation map with Z-transect and met mast locations that takes place in the Alex17 benchmark

Characteristics

The computation speed

During the Alex17 benchmark, less than 2 hours of computation time were required to accurately assess the wind forecasts on a 20 km microscale domain, simulated with a 25 m horizontal resolution and a 4 m vertical resolution, resulting in a 16-million-points mesh.

High accuracy

Our WRF-Meteodyn WT (RANS) model achieved a wind speed bias of less than 1% (-0.55%) in the ALEX17 benchmark, dedicated to diurnal cycles in complex terrain. More information here.


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The method

1

Dynamic downscaling of Global Weather Models to a regional mesoscale WRF model, followed by a statistical downscaling from the regional WRF model to the Meteodyn WT microscale model (RANS**), from 0.1 to 1 km horizontal resolution.

RANS**: Reynolds-Averaged Navier–Stokes

2

The WRF model outputs with a fine resolution up to 1 km are classified according to wind direction and stability classes to obtain input and upper boundary conditions for the Meteodyn WT RANS k − l model.

3

Three classes of atmospheric stability (unstable, neutral, and stable) are simulated via the turbulence kinetic energy and the turbulence length scale. A forcing term is added to relax the microscale flow to mesoscale wind profiles in a mesoscale zone starting at 500 m above ground level for very complex sites. The vertical profiles from the mesoscale model output are therefore assimilated into the CFD model for a forcing of the microscale wind field into the mesoscale zone. Below this level, there is no relaxation applied due to the high resolution of the topography in the microscale CFD model.


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