1. European Wind Energy Conference and Exhibition 2009, Marseille, France
Testing of integrated wind resource modelling system in complex terrain
László Horváth*, Kristian Horváth**, Nikola Karadza*, Alica Bajic**
*Energy Institute Hrtvoje Pozar, **Meteorological and Hydrological Service of Croatia
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Abstract
A new modelling system is under testing in order to explore limits and accuracy of predicted wind resource in complex terrain of eastern Adriatic coast. Modelling system integrates mesoscale NWP model ALADIN and the CFD model WindSim. Ability of ALADIN to model general circulation problems and dynamics extremely well was recognised as important advantage in wind resource assessment on sites with high presence of local gusty, downslope wind Bura. WindSim was used to refine course resolution wind fields of the mesoscale model.
Objectives
From practical point of view it would be nice to have simple, accurate and fast tool for wind resource prediction in any terrain, in any wind regime.
Objective of this work was to perform preliminary testing of wind modelling and prediction system consisting of mezoscale NWP model ALADIN/HR and microscale CFD model WindSim. Both models perform well within limits of their recommended use. Their successful coupling would thus give a new and reliable tool for wind prediction in complex terrain without measurements. However, downscaling of mesoscale model results by microscale model is not an easy task due to different philosophies behind them, especially on sites in complex orography with complex wind regime.
The second objective of this work was to test prediction of the local wind flow during strong Bura event at the site with complex orography.
Methods
ALADIN/HR is limited area numerical weather prediction model at 8 km horizontal resolution, initialized from the downscaled global analysis of the global spectral model ARPEGE/IFS.
WindSim is CFD-based microscale numerical model for simulation of the wind flow over terrain.
For this modelling experiment we selected a case study of an isolated Adriatic island with complex orography. This was expected to minimise boundary condition error of the 3D computational domain. Hourly wind speed and direction time series obtained by 3-day forecast range with ALADIN/HR was used to initialise the WindSim model at the top of the 3D domain.
Results
The modelling system was run for different wind speeds and directions to obtain wind fields over terrain for particular wind conditions. Nesting technique was tested but not chosen at the end due to better performance of the refinement approach. Modelling results of wind speed and direction at 40 m a.s.l. were compared to the measured time series.
Comparison of modelled and measured wind speed time series
Comparison of modelled and measured wind direction time series
ALADIN/WindSim result for v=15.3 m/s; dir=47°
ALADIN/WindSim result for v=4.5 m/s; dir=323°
Problems we have faced during the testing:
- definition of the BL height and conditions for Bura events;
- application of the nesting technique for selected wind directions of the outer model was not understood well; -
- the calculation at 25 m grid resolution was demanding on processing time;
Conclusions
The simplest way of coupling two models of a different scale was tested in order to assess the possibility to predict wind speed and direction more accurately and at finer resolution over complex terrain. Mesoscale resulats on a 8 km grid were downscaled by forcing microscale model to simulate vertical structure of the athmosfere for given boundary conditions at the top of the model domain. Preliminary results of the described modelling system suggest that improvement was achieved in wind direction prediction, while wind speed was predicted less reliably compared to the mesoscale model.
However, this very preliminary results should not be considered as representative of the models skills, but rather as an exercise of possible coupling strategy of different modelling structures.
References
[1] Horvath, L., Horvath, K.: Validation of the wind resource prediction for coupled ALADIN/WAsP modelling system in complex terrain, Proceedings of the EWEC 2006, Athens;
[2] WindSim 4.8.1: Getting Started,
European Wind Energy Conference and Exhibition 2009, Marseille, France
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