Development and validation of a new mass-consistent model using terraininfluenced coordinates

Abstract: Simulations of the wind climate in complex terrain may be useful in many cases, e.g. for wind energy mapping. In this study a new mass-consistent model (MCM), the λ-model, was developed and the ability of the model was examined. In the model an initial wind field is adjusted to fulfill the requirement of being non-divergent at all points. The advance of the λ- model compared with previous MCM:s is the use of a terrain-influenced coordinate system. Except the wind field, the model parameters include constants α, one for each direction. Those constants have no obvious physical meaning and have to be determined empirically. To determine the ability and quality of the λ-model, the results were compared with results from the mesoscale MIUU-model. Firstly, comparisons were made for a Gauss-shaped hill, to find situations which are not caught by the λ-model, e.g. wakes and thermal effects. During daytime the results from the λ-model were good but the model fails during nighttime. From the comparisons between the models the importance of the α-constants were studied. Secondly, comparisons between the models were made for real terrain. Wind data from the MIUU-model with resolution 5 km was used as input data and was interpolated to a 1 km grid and made non-divergent by the λ-model. To study the quality of the results, they were compared with simulations from the MIUU-model with resolution 1 km. The results are quite accurate, after adjusting for a difference in mean wind speed between MIUU-model runs on 1km and 5 km resolution. Good results from the λ-model were reached if a climate average wind speed was calculated from several simulations with different wind directions. Especially if the mean wind speed for the domain in the λ-model was modified to the same level as in the MIUU 1 km. The λ-model may be a useful tool as the results were found to be reasonable good for many cases. But the user must be aware of situations when the model fails. Future studies could be done to investigate if the λ-model is useable for resolutions down to 100 meters.