In atmospheric flows in general and wind resource site assessment in particular, one of the main challenges is dealing with phenomena of different scales. While mesoscale simulations can capture weather trends and the regional orographic effect accurately, their resolution is limited. On the other hand microscale simulations provide a high resolution and can represent local orographic features, but are limited to the local site region. A single point downscaling from meso- to microscale simulations can introduce synoptic changes. The overall change in the wind regime is used to scale the local microscale simulation, similarly to using local measurements. A multipoint method is even able to identify and include situations with local changes of the wind direction (shear flow).
As the method is specifically useful in regimes of a local shear flows, it is important to identify and quantify such situations. Therefore the study includes an analysis based on the simulation data of the New European Wind Atlas (NEWA) .
Fig. 1: Rödeser Berg: Horizontal slice 5 m above ground. Wind speed in colors and wind vanes in arrows. (5-10-2012 6:00pm)
The method we use is motivated by the work of Barcons et al. (2018). It is used as a post-processing analysis, which combines the results of the mesoscale simulation (here: NEWA) and a set of microscale simulations (THETA1). The microscale domain is decomposed into different regions which are all associated to a different mesoscale reference point. A smoothing algorithm has been applied, on the overlapping areas, to avoid discontinuity. General scaling procedures are applied as usual.
The analysis showed, that local shear flows are mainly dominant in very complex terrain. Additionally they can be found at coastlines and some river valleys, but to a smaller extend. Furthermore short term shear flows occur in most places, e.g. when certain weather systems move accross the site. The applied downscaling method can represent these shear flows (see Fig.1). It is important to note, that this effect is not limited to changes in the direction of the flow, it represents changes in wind speed as well.
While the method is working successfully, a local wind shear is a prerequisite to use its full possibilities. At sites with simple terrain, these situation occur on rare occasions only, the benefits are relatively small. Nevertheless it can be used to study certain situations of strong local wind shear. Fig.1 resembles the site Rödeser Berg in Germany. In this example four different grid cells of the NEWA-Database are located within the outline of the chosen site. This would change with a larger site or with a higher resolution of the mesoscale simulation. The downscaled results will furthermore be compared to measurements taken at two different metmasts on site.
This work has been funded by the BMWI in the scope of the ETESIAN project. Thanks to the NEWA project for granting early access to preliminary mesoscale simulation data.
1CFD-Solver developed by the German Aerospace Center in Göttingen, Germany