*2.3. Downscaling of the 10-Year Return Levels of Wind Speed*

The impact of local terrain features on maximum wind speed cannot fully be taken into account in the relatively coarse 0.75◦ × 0.75◦ grid of ERA-Interim, because for example, hills, lakes, and changes in land-use are not considered in detail. For this reason, in order to downscale the wind speed return level from the coarse grid to a high-resolution grid we used a wind multiplier approach tested recently by [41] for boreal forest conditions. In this study, only topographic and terrain (surface roughness) properties are taken into account when assessing local maximum wind speeds (and their return levels) separately for the eight cardinal and intercardinal wind directions. For the application in forested landscapes, the shielding factor, i.e., the effect of upwind buildings providing cover to the place of interest and only relevant in urban areas, was not considered. The wind multiplier method has been presented earlier in [40,48] for more details.

Following the study by [41], the return level of regional maximum wind speed (*UR*) in an open terrain at a 10-m height is downscaled into site-specific return level (*Usite*) by applying two multipliers, i.e., terrain multiplier (*Mz*), and topographic (hill-shape) multiplier (*Mh*):

$$
\mathcal{U}L\_{\rm site} = \mathcal{U}R \times \mathcal{M}\!\_{\mathbb{Z}} \times \mathcal{M}\_{\mathbb{h}} \tag{2}
$$

We used a 20 × 20-m grid, which is in line with the CORINE Land Cover 2012 dataset [49] providing the information on land cover and land use that enabled the calculation of terrain multiplier. When defining the terrain multiplier (*Mz*), we used a 500 m fetch length and weighted the grid points close to place of interest more than the further upwind grid points.

The topographic (hill-shape) multiplier (*Mh*) was calculated by taking into account the variations and change of elevation 1000 m upwind from the place of interest. As well, the elevation of the place of interest was taken into account. Development of the *Mz* and *Mh* multipliers used in this study were described more elaborately in the Sections 2.3 and 2.4 of [41]. According to [41], for areas with no extreme variations of elevation, the wind multiplier approach was a feasible method to identify at a high spatial resolution locations having the highest forest wind damage risks.
