# Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill
## 3 Results and Discussion
### Horizontal Wind Velocity
The profiles of horizontal wind velocity near the windward hill foot were similar, showing a negligible effect of hill height at that site. In addition, the corresponding wall jet thickness at that site was about \(0.26D_{\rm jet}\), which is greater than that recorded during an undisturbed downburst wind (\(0.15D_{\rm jet}\)), indicating an increased wind intensity. In contrast, the wind profiles along the \(45^{\circ}\)-ray ridge, whether recorded at the hill foot or at the crest, were influenced by the hill height, which indicates that the blockage effect on the leeward wind field was relatively significant. The contour maps of \(M_{\rm t,max}\) are depicted in Figure 10 and show the changes in wind intensity. As the hill height increases, the area of the speed-up region becomes increasingly small and the center of the region gradually moves to the windward hill foot lying on the \(135^{\circ}\) ray. In the cases of high hills, it was found that the speed-up regions when \(M_{\rm t,max}\geq 1\) were mainly located at the two sides of the hill. This again indicates that the downburst-induced near-ground strong airflow would be blocked if the hill is sufficiently high, and behaves like a flow around the hill body. In general, provided a high hill, the maximum horizontal wind velocity recorded for the crest was lower than that found at the hill foot, as shown in Figure 10. For a hill with a low height, e.g., the case of \(H=h/12\), in which the near-ground wind is capable of climbing over the hill, the speed-up region comprised a relatively larger area in the vicinity of the crest. These findings are also corroborated by the distribution of the wind velocity vector over the hill (see Figure 11), where the arrows represent the directions and the magnitudes of the horizontal wind velocities. This flowing-around phenomenon was not observed in the previous studies using 2D hill models. Figure 12 and 13 show the wind profiles when the hill models were placed at \(r_{\rm h}=0.8D_{\rm jet}\) and \(r_{\rm h}=1.2D_{\rm jet}\). The variation in the wind field was similar to the foregoing findings in the case of \(r_{\rm h}=1.0D_{\rm jet}\). That is, the profiles for the hill models at these three hill positions show that the speed-up regions were mainly located in the vicinity of the edge on the windward side, indicating that more attention should be paid to wind velocity in this region. In addition, provided \(0.8D_{\rm jet}\leq r_{\rm h}\leq 1.2D_{\rm jet}\), the location of the hill model had insignificant effects on the magnitude of the wind profile near the hill foot. This lack of impact of the hill position is consistent with the findings of work by [PERSON] et al. [9,10]. Generally, the wind profiles in the vicinity of the crest, as well as those on the leeward side, were relatively more susceptible to the hill height. Figure 12: Profiles of horizontal wind velocity (\(L=0.5D_{\rm jet}\), \(r_{\rm h}=0.8D_{\rm jet}\)). It is worth noting that in the cases surveyed, most of the profiles of the horizontal wind velocity have similar shapes to the downburst profile proposed by [PERSON] et al. (2018), i.e., correspondingly with the increase in the relative height from the ground/surface, the velocity increases almost linearly, then rapidly decreases after reaching a peak. Comparatively, the vertical profile of the horizontal wind in the ABL wind (ABL wind profile) usually exhibits an exponential/logarithmic shape. In the cases of high hills, it was found that the shape of the wind profile at the windward side of the \(x\)-axis ridge changed from a shape similar to an ABL wind profile to a shape more characteristic of a downburst profile. This phenomenon can be also observed in other cases, e.g., \(L=1.0D_{\rm jet}\), as shown in Figures 14-16. Furthermore, the similarity to the ABL wind profile was usually but not always observed when \(r/D_{\rm jet}\) was around 0.5, and occasionally when \(r/D_{\rm jet}>1.0\) for high hills (\(H/h>1/6\)).