# Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill
## 3 Results and Discussion
### Vertical Wind Velocity  
For illustration, Figure 19 shows the corresponding profiles obtained experimentally for vertical wind velocity for the hill models with \(L=0.5D_{\rm jet}\) and \(r_{\rm h}=1.0D_{\rm jet}\). In the figure, a negative value indicates a downward flow. It was found that the vertical wind velocities were lower than the horizontal wind velocities in general, and the global maximum of the former was less than \(0.5V_{\rm jet}\), except for the results measured at the hill foot lying on the \(x\)-axis ridge. Focusing on the \(x\)-axis ridge, the profile of vertical wind velocity varied with the altitude of the measuring site. On the windward part, the variation in wind direction with the relative height from the surface of the hill, identified in the same profile, indicates the occurrence of circulation on the ridge. This might be attributed to the flow hitting the hill surface and being forced into an upward flow to ascend the hill. In addition, it was found that the position of the circulation on the windward ridge gradually moved toward the hill foot, as the hill height increased. On the leeward part, a downward flow was observed. Considering the \(135^{\circ}\)-ray ridge (windward), the profiles also show the occurrence of circulation on the ridge, which was relatively weaker compared to that on the \(x\)-axis ridge. In addition, the weak vertical wind found at the hill foot implies a flowing-over horizontal wind at that place. For the sites located at the \(y\)-axis and \(45^{\circ}\)-ray ridges, the wind profiles also showed a downward flow. Moreover, the vertical winds on the \(135^{\circ}\)-ray, \(y\)-axis and \(45^{\circ}\)-ray ridges were relatively weaker, compared to that found on the \(x\)-axis ridge. Figure 20 shows the contour maps of the maximum vertical wind velocity. It was observed that both the global maximum vertical wind velocities corresponding to the upward airflow and those corresponding to the downward airflow initially increased with the increase in hill height before reaching a peak value, then decreasing. The peak value for the upward airflow was \(0.5V_{\text{jet}}\) (\(H/h=1/4\)), whereas that for the downward flow was \(0.65V_{\text{jet}}\) (\(H/h=1/6\)). Regarding the downward wind, it was found that the global maximum typically occurred at the leeward part of the \(x\)-axis ridge if \(H/h\leq 1/6\), and at the hill foot of the \(y\)-axis ridge if \(H/h>1/6\). With regards to the upward wind, the global maximum appeared at the windward \(x\)-axis ridge if \(H/h\leq 1/4\), and at the two sides of the hill if \(H/h>1/4\). These findings again highlight that if the hill is sufficiently high, a circumferential flow occurs on the sides of the hill. Figure 19: Profiles of vertical wind velocity (\(L=0.5D_{\text{jet}}\), \(r_{\text{h}}=1.0D_{\text{jet}}\)). To clarify the impact of the hill position on the maximum vertical wind velocity, Figure 21 illustrates the corresponding contour maps for a typical hill model (\(L=0.5D_{\rm jet}\) and \(H/h=0.25\)) placed at \(r_{\rm h}=0.8D_{\rm jet}\), \(r_{\rm h}=1.0D_{\rm jet}\), and \(r_{\rm h}=1.2D_{\rm jet}\), respectively. These locations of the hill models are around where the strongest near-ground horizontal wind would be observed in a corresponding undisturbed downburst. The three subplots show a similar phenomenon; that is, the upward airflow occurs on the windward side and the downward airflow mainly occurs on leeward side. In addition, correspondingly with the increase in the horizontal distance from the downburst, the vertical wind velocity of the upward flow increased, whereas that of the downward flow decreased. Furthermore, the strong upward flow mainly appeared around the \(x\)-axis ridge, whereas the strong downward flow appeared at the hill foot of the \(y\)-axis ridges.