# Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill
## 1 Introduction  
Downbursts, resulting in a high near-ground wind velocity, are recognized as a primary factor in most transmission tower collapse events [1, 2], and have received much attention in wind engineering. Figure 1 shows several collapsed transmission towers, erected on hilly terrain, due to a severe downburst event in Shaoxing, China. In addition, concerns regarding downburst-induced wind loadings on other structures that are vulnerable to winds, e.g., long-span bridges [3] and rail overhead lines [4], have also been raised recently. Considering downburst effects on structures erected on islands, hills, or mountains, a large number of studies [5, 6, 7, 8, 9, 10, 11, 12, 13] utilizing various idealized hill models have been conducted in the past decades, and have reached a consensus that the aerodynamic interference of hilly terrain leads to a speed-up phenomenon, which a priori increases the risk of structural failure. Nonetheless, [PERSON] et al. [5] recently emphasized that the effects of topography on downburst wind fields require further research, as most studies have been limited to two-dimensional (2D) topographic features, and the changes in the vertical downburst wind speed component are not well understood. To the best of the authors' knowledge, no field measurement-based study has been performed to examine the effects of hilly terrain on a downburst wind field from the perspective of wind engineering. Moreover, in terms of wind profile, which is a factor in determining wind loadings on structures, the relevant field test results are limited and scattered in the literature. The earliest study of downburst phenomena can be traced back to the pioneering work of [PERSON] [14; 15], who named the downburst and reported an impressive wind record in a downburst event near Washington, DC, during which the peak velocity reached 67 m/s at 4.9 m above the ground [16]. [PERSON] [17] investigated variation in wind velocity with height during a thunderstorm event in Singapore, and documented that the measured wind profiles matched the results from the impinging jet model. Using the data of the Joint Airport Wind Shear (JAWS) Project, [PERSON] [18] presented the ring vortex structure of the downburst flow, and pointed out that the outflow was found to resemble the impinging jet model in terms of wind profiles. [PERSON] and [PERSON] [19] reported the full-scale vertical profiles of full-scale nonstationary downburst wind speeds via seven evenly deployed towers at the Reese Technology Center, Texas, USA, and found that the time function for the time-varying mean speed is significantly different from event to event. Additionally, using data collected via Doppler radars, [PERSON] and [PERSON] [20] showed the evolution of the mean wind profile from a uniform to an impinging jet shape. In contrast, substantial laboratory tests and numerical simulations, most of which are based on an impinging jet model, have been conducted to reveal insights into hilly terrain-disturbed downburst wind fields. Note that the impinging jet model conventionally employed for these studies makes it difficult to reproduce the dynamic characteristics of downbursts, particularly the ring vortex [16]. Selected experimental results and numerical simulation results from the literature are tabulated in Table 1, where \(D_{\rm jet}\) and \(h\) are the jet diameter and the jet height of the downburst, respectively; \(H\), and \(\phi=H/2L_{\rm u}\) are the height and the representative slope of the terrain, respectively; \(L_{\rm u}\) is the horizontal distance from the crest to the site with altitude of \(H/2\); \(r_{\rm h}\) is the horizontal distance from the crest to the stagnation point of the downburst; and \(M_{\rm t,max}\) is the topographic multiplier for the maximum velocity amplification [9; 10]. In the table, for the studies in which the maximum of \(M_{\rm t,max}\) is not declared, the data are read directly from the relevant figures, which may lead to the existence of a small inevitable error.