# Experimental Simulation of Thunderstorm Profiles in an Atmospheric Boundary Layer Wind Tunnel
## 1 Introduction  
Thunderstorm (TS) is a weather phenomenon characterized by an intense form of convection that in some cases would produce a downburst. The wind velocity associated with downbursts is non-stationary in time intervals of 10 min to 1 h, and the vertical profile of the slowly varying mean component has a characteristic nose-shape [1]. In particular, the maximum wind velocities (i.e., the nose) tend to occur at heights of around 50 m to 120 m [2]; however, there are uncertainties regarding the parameters that govern the height of the nose. In some cases, wind speeds due to thunderstorms are comparable in magnitude to those of strong synoptic events, leading to the collapse of many structures around the world, especially low-rise structures such as transmission towers, harbor cranes, warehouses, signboards and canopies, which are currently being designed for standard synoptic events. This may be due to the fact that, typically, in structural design, reference wind velocities evaluated from databases and codes of practice containing synoptic and thunderstorm events are considered in conjunction with standard boundary layer wind speed profiles. For low-rise structures this approach could give rise to unsafe designs, due to the coincidence of their average heights with the height of the nose [3]. For tall buildings, this methodology is most likely conservative, since assuming a typical logarithmic mean wind profile could provide an overestimate of wind actions with respect to downburst profiles, characterized by a maximum velocity at intermediate heights above the ground. The effect of thunderstorm downbursts on civil engineering structures has become a focus of several investigations and studies around the world with the ultimate goal of codifying methodologies that would allow designers to safely design structures against these events. Despite the effort in the study of these phenomena, a comprehensive analytical model for thunderstorm outflows and their loading on structures, similar to that provided for synoptic events, is still not available [4]. Analytical models for the mean wind speed profile have been developed through the analysis of field measurements, laboratory tests and numerical simulations. The key parameters in the vertical profile representation are the height of the nose and the maximum velocity at this height. A literature review of several research studies [2; 5; 6] indicates that the maximum horizontal velocity appears in the interval 50-120 m above ground level. In terms of turbulence intensity, a consolidated model to recreate the vertical profile is not available due to the shortage of multi-point full-scale measurements for these phenomena. Therefore, some papers in the literature propose a constant value of turbulence intensity independent on the height (e.g., [7]). Wind tunnel testing is an effective tool to estimate wind loading, especially on tall buildings [8; 9]. Experimental simulation of thunderstorm downbursts can be carried out in two types of facilities: 3D wind simulators and Atmospheric Boundary Layer (ABL) wind tunnels. The most widespread technique aimed to reproduce downburst is the impinging wall jet, in which a jet impinges on a flat surface to create a wall radial outflow and a vortex ring, using equipment built for this specific purpose (e.g., [8; 10]). However, these types of facilities are expensive and normally available in research centers, where standard civil engineering projects do not have an easy access to them. Therefore, more practical and cost-effective approaches have been proposed in order to reproduce thunderstorm outflow conditions in ABL wind tunnels. Investigations have been developed to reproduce downburst outflow in traditional wind tunnels, using active devices such as multi-blade flow systems (e.g., [11; 12]) or in a new generation of facilities such as multiple-fan wind tunnels (e.g., [13; 14]). These techniques can simulate both the nose-shaped profile, and some non-stationary characteristics of thunderstorm downbursts.