## 4 Wind Field Characteristics  
### Mean Wind Speed and Reduced Turbulent Fluctuation  
The slowly-varying mean wind speed of the thunderstorm was obtained by a moving average filter, or running mean, the time interval of which could be 30 s according to previous research results [19; 25; 31]. Figure 7 shows the time history curves of the slowly-varying mean wind speeds of the thunderstorm, obtained by 9 anemometers at different heights. Slowly-varying mean wind speeds did not change the basic shape of the instantaneous wind speeds, and there were still obvious jumps at all heights. The thunderstorm wind speeds varied greatly with time, presenting clear non-stationary characteristics. With the increase in wind speed, the thunderstorm wind directions at different heights gradually changed from south to northwest, and then remained almost unchanged during the process of the thunderstorm. It was found that the duration of the thunderstorm wind gradually increased with elevation. Figure 6: Vertical wind speed (blue) and wind attack angle (red) time history of the thunderstorm recorded at 16 m (**a**) and 64 m (**b**). Figure 7: Slowly-varying mean wind speed (**a**) and direction (**b**) time history. Figure 8 shows the slowly-varying mean wind speed (a) and wind direction (b) profiles in the 30 min time duration. Before the thunderstorm, the wind speed was small and the wind speed profile fluctuated slightly along the height. When the thunderstorm occurred, the slowly-varying mean wind speed changed abruptly. Especially around 14:29, the wind speed reached its peak, and the slowly-varying mean wind speed did not increase monotonically with the height. This is depicted in Figure 8 with the color change, and it is clearly different from the typical characteristic of wind speed profiles of synoptic winds. Figure 8 represents the changes of wind direction during the rising phase of the thunderstorm wind. Combined with Figures 5 and 7, the low-level wind direction changes when the thunderstorm wind speed begins to increase, and then remains stable, while the high-level wind direction gradually shifts as wind speed increases. It can be inferred that the upper wind direction was mainly controlled by the moving wind speed of the thunderstorm wind, while the lower-level wind direction was mainly controlled by the scattered outflow vortex caused by the collision between the thunderstorm downdraft and the ground, and the moving wind speed. Figure 9 illustrates the evolution of the slowly-varying mean wind speed (a) and wind direction (b) profiles at a series of key instants during the passage of the gust front. At 14:15 and 14:20, i.e., before the occurrence of the thunderstorm event, the background wind was dominant, the wind speed changed slightly with respect to height, and the wind direction was varied according to altitude, especially near the ground and at high altitude. This may be related to the effect of ground roughness. When the thunderstorm started, at 14:25 and 14:27, the wind speed increased with height and time, and at the same time, the wind direction tended to turn clockwise. Near the peak of the thunderstorm wind, i.e., at 14:29, the nose shape profile could be clearly observed, and the maximum value of slowly-varying mean wind speed at the tip of the nose appeared at the height of 64 m. At 14:30, the wind speed decreased at low altitude and increased at high altitude. Meanwhile, the nose profile gradually diminished. Subsequently, the wind speeds gradually decreased at all altitudes and the nose profile disappeared. The occurrence of the nose shape of the wind speed profile in the thunderstorm event was limited to quite short time intervals during the ramp-up and peak stages, which is consistent with those evaluated for the thunderstorm outflows in the Northern Tyrrhenian [21]. During this process, only the wind direction near the ground changed greatly with the height. The wind direction at the upper level remained almost unchanged with respect to height, and gradually turned clockwise over time.