## 3 Measurement of Thunderstorm Winds
### Typical Event  
In order to study the vertical variation properties of the thunderstorm wind field with time, a typical case was chosen, and this was a thunderstorm event observed on 8 June 2014 by the Beijing 325-m-high meteorological tower. Table 1 illustrates the 1 s peak wind speed of the resultant horizontal wind speed (classical method) and longitudinal wind speed (directional method). It can be observed that the 1 s gust wind speed of longitudinal wind speeds obtained by the directional model was slightly smaller than that of the resultant horizontal wind speed obtained by the classical method. The 1 s gust wind speeds of thunderstorms detected at all heights were high, while the values did not increase monotonically with the increase in height. The maximum value appeared at the height of 64 m, which is consistent with the unique vertical distribution characteristics of thunderstorm wind speed [28,29].  
Figure 5 shows the 1 h wind speed (blue) and wind direction (red) time histories recorded by the ultrasonic anemometers at 16 m (a) and 64 m (b), respectively. As shown in the figure, when the thunderstorm occurred, there was a rapid and strong jump in terms of wind speed, and the wind direction changed about 180\({}^{\circ}\). In addition, according to the temperature record, the temperature dropped by about 10 \({}^{\circ}\)C with the sudden change of wind speed, which is consistent with the typical weather feature of thunderstorm wind.  
Figure 6 shows the 1 h vertical wind speed (blue) and wind attack angle (red) time history curves recorded by the ultrasonic anemometer at 16 m (a) and 64 m (b), respectively, in which wind attack angle is the angle between the wind flow and the vertical line and the positive direction is directed upwards. It can be observed that the vertical wind speed increased when the peak of the thunderstorm gust wind speeds was reached. At both 16 m and 64 m, when the instantaneous wind speed began to increase at the initial stage of the thunderstorm, the wind attack angle diminished almost to 0. Then, the wind attack angle  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**h (m)** & **8** & **16** & **32** & **47** & **64** & **80** & **140** & **200** & **280** \\ \hline \(\hat{U}\) & 14.76 & 19.00 & 23.25 & 23.71 & 24.75 & 19.87 & 21.22 & 21.20 & 23.68 \\ \(\hat{u}\) & 14.30 & 18.38 & 23.08 & 23.68 & 24.73 & 19.78 & 21.11 & 21.17 & 23.62 \\ \hline \end{tabular}
\end{table}
Table 1: Peak wind speed.  
Figure 5: Wind speed (blue) and direction (red) time history of the thunderstorm event recorded at 16 m (**a**) and 64 m (**b**).  
increased and appeared to be oscillating strongly, indeed more at 16 m than at 64 m. After the jump of the wind speed, the vertical wind speed decreased at the height of 16 m, while it continued to increase at the height of 64 m. On the whole, the meteorological tower was not in the core area of the downdraft of the thunderstorm wind. This may be because of the movement of the thunderstorm downburst along the northwest after the occurrence. During the movement, the scattered outflow vortices passed through the weather tower.  
According to the upper air map and sounding data of the day, the severe convective weather was mainly caused by the northeast cold vortex, and the weather was supposed to be a thunderstorm gale of the northwest flow type. From 8:00 to 14:00, the convective effective potential energy increased from 0 to 954.8 J/kg, and the wind shear at the lower level was moderately strong vertical, reaching 3.1 \(\times\) 10\({}^{-3}\) s\({}^{-1}\) between 0 and 2 km. The wind index reached 28 m/s, the temperature difference between 500 and 850 hPa was 32 \({}^{\circ}\)C, and the temperature decline rate was 0.8 \({}^{\circ}\)C/hm, presenting the typical weather features of thunderstorm wind [30].