## 3 Wind Climate Assessment of Transmission Tower Site  
A wind climate assessment was carried out to determine the multidirectional extreme wind speed at the transmission tower site with different return periods. The Ningbo area belongs to the mixed wind climate region, which is influenced by East Asian mansoons and Northwest Pacific typhoons all year round. The daily maximum wind speed data (10-min time interval) and the corresponding wind direction records of the Ningbo ground meteorological station (58562 Yinzhou) from 1 January 1967 to 31 December 2019 were used as samples, and the data were extracted from the China Meteorological Data System ([[http://data.cma.cn/](http://data.cma.cn/)]([http://data.cma.cn/](http://data.cma.cn/)), accessed on 1 July 2021). According to the meteorological data specification, the collected wind speed data was carefully calibrated by adjusting the observation height, observation time interval and so on to the standard conditions. Figure 9 shows the division of 16 wind angles. D1 is the northerly wind direction, rotated clockwise every 22.5 \({}^{\circ}\) as a wind direction, divided into D1, D2,..., D16. The observed daily maximum 10-min average wind speed series were categorized into 16 wind direction angles. By excluding the typhoon wind speed data, the annual maximum wind speed series of constant winds in each wind direction can be filtered. Figure 10 shows the annual maximum wind speed series of constant winds in wind direction D14. Figure 8: Long-term corrosion depth prediction curve for carbon steel in Ningbo. Figure 9: Definition of the wind direction azimuth. Due to the limitation of the sample size of historical typhoon wind speed observations, this paper adopted the typhoon full-track simulation and applied the [PERSON] wind field model to obtain the 600-year near-surface typhoon wind speeds affecting the Ningbo area [31]. The full-track simulation process of a single typhoon is shown in Figure 11, which can be divided into five steps, i.e., establishing the genesis, track and intensity models, calibrating the track and intensity simulation results and activating the typhoon wind field model. The Vickery empirical model [32] was used to determine the values of the key wind field parameters, such as the maximum wind speed radius \(R_{max}\) and Holland pressure profile parameter \(B\). The roughness length was taken as 0.05 m. The 600-year typhoon wind speed series obtained from the simulation was filtered and categorized according to the definition of the wind azimuth in Figure 9, and the annual maximum typhoon wind speed series under wind direction D14 is shown in Figure 12.  
Figure 11: Flow chart of the full-track simulation of a single typhoon. Figure 10: Annual maximum wind speed series of the constant winds in wind direction D14. Based on the \(t\)-Copula function, the joint probability distribution models of the annual maximum wind speed series of constant wind and typhoons under 16 wind directions were constructed, respectively, and the multidirectional extreme wind speeds under different return periods were calculated by using this joint distribution model, and the specific procedures can be referred to the literature [33]. Figure 13 shows the marginal probability distribution of the annual maximum wind speed in wind direction D14. It can be seen that this wind direction is dominated by typhoons. In fact, all 16 wind directions in the Ningbo area are controlled by typhoons. In this paper, the parameters of the \(t\)-Copula function were estimated by the Maximum Likelihood Estimation (MLE). Table 2 shows the correlation coefficients of the adjacent wind directions in the correlation matrix. It can be seen that the correlation coefficients of the adjacent wind directions of the typhoons in the Ningbo area all exceed 0.7. The correlation coefficient of the adjacent wind directions between wind direction D2 and wind direction D8 even reaches 0.99, which proves that the correlation of the adjacent wind directions is very significant.