Study on wind field characteristics of large aperture radio telescope site based on average wind profile index

Wind load is an important factor that affects the observation performance of radio telescopes. Along with the increase of the telescope aperture, the wind load acting on the observed performance of the telescope appears more obviously. Therefore, it is important for obtaining the wind field characteristics accurately, which would improve the observation performance of the telescope under the wind disturbance. The average wind profile index ( $\alpha$) shows the varying relation between the average wind speed and the vertical height, which is one of the important parameters of the wind field character at a telescope site. This work focuses on the wind field of 110 m aperture Qitai radio telescope(QTT), located in Xinjiang. With the gradient wind tower data of the QTT site, we use the logarithmic replacement way to compute the value of $\alpha$ for the selected data after comparing four calculation methods. The values of $\alpha$ are deduced in condition of 16 standard wind directions. Compared with the standard value of $\alpha$ of class B of the ground roughness ( $\alpha$=0.15), the reference values in our exact evaluation are improved to be 26.65%, 15.76 % and 11.58% in the range of the wind speed of [0, 2.00), [2.00, 4.00) and [4.00, 6.00) m/s, respectively. Based on the analysis of the wind field characteristics of the QTT site, we suggest that the value of $\alpha$ changing with the wind speed is strongly related to the topography of the telescope site and the frequency of incoming wind.