## 3 Instruments and Data  
To verify the role of troposcatter in radar, we selected one of the most widely used weather radars in China, China's New Generation Weather Radar (CINRAD)/SA (HuaYun Metstar, Beijing, China), which was developed from the American NEXRAD-WSR-88D (Lockheed Martin, Maryland, America) through a joint agreement between the USA and China [31]. The CINRAD products have a radial distance resolution of 250 m and an azimuthal resolution of 1 degree. The radar uses the scan mode of Volume Coverage Pattern 21 (VCP21), which sweeps nine elevation angles in 6 min. Some system characteristics of the CINRAD radar are listed in Table 2.  
Due to the fact that tropospheric scattering is often weaker compared to scattering from precipitation particles, the influence of attenuation is not considered. The limitation of the minimum detectable reflectivity factor also needs to be paid more attention in the pretreatment of clear-air echoes. Another potential source of error comes from signals with a signal-to-noise ratio (SNR) lower than 6 dB. These restrictions need to be taken into account with radar data to ensure their relative reliability. Although our study focused on S-band weather radar, it is important to note that long-standing misconceptions about clear-air echoes require the use of a number of other devices to change people's perceptions. Therefore, we used some observation equipment other than S-band weather radar. Firstly, due to the specificity of Rayleigh scattering (the scattering intensity is inversely proportional to the fourth power of the wavelength), we used the Beijing X-band weather radar network (called BJ-Xnet) to observe echoes caused by Rayleigh scattering. The Beijing X-band weather radar network was built and is supervised by the Beijing Meteorologic Service. All radars of the network are the same radar type and use the same scan strategy as the CINRAD/SA radar. The Figure 2 shows the location of weather radar and the vicinity topography. Secondly, we tried to use some of the observing equipment to obtain the wind field. Comparison of radar-based Doppler velocities with the wind field is one of the most important basic means of determining the type of echo. However, there are problems in obtaining true wind field velocities. Usually, the most direct way to measure the wind field is to use radiosonde. The World Meteorological Organization (WMO) requires radiosonde to be launched at the same time all over the world (within one hour before 00:00 and 12:00 UTC). Thus, in China, radiosonde is launched every day at 7:15 and 19:15 local time. However, these timings of radiosonde observations cannot include the night-time strong clear-air echoes in summer. Therefore, it is not possible to use the wind field information from the radiosonde data. Doppler wind lidar (DWL) uses the optical Doppler effect to measure atmospheric wind speed with high spatial-temporal resolution and a long detection range and has been widely applied in scientific research and engineering applications. In addition, since lasers do not have the electromagnetic wave's ability to penetrate and bypass, if the measurement \begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **CINRAD/SA Radar** & **X-Band Radar** \\ \hline Frequency & 2700–3000 MHz & \\  & (2870 MHz @ Beijing) & 9300–9500 MHz \\ Antenna coverage diameter & 11.9 m & 2.4 m \\ Polarization & Linear H and V & Linear H and V \\ Volume coverage patterns & VCP21 & VCP 21 \\ Time of VCP21 & 6 min & 3 min \\ Range resolution & 250 m & 75 m \\ Observation range & 230 km & 75 km \\ Azimuthal resolution & \textless{}1 degree & \textless{}0.99 degree \\ Antenna gain & \textgreater{}44 dB & \textgreater{}44.1 dB \\ Minimum detectable reflectivity & \textasci{}7.5 dBZ @ 50 km & 5 dBZ @ 60 km \\ \hline \hline \end{tabular}
\end{table} 
Table 2: System characteristics of CINRAD/SA radar and Beijing X-band radar.