# Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night
## 4 Results  
### Plan Position Indicator  
For the sake of discussion, the radar data from May 2 were taken as the case for further study. The focus of this case was from 11:30 (UTC), 2 May 2021, when the clear-air echo first appeared in the radar display, until it began to disappear at 20:30, when the echo became less distinct. The products in Figure 4 show that the echo was characterized by a low \(Z\), a \(Z_{DR}\) higher than that typically observed in meteorological echoes, and a correlation coefficient \(\rho_{HV}\) lower than that observed in meteorological echoes. Generally, similar echoes have been observed in bird scattering comprising many species flying over Southern Kansas from Wichita, KS, USA [5]. Figure 4: Radar products from the CINRAD/SA radar (SDX), Daxing, Beijing, on 2 May 2021 at 13:00 UTC for an elevation angle of 2.4\({}^{\circ}\). The mapped domain is 75 km by 75 km. Figure 3: The simulated vertical profiles of the DWR between the S-band and X-band with the vertical height. The vertical profiles of turbulence fall in the grey-shaded area because of the different turbulence theories. The parameters of the three species are referenced from Table 1. All species are major agricultural pests in North China. Some researchers have indicated that the \(\rho_{HV}\) varies with scatterer orientation, with birds flying away producing low \(\rho_{HV}\) values and birds flying toward producing higher \(\rho_{HV}\) values. However, the velocity-azimuth display (VAD) confirmed that there was no intense bird migration. Analysis of the VAD is used to determine spatially averaged kinematic properties of the velocity field. According to the function of the radar azimuth angle, the VAD of the wind field resembles a sine function. Moreover, during intense bird migrations, a huge difference in scattering of the observed radial velocity around the modeled radial velocities will be evident. Figure 5 shows a high-quality wind VAD, which was the same passage of a cold front as observed by an operational weather radar in De Bilt [45], Netherlands. Only a small part of the residual error exhibited signs of bird activity. Since the nonzero value of the Doppler velocity due to bio-scatterers is representative of biological target movement, the temporal change in \(Z\) is shown in Figure 6 to check for movement. Figure 6 shows a continuous change in \(Z\); the echoes began to increase rapidly at 11:30 and then remained basically unchanged after 12:06. It is strange that the echoes changed with the radar station as the center but not the \"habitat\", and the echoes with a larger \(Z\) also maintained their appearance. Whereas the value of the Doppler velocity was rhythmical, shown in Figure 7, the characteristics of the spatial distribution were also unchanged after 48 min, indicating that the scatterers did not move. To confirm the state of the scatterers' movement, in Figure 8, the range-height cross-section of \(Z\) is displayed in the azimuth of the wind direction. As shown in Figure 8, the signal was enhanced along the height at each range and did not change following wind direction. The phenomena discovered by [PERSON] were not observed [46]. Paradoxically, the scatterer remained in the air, but the Doppler velocity was a nonzero value. Figure 5: **(a)** An example of VAD data from the CINRAD/SA in Beijing. The line is the modeled radial velocities as a sine function of the azimuth, and the dots are the data of the Doppler velocity. The data samples are in the range of 30 km and the elevation of 2.4 deg. **(b)** The residual error (the dot) is essentially less than 2 m/s. The root mean squared error is 1.278, and the adjusted R-square is 0.9924. Figure 6: Combined reflectivity factor for 2.4 h from the CINRAD/SA, 2 May 2021. (**a**–**i**) Continuous observations from the CINRAD/SA at 12 min intervals from 11:12 to 13:36. The horizontal and vertical coordinates are, respectively, the ranges (km) in the west–east direction and the south–north direction.