## 4 Results
### Paradox of Biology and Clear-Air Echo  
Meteorological clear-air echoes are often confused with other types of echoes. For example, there is a common misconception that the echoes in Figure 5 are caused by the super-refraction of radar waves. It is known that super-refraction increases ground clutter  
Figure 5: Radar products from the CINRAD/SA radar, on 7 May 2021, at 15:00 UTC for an elevation angle of 1.45\({}^{\circ}\). Figure 6: Total precipitation of ERA5 reanalysis product (**a**) and brightness temperature of band-14 (11.2 \(\upmu\)m) observed by Himawari-8 satellite (**b**), on 7 May 2021, at 15:00 UTC. The red dot represents the location of the CINRAD/SA weather radar. at the lowest elevation and is the cause of what is normally referred to as anomalous propagation or AP echoes. In reality, the echo of super-refraction is one kind of ground clutter generated under special atmospheric conditions. The majority of clear-air echoes have significant Doppler radial velocities, which are not characteristic of ground clutter. For a long time, the study of meteorological clear-air echoes has been based on an incomplete understanding of echoes and on the results of echo statistics. Clear-air echoes are inconsistently related to the activity of living things, but few studies have focused on this problem. [PERSON] et al. found that the progression and movement of clear-air echoes do not conform to the rules of biological activities. The frequency distribution of dual-wavelength ratio peaks is 21.5 dB, which is in accordance with [PERSON]'s turbulence theory [36]. In this study, from 1 May to 20 May, the 58% dual-wavelength ratio between the S-band and the X-band was distributed between 18 dB and 24 dB. These results show that more than half of the clear-air echoes of CINRAD/SA at night were caused by turbulence in Beijing. However, some researchers still assume that the low correlation coefficient values and azimuthal dependencies of differential reflectivity and differential phase point to reflections from birds. However, when the carrier signal of communication passes through the wireless channel, it will also generate carrier phase deviation due to the multipath effect, propagation path bending, and other reasons. In fact, the differential phase shift is a cumulative quantity. The differential phase \(\mathcal{O}_{\text{DP}}\) is written as  
\[\mathcal{O}_{\text{DP}}=\delta+2\int_{0}^{\tau_{0}}K_{\text{DP}}(r)dr \tag{26}\]  
where \(\delta\) is a backscatter differential phase shift, and \(K_{\text{DP}}\) is the derivative of the differential phase shift, which is commonly used to estimate rain attenuation. [PERSON] and [PERSON] state that the estimates of the backscatter differential phase \(\mathcal{O}_{\text{DP}}\) are quite noisy for insects and meteorological scatterers for which \(\rho_{h\upsilon}\) is usually larger than 0.8. Further, the variation in the differential phase shift of the echoes for meteorological scatterers is small [3]. [PERSON] and [PERSON] stated that biological scatterers can have potentially large and highly variable \(\delta\) values, which, combined with the continuous cyclic nature of the phase, results in erratic discontinuities in azimuthal functions of \(\mathcal{O}_{\text{DP}}\)[37]. [PERSON] et al. also stated that the \(\mathcal{O}_{\text{DP}}\) texture shows a clear delineation between biological and non-biological signals, with weather having much less phase variation along the radial [38]. Thus, if clear-air echoes are produced by birds or insects, the value of \(\mathcal{O}_{\text{DP}}\) changes drastically and the gradient of the \(\mathcal{O}_{\text{DP}}\) (or \(\mathcal{O}_{\text{DP}}\) texture) value is large and discrete. In Figure 7, only a few areas have large \(K_{\text{DP}}\) values, and most echoes are devoid of bio-particle scattering. There are usually larger values of \(K_{\text{DP}}\) close to the radar region.