# Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night
## 3 Instruments and Data
### Preprocessing
#### 3.2.3 Dual-Wavelength Ratio  
Unlike the differential reflectivity measured by means of two orthogonal signals that differ in polarization, the DWR describes the ratio of the radar reflectivity measured with two signals of different wavelengths. In Section 2, it has been found that biological scattering and turbulent scattering have different characteristics of the DWR. The DWR of Braggscattering is dependent on the radar wavelength, as shown in Equation (9). Moreover, the DWR change in biological scattering is unpredictable because of the biologically complex shape. Hence, the predominant scattering mechanism can be determined by the DWR.  
However, it is also known that the value of \(Z\) from biological scatterers has an asymmetric pattern which depends on the angle between the radar beam and the main orientation of the biological scatterers. Moreover, values of the DWR may be more complicated and confusing because of the asymmetric pattern. Thus, it needs to examine the effect of the asymmetric pattern.  
For the examination, the RCS of biological scatterers is simulated by the computer utilizing FEKO simulation software in this study [35]. The simulative value of RCS is dependent on many factors. One of the factors is the incident angle of the radar beam. Although biological scatterers appear randomly at each azimuth angle of the radar, the incident angle of the simulation is only decided by the angle between the incident beam and the biological scatterer. By changing the coordinate origin of the simulation from the location of the scatterer to the location of the radar, the incident angle can be converted to the azimuth angle when the directions of the biological scatterers are same.  
Other parameters of the simulation are obtained from Table 1 based on the model of prolate spheroids of the spinal cord dielectric. Further, supposing that insects fly horizontally in the east-west direction and the density is one per cubic meter, the \(Z\) value can be calculated by the simulated RCS shown in Figure 2. Figure 2 exemplifies the asymmetric pattern of the \(Z\) values and is almost consistent with previous studies [42]. It is found that the asymmetric pattern may lead to confusion about the conclusions of Section 2.  
However, the asymmetric pattern does not affect the results of the method in Section 3.2.3. The vertical profiles of the DWR between the S- and X-band are calculated and shown in Figure 3. The computation is deduced in terms of the scan mode of VCP21. Figure 3 shows that the vertical profiles of the DWR are same on each level of height, and the values are close to the simulation values of the insects' body sizes. This is because the RCS of the insect size changes more slowly with angle. However, it is also found that the \(Z\) difference values of large insects and the values of turbulence are close. Thus, the echo of turbulence is likely confused with that of large insects in the DWR.  
Figure 2: Azimuthal dependencies of the reflectivity factor for three species at the S-band (**a,c**) and X-band (**b,d**). As in (**a,b**), the elevation of the radar beam is 0.5\({}^{\circ}\), which is the minimum elevation of VCP 21, and in (**c,d**), the elevation is 19.5\({}^{\circ}\), which is the maximum elevation of VCP 21.  
Fortunately, large insects have a much stronger flight ability, and flight speed is correlated with body size in animals [43]. Research on the migratory behavior of armyworms expresses that the flight speed of armyworms during migration is greater than 4 m/s [44]. Thus, large insects can be easily distinguished by the velocity azimuth display (VAD). Analysis of the VAD is necessary to exclude confusion with large insects.