# Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night
## 2 Concepts and Theory
### Biological Scattering  
Unlike raindrops, biological scatterers have complex shapes that result in highly aspect-dependent scattering characteristics. As a result, radar cross-sections (RCS) are normally modeled by prolate spheroids of equivalent mass [7; 31]. It is apparent that insects and birds, which are highly non-spherical, would exhibit large \(Z_{DR}\) signals and low copular correlation coefficient values.  
However, there are polarimetric differences between birds and insects. Insects often have a high \(Z_{DR}\) (up to 10 dB) and a relatively low differential phase, while birds may have a lower \(Z_{DR}\) (1 to 3 dB) and a much larger differential phase [5]. Moreover, for both types of echoes, the cross-correlation coefficient is between 0.3 and 0.5, which is lower than the hydrometeorological signal. Polarimetry becomes a technical standard of the application of a dual-polarization radar to delineate meteorological and non-meteorological signals in the areas of aeroecology.  
Based on fuzzy logic, some researchers have differentiated bio-scatterers by using typical values of polarimetric variables [32]. [PERSON] proposed an estimate of the depolarization ratio for separate types of echoes [33]. Overall, the polarimetric characteristics of bio-scatterers are the primary means by which to solve this problem.  
For radars that cannot measure the polarization characteristics, the difference derived from the Doppler velocity is another indicator of the scatterer type. Insects reasonably represent actual winds, whereas birds have a much larger independent velocity component [34]. Therefore, the radial velocity standard deviation obtained from the velocity-azimuth display retrieval is an indicator of migration. For high migration densities, the raw weather radar wind vectors could be up to 15 m/s for birds and 6 m/s for strong insects [7]. However, for micro-insects, which are weak flyers, their motions are primarily wind-driven and behave as quasi-passive wind tracers. Thus, migrating and wandering birds are the main targets of Doppler wind measurement.  
Biological scattering and Bragg scattering are always rivals. Compared with Bragg scattering discussed above, the ratio of the \(Z\) values for two radar wavelengths is more complicated. When the physical diameter of the spherical particles is considerably smaller than the radar wavelength (approximately 6.25 mm for the S-band), Rayleigh scattering can be used. On the other hand, for larger particles, Mie scattering occurs. The different scattering makes the relation between their size and \(Z\) values far from straightforward. Provided that Mie scattering is occurring at one or both of the wavelengths, the ratio of the \(Z\) values with a spherical diameter is nonlinear [3].  
For example, using prolate spheroids of a spinal cord dielectric, a new model that is closer to real insects [35], the RCS could be simulated by the method of moments, as shown in Table 1, according to biological datasets provided by the Chinese Academy of Agricultural Sciences. Additionally, based on the simulation, the DWRs were 1.9 dB, 13.8 dB, and 17.0 dB between the wavelengths of the S-band and X-band. Further, all birds and many common insects are above the 2 mm size threshold for Mie scattering at the X-band. Measurements of volumes containing multiple scatterers are likely often biased toward the characteristics of the largest scatterers [31]. Thus, comparing the value of the DWR is a valid way to be certain of the dominant mechanism of scattering.