# Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night
## 5 Discussion
Additionally, the unmixed air masses with relatively sharp gradients become the reflecting layer and generate the scattering signal of the clear-air echoes. In contrast, in the daytime, the strong turbulent mixing breaks the structure of the reflecting layer, and only the turbulent  
Figure 16: Turbulence intensities (**a**) and Richardson number (**b**) for 20 h at Daxing, Beijing, 2 May 2021. The black isopleth is the time–height cross-section of \(Z\) (unit: dBZ), which is the same as the fill color in Figure 6.  
scattering remains. Thus, turbulence has a greater impact on the echoes at night than in the daytime. The same diurnal variation has been observed in the field-strength variation in the short radio wave propagation in Arizona [58]. Other studies have found that troposcatter propagation only occurs when the turbulence scale is larger than the wavelength of the radio signal, and the signal level with a wavelength of 9 cm is much larger than that with a wavelength of 3 cm [58; 59]. These phenomena have the same characteristics as clear-air echoes, with the value of the reflectivity factor at the S-band being bigger than that at the X-band. Thus, we propose that the reflecting layers cause a diurnal variation in clear-air echoes and enhance the signal of clear-air echoes at night. Therefore, the coexistence of the reflecting layers and Bragg scattering is the reason why the value of \(C_{n}^{2}\) calculated from the echoes is larger than the value from the theoretical calculations. Further, the reflecting layers explain some of the characteristics of \(Z_{DR}\). For example, the vertical signal of clear-air echoes is weaker than the horizontal signal because turbulent mixing is stronger in the vertical direction, and the vertical reflecting layers cannot easily survive. Thus, the values of \(Z_{DR}\) are generally greater than zero. An interesting note is that the high reflectivity area in the upper-left region of Figure 4 analyzes the characteristics of a low correlation coefficient, and in this area, \(Z_{DR}\) presents a mixture of high and low values. This characteristic is ascribed to the atmospheric response to the underlying surface, which belongs to a mountainous region and is quite different from the others in Figure 1.  
The rough terrain of mountains makes the turbulence more chaotic and intense, which produces a stronger refractive index and scattering. A thin layer of shear turbulence excited on the shear plane makes the \(Z_{DR}\) larger, but the layer is not stable, bringing a mixture of high and low values to the \(Z_{DR}\). However, it is noted that, by upthrust, too high mountains may affect the turbulent scales, which are too large to match up to the radar wavelength for scattering. The relationship between the underlying surface and clear-air echoes is worth further exploration in the future. Another interesting characteristic of the \(Z_{DR}\) is that its value grows weak and becomes close to zero in the day-night shift scenes. We speculate that the turbulent mixing reaches a quasi-equilibrium state, and this will be further investigated in future studies. Yet despite all this, some scholars still express a slightly different point of view. They claim that the reflection from turbulent air at the S-band has been studied by some studies [60; 61; 29; 62; 63], and it has been well established with polarization radars that turbulent air has \(Z_{DR}\) values close to 0 dB at the S-band. However, these echoes were commonly observed at the top of the convective boundary layer (also called the entrainment layer). The entrainment layer is essentially static in stability. Due to the effects of penetrative convection and entrainment, thermals reciprocate in the layer [64]. The reciprocation brings turbulence generation on the one hand; on the other hand, the stable air brings the turbulence closer to a locally homogeneous isotropic state. Thus, the observed echoes can be regarded as special cases and cannot represent the whole characteristic of clear-air turbulent echoes.