# Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night
###### Abstract  
It is commonly believed that clear-air echoes detected by weather radars are caused by atmobios migration. However, clear-air echoes are sometimes inconsistently related to the activity of living creatures. In some cases, the characteristics of radar products seem to conform to biological scattering, but the movement of echoes cannot be observed. For these reasons, we sought to expand the cause of clear-air echoes from a Chinese Doppler S-band Weather Radar (CINRAD/SA) in Beijing. Some contradictions were discovered in a case which diverged from previous conclusions. It was 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. 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. A new model of troposcatter propagation, the reflecting-layers model, was then introduced to explain the radar observations. According to the reflecting-layers model, the echoes' diurnal variation and reflectivity characteristics are attributed to the effects of turbulent mixing. Excessive turbulent mixing affects the generation of the reflective layer, thereby weakening the echo signal. It is necessary to re-examine the position of turbulence in clear-air echoes. weather radar; clear-air echo; turbulence; troposcatter propagation; aeroecology 202315781.1223315781.1223315781.  
the scattering process of clear-air echoes [7], it is recognized that most of the echoes at centimeter wavelengths are primarily caused by insects and birds. This viewpoint is based on the theory of locally homogeneous isotropic turbulence developed by [PERSON] [8; 9]. The [PERSON] theory holds that turbulent motion is homogeneous and isotropic, and its average properties are uniquely determined by the average rate of dissipation of the turbulent kinetic energy per unit mass of fluid within a subrange or regime of turbulent eddy sizes [9]. According to this theory, [PERSON] clarified the relationship of turbulence scattering (frequently referred to as Bragg scattering) from refractive index irregularities to the atmospheric structure [8]. In light of this relationship, [PERSON] compared the reflectivity at different wavelengths and examined the differential reflectivity at the S-band, concluding that biological scatterers dominate clear-air echoes because a smaller reflectivity difference and a nonzero value of the differential reflectivity are not consistent with Bragg scattering [3]. However, [PERSON]'s query (1957) caused [PERSON] and [PERSON] (1961) to introduce important modifications to [PERSON]'s theory. They took into account spatial fluctuations in the turbulent energy dissipation and chose a specific form (lognormal) for the probability density as a third hypothesis [10]. However, [PERSON] (1976) pointed out that the lognormal assumption is only a special, probably physically unrealistic, case of weighted curdling [11]. Moreover, many experimental cases for the fine-structure intermittency of turbulence showed the inhomogeneity of physical quantities in space and time [12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. [PERSON] and [PERSON] firstly observed that the turbulence and its energy dissipation were very unevenly distributed over space, and the velocity gradients became increasingly intermittent as the Reynolds number increased [13]. [PERSON] made a numerical simulation and found that 95% of the energy dissipation is concentrated in a tiny region of space [15]. Turbulent mixing of a passive scalar is an extremely intermittent phenomenon [21]. Intermittency has been shown to be one of the fundamental mechanisms of turbulence. Conclusions regarding clear-air echoes are overshadowed by theoretical defects.