# Turbulence: A Significant Role in Clear-Air Echoes of CINRAD/SA at Night
## 6 Conclusions  
In this study, clear-air echoes detected by CINRAD were analyzed to find their causes. Some observations diverge from the previous conclusion that bio-scatterers are the main reason for clear-air echoes.  
Echoes with a larger Z are not deformed in the air, even if the scatterers are moving. The change in echoes in the vertical direction is also closer to the switching of the physical mode rather than biological flying. The analysis results of the DWR and the VAD support that turbulence plays an important role in clear-air echoes. In the case of May 2, the frequency distribution of the DWR peaks at 21.5 dB, which is consistent with the theory of turbulence. From 1 May to 20 May, 58% of the DWR between the S-band and the X-band is distributed between 18 dB and 24 dB, which means that more than half of the echoes at night were caused by turbulence. It was confirmed that the influence of turbulence is erroneously ignored in clear-air echoes. The reflecting-layers model of troposcatter propagation is the cause of the clear-air echoes, and this model can explain the main phenomena of the radar observations.  
Figure 17: Time–height cross-section of the Z values (**a**) from 22:30 to 24:00, and the vertical profiles of the radar products (**b,c**) and rawinsonde products (**d,e**) at 23:15 UTC on 2 May 2021. The profiles of (**d,e**) are measured by rawinsondes. The estimated entrainment layer is based on the maximum vertical gradient in each variable (orange line).  
This study provides initial evidence for a case study in Beijing that the model based on [PERSON]'s theory may not be tenable for all clear-air echoes, highlighting the need for an expanded set of causes of clear-air echoes. The reflecting-layers model, which is one of the three models of tropospheric scattering, cause a diurnal variation in clear-air echoes and enhances the signal of clear-air echoes at night. Unmixed air masses with relatively sharp gradients become the reflecting layer and generate the scattering signal of clear-air echoes.  
With the help of the theory of troposcatter propagation, rapid progress will be made in the ecological monitoring method of weather radars. A more objective and comprehensive study of clear-air echoes can effectively make weather radars acceptable in biological research instead of ignoring irrationalities. With the help of weather radars, ecology will be able to develop strongly and continuously in the near future.  
Conceptualization, Y.T. and S.M.; methodology, Y.T.; software, Y.T.; validation, Y.T.; formal analysis, Y.T.; investigation, Y.T.; resources, S.M. and H.C.; data curation, S.M. and H.C.; writing--original draft preparation, Y.T. and T.L.; writing--review and editing, Y.T. and T.L.; visualization, Y.T. and T.L.; supervision, S.M. and H.C.; project administration, Y.T., S.M. and H.C.; funding acquisition, Y.T., S.M. and H.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 42205145 and 31727901.  
Not applicable.  
The authors thank [PERSON], Beijing Meteorological Observation Center, China, for providing the RPG-HATPRO-G5 and the Windcube 100 s data.  
The authors declare no conflict of interest. The funders had no role in the design of this study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.