**5. Conclusions**

In this study, the impacts of the ASMA and high-pressure activities in the 500 hPa layer on *C*2*n* were analyzed under different ASMA strength states over the Lhasa during the summer based on precious in situ sounding data.

The atmospheric refractive index structure constant *C*2*n* characterizes the optical turbulence intensity, which is directly affected by the atmospheric temperature. The ASMA is a warm, high-pressure system in the upper troposphere that causes the TP to be a strong heat source during the summer, heating the air over the TP. The upper atmosphere diverged to

form a high-pressure circulation system, and the lower atmosphere converged to form a low-pressure circulation system. The "upper highs and lower highs" pressure structure enhances the potential temperature lapse rate, which is conducive for the reduction in static stability and development of optical turbulence in the UTLS. However, once strong highpressure activity exists in the lower pressure layer, such as at 500 hPa, the high-pressure system is dominant from the 500- to 100-hPa layer, constituting an "upper highs and lower highs" pressure field structure. In comparison with the "upper highs and lower lows" pressure field structure observed in most cases, this particular pressure field structure suppresses the vertical potential temperature lapse rate and vertical upward movement, and weakens the atmospheric convective activity. Under the combined action of the ASMA and low-pressure activity over 500 hPa, the potential temperature lapse rate decreased rapidly, and *C*<sup>2</sup> *n* increased by an order of magnitude in the upper-troposphere.

The situations in both the tropopause and lower stratosphere are different from those in the upper troposphere, where atmosphere is almost unaffected by high-pressure activities at 500 hPa. The difference in the potential temperature lapse rate caused by the ASMA is particularly manifested in the region adjacent to the tropopause. The best evidence is that the potential temperature lapse rate in high-intensity ASMA situations is twice as high as that in low-intensity states. In other words, the potential temperature gradient can not only reflect the static structure of the atmosphere represented by buoyancy frequency, but also qualitatively analyze the variation tendency of *C*<sup>2</sup> *n*. Under different ASMA intensities, the potential temperature lapse rate is consistent with the variation tendency of *C*<sup>2</sup> *n*, and the profile on 13 August 2018, was the most evident.

The tropopause height over the TP is close to the 100-hPa layer, corresponding to the scope of activity of the ASMA. The tropopause height is closely related to the turbulence intensity. Strong turbulence elevates the CPT to a higher position, and the CPT temperature is lowered [42,53]. When the impact of the ASMA is greater, the CPT height rises by approximately 1.5 km.

In general, the ASMA has different mechanisms of influence on the atmospheric refractive index structure constant *C*<sup>2</sup> *n* in the upper troposphere, tropopause, and lower stratosphere. It was found that during high-intensity ASMA, turbulent activity in the tropopause and lower stratosphere (in the upper-troposphere) is suppressed (promoted), which may be not conductive to the STE process (astronomical observations).

The extent of the promotion of convection by ASMA is not only related to the position of its center and strength but is also inseparable from the high-pressure activities of the lower atmosphere. Clearly, because of the limited radiosonde data considered in this study, determining whether our discussion is regional and limited and the reasons for the short-term *C*<sup>2</sup> *n* fluctuations necessitate further exploration and analyses with various and abundant data, such as Stereo-SCIDAR measurements [26].

**Author Contributions:** K.Z.: methodology, software, data analysis, writing—original draft. F.W.: investigation. X.W.: data curation, project administration. N.W., X.L.: writing—review and editing. T.L.: project administration, funding acquisition, formal analysis, conceptualization, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA17010104), and the National Natural Science Foundation of China (Grant Nos. 4157685 and 91752103).

**Data Availability Statement:** In this section, the data underlying this article cannot be shared publicly due to the confidentiality requirements of the project in the study.

**Acknowledgments:** Thanks to the reanalysis data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) for the purposes of this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
