**1. Introduction**

The unique features of the Tibetan Plateau (TP), such as its complex terrain formed by high mountains and valleys, dramatic changes in the atmospheric environment, differences in atmospheric composition, and unique geographical climate and circulation characteristics, form different atmospheric optical turbulence characteristics over the TP from those of low-elevation plain regions [1]. The TP is the major energy source providing sensible and latent heat fluxes to the atmosphere depending on the turbulence processes that occur during land–atmosphere interactions for mass and energy exchanges [2]. The combined effect of the complex terrain of the TP and the heat source enables the development of turbulence in the middle and upper atmosphere over the region.

Strong Asian summer monsoon circulations exist above the TP, including deep convective activities and planetary-scale anticyclones, such as the South Asian high, SAH (hereinafter referred to as the Asian summer monsoon anticyclone (ASMA)) [3,4]. The ASMA is stable and strong in the vertical direction at 70–300 hPa and occupies almost

**Citation:** Zhang, K.; Wang, F.; Weng, N.; Wu, X.; Li, X.; Luo, T. Optical Turbulence Characteristics in the Upper Troposphere–Lower Stratosphere over the Lhasa within the Asian Summer Monsoon Anticyclone. *Remote Sens.* **2022**, *14*, 4104. https://doi.org/10.3390/ rs14164104

Academic Editor: Michael E. Gorbunov

Received: 16 July 2022 Accepted: 18 August 2022 Published: 21 August 2022

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the entire upper troposphere–lower stratosphere (UTLS) area [5], which is closely related to plateau land-atmosphere heat transfer [6,7]. The ASMA is closely related to frequent convective activities, particularly during the period from June to September. The geographic location of the ASMA center varies over periods of a few days or even over longer periods [8]. The coupling of atmospheric circulation and convection that prevails over the TP during the summer results in the frequent occurrence of convective activities in the lower atmosphere. Less details are known about the influence of the ASMA on the thermodynamic structure of the atmosphere in the stratosphere.

The strong convective activities and ASMA on the TP affect the atmospheric components and their distribution in the UTLS of the Asian monsoon region by uplifting the lower atmosphere [9–11]. A turbulent atmosphere is an important transport medium in stratosphere–troposphere exchange (STE). First, convective injections can impact air and aerosol transport from the atmospheric boundary layer (ABL) to the UTLS [12,13]. In contrast, deep convection activities carry low concentrations of ozone and high concentrations of water vapor into the ASMA, which remain inside the ASMA for a period of time, and they are relatively isolated from the outside air and subsequently uploaded to the UTLS [14,15]. Moreover, the vertical distribution of atmospheric turbulence is one of the factors that must be inevitably considered in the astronomical site testing of ground-based astronomical optical telescopes [16]. Atmospheric turbulence is the main reason for the serious degradation of optical imaging quality, and it is also an important indicator for comparing the quality of astronomical sites [17,18].

TP has garnered significant attention as the third pole of the Earth. In the past three decades, researchers have conducted several atmospheric scientific experiments on the TP combined with numerical simulations. Some studies have been conducted mainly on the structure of the ABL over the TP and its surrounding areas, including studies on the occurrence and development of weather systems and the impact of the TP on atmospheric circulation [19,20].

However, the relationship between the optical turbulent structure and meteorological parameters in the UTLS has rarely been studied [21]. The results of observations and numerical simulations obtained in recent years indicate that the ASMA region surrounding Lhasa as the core area is an extremely important area, through which surface pollutants enter the global stratosphere. The transport of these materials into the stratosphere through atmospheric turbulence has important effects on the global climate and environment through microphysical, chemical, and radiative processes [22–24].

At present, most research on atmospheric turbulence structure in UTLS over the TP are studied based on model simulations or reanalysis, because in situ observed turbulent data are scarce. In consequence, details regarding the structure of atmospheric turbulence in the UTLS, STE process, and characteristics of atmospheric composition budgets are still lacking. The widely used measurement techniques of high-altitude atmospheric turbulence characteristics are roughly divided into two categories. One is the path-averaged turbulence intensity measurement technique, such as the optical triangulation method [16,25] and extension technology [26]; the other is the localized turbulence intensity measurement technique, such as the micro-thermal pulsation method. In situ turbulent measurement based on radiosonde is a simple but very reliable, effective, high-precision, and commonly used atmospheric detection method, particularly the micro-thermal sensors mounted on the balloon, which realizes the measurement of atmospheric turbulence in UTLS [27–30]. As such, not only basic atmospheric parameters such as temperature and humidity can be measured, but also the value of atmospheric refractive index structure constant *C*<sup>2</sup> *n* in the middle and upper atmosphere is obtained simultaneously, which is an important parameter to measure turbulence intensity. This study analyzes the reasons for the large short-term fluctuations of *C*<sup>2</sup> *n* in the Lhasa region from the aspects of atmospheric turbulence parameters, the ASMA, high-pressure activities at 500 hPa, and atmospheric circulation.
