**3. Results**

### *3.1. ASMA Activities during the Experimental Period*

SAH has significant multicenter characteristics [33], particularly bimodality [5,34], which is attributed to the warm preference of the SAH. Only the Tibetan mode is considered in this study. Figure 2 shows the distributions of the ECMWF geopotential height at 200-hPa pressure level on 12 August (Figure 2a), 13 (Figure 2b), 14 (Figure 2c), and 16 (Figure 2d), overlaid with the wind fields. The black square in Figure 2 denotes the location of Lhasa, whereas the black circle denotes the center of the SAH that is associated with the largest geopotential height. As shown in Figure 3a, the center of the SAH gradually moved northeastward during the experimental period. After 14 August, two SAH centers formed, with one located over the TP and the other over the Iranian Plateau.

**Figure 2.** Distribution of the 200 hPa geopotential height (shaded), wind (vector), the 500 hPa geopotential height (contour), and the central region of the ASMA (the black square represents the geographic location of Lhasa, and the black dots represent the strongest negative vortex region. The same symbols are used below). The weather conditions during experimental periods were analyzed using the ERA-interim reanalysis data (http://www.ecmwf.int/, accessed on 17 January 2019) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The horizontal resolutions of the meridional wind and relative vorticity were 2.5◦ × 2.5◦ and 0.25◦ × 0.25◦, respectively. (**a**) 12 August 2018; (**b**) 13 August 2018; (**c**) 14 August 2018; and (**d**) 16 August 2018.

**Figure 3.** (**a**) The geographic locations of the ASMA centers from 12–16 August 2018. A-area (27.5–32.5◦N, 65–80◦E), B-area (27.5–32.5◦N, 80–95◦E), C-area (22.5–27.5◦N, 65–80◦E), and D-area (22.5–27.5◦N, 80–95◦E) were divided to identify the ASMA center (the black square represents the geographic location of Lhasa; the same symbols are used below); (**b**) the mean vorticity of 20 × 20 grids with the ASMA center considered as the geometric center, representing the strength of the ASMA; (**c**) the vorticity at 200 hPa over Lhasa from 12–20 August 2018; and (**d**) the geopotential height anomalies at 500 hPa and 100 hPa over Lhasa from 12–20 August 2018.

Figure 3 shows the development of the ASMA and its influence on Lhasa during the experimental period. The geolocation of the strongest ASMA center (dots in Figure 3a) is defined as the position of the greatest potential height in the strongest anticyclone area among the four areas (denoted as the A-area (27.5–32.5◦N, 65–80◦E), B-area (27.5–32.5◦N, 80–95◦E), C-area (22.5–27.5◦N, 65–80◦E), and D-area (22.5–27.5◦N, 80–95◦E)), according to the literatures [35,36]. In the physical sense, the criterion used to measure the strength of the ASMA is that the potential vorticity (PV) at the center of the anticyclone is smaller than that in the surrounding region [15,37,38]. The intensity of ASMA is shown in Figure 3b. The strength of the high-pressure system over the TP gradually increased with increasing geopotential height. Along with the development of a high-pressure system, the ASMA center moved to the TP after 12 August (onset), and its intensity was generally enhanced (13–15 August, early stage formation). After 16 August, the ASMA was fully established over the TP, with its intensity reaching the highest value.

The variations in relative vorticity at 200 hPa (Figure 3c) and geopotential height anomalies at 500 and 100 hPa over Lhasa (Figure 3d) are also presented. Along with the approach of the ASMA center toward Lhasa, the strength of the PV at 200 hPa over Lhasa increased, reached a maximum on 14 August, and thereafter gradually decreased. The anomalies at 100 hPa over Lhasa (Figure 3d) gradually increased from 12–15 August 2018, indicating that the transition from low to high pressure activity gradually occurred. Although the high-pressure activity at 500 hPa started to form with a one-day lag, it showed a more rapid growth trend and lasted longer. It can also be seen from the 500 hPa geopotential height field (Figure 2) that the high-pressure activity at 500 hPa gradually increased from the 13th, enveloping the entire TP on 16th. Notably, the geopotential height anomaly changed from negative to positive on 14 August, and the subsequent growing tendency was fiercer, which indicated that the high-pressure activity at 500 hPa became abnormally strong from 14 August.

### *3.2. Characteristics of C*<sup>2</sup> *n under Different ASMA Strength States*

We selected four representative profiles obtained at night on 12–14 and 16 August 2018, as shown in Figure 4, to analyze the variation characteristics of *C*<sup>2</sup> *n* under different ASMA strength states over the Lhasa. There were strong maximum peaks of *C*<sup>2</sup> *n* at 17–18 km (about 100 hPa). Certainly, a weak and thin maximum peak of Cn2 appeared at around 12 km (about 200 hPa), such as the 16 August 2018.

*C*<sup>2</sup> *n* was largest on 13 August and smallest on 16 August. On 12 August, ASMA had a subtle impact on the *C*<sup>2</sup> *n* profile over Lhasa. As the ASMA center approached Lhasa and its intensity increased, *C*<sup>2</sup> *n* increased correspondingly on 13 and 14 August. Although these two days are in the middle stage of the ASMA, *C*<sup>2</sup> *n* decreased on 14 August to a lower value than that recorded on 13 August in the range of ~15–20 km. With the departure and attenuated intensity of the ASMA, *C*<sup>2</sup> *n* decreased rapidly on 16 August 2018.

In general, when the ASMA intensity was higher or the ASMA center was closer to Lhasa, a more pronounced "upper highs and lower lows" pressure field structure appeared over Lhasa. The stronger the convective activity, the greater the value of *C*<sup>2</sup> *n*. However, changes in the low-level pressure field, such as at 500 hPa, may have had a crucial impact on the vertical profile of *C*<sup>2</sup> *n*. A turning point occurred on 14 August, when the geopotential anomaly value changed from negative to positive, that is, the pressure field constructed in the UTLS changed from "upper highs and lower lows" to "upper highs and lower highs".

**Figure 4.** Vertical profiles of the atmospheric refractive index structure constant *C*<sup>2</sup> *n* in the UTLS.

In comparison with that recorded on 16 August 2018, the high-pressure activity at the 500 hPa layer was weaker on 14 August, and the upward movement of the atmosphere was only slightly suppressed [37]. Therefore, *C*<sup>2</sup> *n* on 14 August was higher than that on 16 August, but lower than that on 13 August.

#### *3.3. Contribution of Atmospheric Turbulence in UTLS to the Total Integrated Parameters*

The turbulent energy ratio (TER) in the range of 10–20 km, describing the contribution of atmospheric turbulence in this layer to *ε* and *θ*0 of the total layer, can be calculated using the following equation [29]:

$$\text{TER}\_{\varepsilon} = \frac{\varepsilon (i)^{5/3}}{\varepsilon (\text{total})^{5/3}} \times 100\% \tag{6}$$

$$\text{TER}\_{\theta\_0} = \frac{\theta \imath (i)^{5/3}}{\theta \imath (\text{total})^{5/3}} \times 100\% \tag{7}$$

where, *i* stands for 10–20 km turbulent layer.

Table 2 summarizes the integrated contribution of seeing (*ε*) and isoplanatic angle (*θ*0) from the range of 10–20 km and the total integrated parameters. The atmospheric turbulence in the range of 10–20 km has a more significant proportion of *ε*(total) (more than 60%) and *<sup>θ</sup>*0(total) (more than 70%) over the Lhasa, which is consistent with the results of Gaomeigu site, Yunnan observatories, Chinese Academy of Science [29]. The *ε*(*i*) (*θ*0(*i*)) differs 0.5" (0.21") between 13 and 16 August 2018, and TER*<sup>ε</sup>*, as well as TER*θ*0 , varies so widely (more than 10%), which indicates that the variations of *C*<sup>2</sup> *n* under different ASMA strength states are related with the astronomical observations.


**Table 2.** The contribution of seeing (*ε* ) and isoplanatic angle (*θ*0 ) from the range of 10–20 km to total height layer.
