# A Probabilistic Forecast Algorithm of Nonconvective Turbulence over the Tibetan Plateau
## 4 Turbulence Probability Index
However, for the 2006 event, the PICAT on the route is only approximately 0.4 (Figure 4a), and the high-value zone of PICAT is not within the range of the route. For the 2009 event, the PICAT on the route reached above 0.7 (Figure 4b) but was still slightly smaller than the PI result (Figure 3c). This also indicates that turbulence related to mountain waves cannot be ignored in the Tibetan Plateau region. Figure 5 displays the PI, PICAT, and PIMWT (probability index based on six MWT diagnostics) averaged over 2480 instances from August 2012 to 2021. Figure 5a indicates that the edges of the Tibetan Plateau, including southern and northern Tibet, western Sichuan, and east-central Qinghai, are regions of high turbulence probability. Western Sichuan, southern Tibet, and the northern slope of the Tibetan Plateau between 36-40\({}^{\circ}\) N are likely to experience moderate or greater (MOG) turbulence (Figure 5d). Severe or greater (SOG) turbulence is more likely to occur on the western edge of the Tibetan Plateau and in north-central Sichuan (Figure 5g). Despite being directly linked to aircraft turbulence, statistics that specifically address turbulence at high altitudes in China are limited. According to a statistic released by the Chinese civil aviation authorities [31], bumps at high altitudes (>7500 m) are most frequently reported in the Gansu and Ningxia regions, Chongqing and Chengdu regions, and the Lhasa region. [PERSON] et al. [21] studied the  
Figure 3: PI for light or greater turbulence at 200 hPa at **(a)** 0600 UTC 26 February 2006, **(b)** 0900 UTC 26 February 2006, **(c)** 0600 UTC 13 October 2009, **(d)** 0900 UTC 13 October 2009, **(e)** 0600 UTC 11 February 2023, and **(f)** 0900 UTC 11 February 2023. The black squares represent the range of the Chengdu–Lhasa route. temporal and spatial characteristics of aircraft turbulence from 2011 to 2015 and found that turbulence in eastern Tibet and northern Sichuan had a greater likelihood of occurrence. The PI in Figure 5 (left column) indicates a high probability of turbulence in the Lhasa area (91.1\({}^{\circ}\) E, 29.6\({}^{\circ}\) N), which is consistent with the findings of previous studies. [PERSON] [32] conducted a statistical analysis of turbulence on high-altitude flight routes in China in 2014 based on airborne detection data and analyzed the causes of turbulence. The results showed that the Lhasa route was the most severely affected, with a turbulence frequency as high as 78%. This route was most affected by turbulence induced by jet streams, with a frequency of 42.82%, as well as by turbulence from mountain waves. Our results from Figure 5 show that the Lhasa route (black box in Figure 4) is located at the intersection of the high probability areas for CAT and MWT (middle and right columns in Figure 5), which is consistent with [PERSON]'s conclusion. The characteristics of the turbulence probability index have been analyzed above. Figure 6 shows the average PI in August from 2012 to 2021 at 200 hPa. The Tibetan Plateau region has interannual variability in the upper-level turbulence spatial distribution. Due to the persistent impact of mountain waves, turbulence is always prone to occur around the edges of the Tibetan Plateau. In some years, there are significant differences in the turbulence probability between the northern and southern edges, such as in 2016 and 2019 when the probability was higher in the north, and in 2018 when it was higher in the south. Certain regions within the Tibetan Plateau, including the eastern part of Tibet, and the central-western part of Sichuan Province (black box in Figure 6) show interannual variations in the turbulence probability index. The high PI (such as 0.44) in this region was highest in 2012, 2014, 2015, 2017, and 2021, but less in other years such as in 2013, 2016, 2019, and 2020. Figure 4: PICAT for the intensity of light or greater at 200 hPa at **(a)** 0600 UTC on 26 February 2006, **(b)** 0600 UTC on 13 October 2009, and **(c)** 0900 UTC on 11 February 2023.