## 3 Results and Discussion
### Monthly Variation Characteristics  
The transmission of laser beams in the atmosphere is influenced by aerosol particles, whose concentration is closely related to boundary layer height and turbulence intensity. Observational studies have shown that the boundary layer height over the Qinghai-Tibet Plateau is greater than over plain regions [21], and turbulence is also stronger than in plains [22]. Moreover, the boundary layer height exhibits notable seasonal and diurnal variations [23; 24; 25]. Therefore, it is essential to analyze further the monthly and diurnal variations in the detection efficiency of DWL. During the experiment, the detection efficiency in PPI mode (Figure 3a) and DBS mode (Figure 3b) generally showed an upward trend. For the PPI mode, the detection efficiency at 4 km dropped below 50% in August, while in September, it dropped below 50% at 5 km. Significant improvements were observed in October and November, with the detection efficiency falling below 50% at 6 km and remaining over 10% at 7 km. From November to February, the detection range was further extended, with nearly 50% detection efficiency at 6 km and over 20% at 7 km. By April 2024, the detection efficiency showed a marked improvement, maintaining 50% efficiency at 7 km and over 10% at 9 km. The monthly variation in detection efficiency for DBS mode followed a similar trend to that of PPI mode. In August and September, the detection efficiency exceeded 90% at 1 km, while from October to December, it remained around 90% at 2 km, with a slight decrease in December. From January to March 2024, the detection efficiency at 2 km stabilized above 95%, and in April, the detection efficiency at 3 km remained over 90%, with nearly 60% efficiency still achieved at 4 km. By combining the monthly variations in meteorological elements shown in Figure 3c, it is evident that precipitation has the most significant impact on DWL performance. In August 2023, the temperature and humidity were the highest of the year, with relative humidity nearing 100% and cumulative precipitation approaching 100 mm, far higher than in other months. In particular, the number of days with precipitation greater than 1 mm reached twelve. The significant transmission attenuation of laser pulses in rainfall directly impacts the detection range of the DWL [16; 26]. Additionally, the washing effect of precipitation reduces the aerosol particle concentration in the air, indirectly affecting the detection range. Figure 3: Monthly detection efficiency heatmaps for PPI scanning mode (**a**) and DBS scanning mode (**b**) from August 2023 to April 2024, with specific detection efficiency values labeled. (**c**) shows cumulative monthly precipitation (bar chart), average wind speed (green line), average temperature (yellow line), and average relative humidity (blue line) during the same period. (**d**) presents a box plot of DBS detection ranges for the same period, where the edges of the box represent the interquartile range (upper and lower quartiles), and the line in the middle represents the median detection range. Studies have shown that the boundary layer height over the Qinghai-Tibet Plateau is lower during the wet season (May to September) and higher during the dry season (October to the following April) [23; 27]. The average detection height in DBS mode shown in Figure 3d) reflects this seasonal variation in boundary layer height to some extent. According to the surface cloud observations at Gonggar Airport (Table 3), in August the detection range in DBS mode was notably smaller than in other months, and the variation in the detection range was more significant. This suggests that the boundary layer height was relatively low that month, with frequent low-cloud activity being the primary cause of the fluctuations in the DBS detection range. In contrast, in September, there was no precipitation, and low-cloud cover quickly diminished.