## 3 Results and Discussion
### Diurnal Variation Characteristics  
To analyze the diurnal variation in detection efficiency, we compared the months of January and April, which exhibited significant differences in detection range and minimal influence from low clouds. In January, the detection range in DBS mode showed a typical \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Low-Cloud Cover** & **Aug** & **Sep** & **Oct** & **Nov** & **Dec** & **Jan** & **Feb** & **Mar** & **Apr** \\ \hline
0.0–1.9 & 1 & 16 & 19 & 22 & 27 & 22 & 13 & 17 & 3 \\
2.0–3.9 & 10 & 9 & 7 & 8 & 3 & 6 & 14 & 12 & 20 \\
4.0–6.0 & 10 & 5 & 5 & 0 & 1 & 3 & 2 & 2 & 7 \\
6.1–8.0 & 10 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Number of days with different ranges of low-cloud cover (octas) observed at Gonggar Airport (by month). diurnal pattern, with a trough and peak (Figure 4a). Around 9:00 a.m., shortly after sunrise (local sunrise in January is approximately 8:50 a.m.), the detection range reached its minimum of approximately 3.3 km. At this time, temperature and wind speed at Gonggar Airport were at their lowest points of the day, with temperatures around \(-\)7 \({}^{\circ}\)C (Figure 4e). As the temperature gradually rose, reaching a maximum of around 6 \({}^{\circ}\)C in the afternoon, wind speed increased slightly from 1 m/s to 2.7 m/s. However, the average detection range in DBS mode did not significantly improve before 2:00 p.m. (local solar noon), reflecting minimal changes in boundary layer height under stable, cold, and low-wind conditions. After 3:00 p.m., the average detection range increased significantly, reaching a peak around 7:00 p.m., with fluctuations persisting but remaining higher than during the daytime. Notably, the magnitude of the fluctuations during this period also increased. Studies have shown that total cloud cover over Lhasa tends to increase significantly in the afternoon, peaking between 2:00 and 6:00 p.m. before rapidly decreasing after 6:00 p.m. [28]. Low-cloud observations at Gonggar Airport in January 2024 exhibited similar characteristics, with cloud cover exceeding 20% between 3:00 and 8:00 p.m., compared to less than 20% during other periods. The increase in low-cloud cover is the main reason for the fluctuations in the DBS detection range during this time. Additionally, from late afternoon to midnight, the upper quartile of the maximum detection range reached the DWL's maximum range, indicating a higher boundary layer height during these hours. In contrast, the trend in the PPI mode was inverse to that in DBS mode (Figure 4c). The detection range in PPI mode was slightly higher in the morning than during other periods, suggesting that the boundary layer height may have been lower, which helped to increase the detection range in PPI mode to some extent. In April, the diurnal variation in the detection range for both the DBS and PPI modes is similar to that observed in January. Still, the detection ranges are generally higher, and the diurnal variation is more pronounced. For DBS mode, both the average value and the upper and lower quartiles of the detection range reach their minimum around 9:00 a.m., with an average of approximately 4.2 km (Figure 4b). Afterward, as the average temperature rises from 5 \({}^{\circ}\)C in the morning to 18 \({}^{\circ}\)C in the afternoon, the average wind speed also increases from 2 m/s in the morning to 7 m/s in the afternoon. The peak in wind speed slightly lags behind the temperature peak. After 6:00 p.m., a southerly wind perpendicular to the runway indicates local solid circulation in the river valley where the airport is located (Figure 4f). As a result, the DBS mode detection range increases significantly, stabilizing above 5 km from 4:00 p.m. to 8:00 p.m., suggesting that aerosol particles may have diffused to higher altitudes.