## 2 Field Experiment and WRF Model Setup
### Description of the Case and Ambient Conditions  
On 27 July 2019, most of the Tian Shan Mountains were covered by orographic clouds during the daytime, as can be seen from the satellite visible image at 9:00 UTC (Figure 2a). The entire storm moved west as time went on. A streamline analysis carried out at 800 hPa using the reanalysis data suggests the lifting of the flow from the opposite sides of the mountain, resulting in convergence over the mountain (Figure 2a). A sensitivity test conducted using model simulation suggests that the convergence disappeared if the sensible heat flux over the mountain was turned off (not shown), indicating the presence of anabatic winds over the Tian Shan Mountains during the daytime. This pattern can be also seen in the 700 hPa wind field (Figure 2c), which shows mainly southwesterly wind over the southern slope and northwesterly wind over the northern slope. However, it can also be seen that some wind vectors were in a southwesterly direction over the northern slope, such as near the observational site (Figure 2c), suggesting a cross-ridge flow from the southwest. This was demonstrated using the model simulation (Figure 2b), which showed that strong vertical wind shear existed at an approximately 4-5 km height. The wind was in a southwesterly or westerly direction above the wind shear level, and the low-level northwesterly wind was blocked as it reached the ridge (Figure 2b). At the upper levels (Figure 2d), the wind was westerly and controlled by a weak trough. The relative humidity at 700 hPa and 500 hPa was high over the Tian Shan Mountains, indicating a thick moisture layer, providing optimal conditions for cloud formation. Precipitation was detected by the radars in the middle of the Tian Shan area from 8:50 to 12:20 UTC (i.e., 14:50 to 18:20 local solar time) and was mostly light to moderate (weaker than 1 mm h\({}^{-1}\)) in the middle of the Tian Shan area. Note that while the radar measurements can only represent clouds and precipitation within the detection range, which was small compared to the entire mountain, they are useful for investigating the fine structures of clouds. Figure 3 shows the time series of the temperature near the surface, the LWP, the freezing level height, and the lifted condensation level (LCL) height from 0:00 to 14:00 UTC derived from the radiometer measurement. It can be seen that the temperature near surface increased from 0:00 UTC (i.e., 6:00 local solar time) and reached 23 \({}^{\circ}\)C at noon. The freezing level varied between 3.8 km and 5.2 km. The LCL was high before the precipitation started. After 9:00 UTC, the LCL decreased to \(\sim\)3.1 km, which was close to the surface; this is because the low-level relative humidity rapidly increased (not shown) and the surface temperature rapidly decreased after the precipitation started, probably due to the evaporation of rain drops and the stronger shortwave reflection by the clouds. The LWP remained about 120 g m\({}^{-2}\) before the precipitation started; this was a fairly high value [35], indicating that the clouds were mostly thick. After the precipitation started, the LWP rapidly decreased, indicating a conversion from cloud to rain. After 12:20 UTC, the LWP increased again but the precipitation stopped. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **D01** & **D02** & **D03** \\ \hline Resolution and grids & 9 km, 600 \(\times\) 500 & 3 km, 511 \(\times\) 400 & 1 km, 697 \(\times\) 535 \\ Vertical levels & & 33 & \\ Cumulus & Grell–Freitas scheme & None & \\ Microphysics & & Thompson scheme & \\ Longwave radiation & & RRTMG scheme & \\ Shortwave radiation & & RRTMG scheme & \\ Land surface & & Noah scheme & \\ Surface layer & Monin–Obukhov (Janjic Eta) scheme & \\ PBL & & MY scheme & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Configuration of the model simulation. To sum up, the ambient conditions were suitable for cloud formation, and convergence was generated due to the anabatic winds over the Tian Shan Mountains.