## 2 Data and Methodology
### WRF Setup and Experiment
#### 2.2.2 Model Configuration  
Ya'an, with a subtropical monsoon climate, is located in the western edge of Sichuan Basin. The area from Kangding to Litang belongs to the western Sichuan high mountain plain, with a wide and flat or hilly top surface, and mainly has a continental plateau-type and mountain-type climate. On the other hand, the area from Litang to Changdu belongs to the southeastern Tibetan Hengduan Mountains area, with high mountains, deep valleys, complex terrain, and mainly plateau climate. Since the terrain of Ya'an and Kangding is flatter and the climate is warmer than that of Litang Changdu, in order to adapt these differences, this paper divides the railway section from Kangding to Changdu into two regions with 100.8\({}^{\circ}\) E as the boundary and performs double-layer nested simulation. The outer grid is marked as d01, and the inner grid is marked as d02. Region 1 mainly includes Ya'an to Kangding and other places, and region 2 includes Litang to Qamdo and other places. The center points of the two regions are 30.03\({}^{\circ}\) N, 101.97\({}^{\circ}\) E and 30.6\({}^{\circ}\) N, 98.62\({}^{\circ}\) E, respectively. Figure 3 shows the specific simulation range. The National Centers for Environmental Prediction (NCEP) Final Operational Global Analysis (FNL) ([[https://rda.ucar.edu/datasets/ds083.2/](https://rda.ucar.edu/datasets/ds083.2/)]([https://rda.ucar.edu/datasets/ds083.2/](https://rda.ucar.edu/datasets/ds083.2/)) (accessed on 13 February 2023)) with a horizontal resolution of 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) and a frequency of 6 h are selected as the initial conditions of the model. The simulation covers the period from 12 December 2016 to 28 February 2017, and the specific grid parameters of the two regions are shown in Table 1. In region 2, which has many mountains, the simulation accuracy is improved by using MCD12Q1v006 land use data with a horizontal resolution of 500 m instead of the original data. Moreover, to simulate the atmospheric processes within the boundary layer more precisely, the vertical direction is divided into 41 layers, with 15 layers densely distributed at a height of about 1.5 km near the ground. Table 2 shows the heights of each layer grid, and Figure 4 illustrates the elevated layers in both regions, the red line shows the position of the 15 th layer. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multicolumn{2}{c}{**Region 1**} & \multicolumn{2}{c}{**Region 2**} \\ \cline{2-5}  & **d01** & **d02** & **d01** & **d02** \\ \hline Grid space & 9 km & 3 km & 9 km & 3 km \\ Grid points & 100 \(\times\) 100 & 88 \(\times\) 88 & 100 \(\times\) 100 & 148 \(\times\) 130 \\ ETA levels & 32 & 32 & 41 & 41 \\ Time step & 27 & 27 & 27 & 27 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The grid parameters of two regions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline Mesh layer & 1 & 2 & 3 & 4 & 5 \\ \hline Height/m & 4 & 30 & 64 & 93 & 131 \\ \hline Mesh layer & 6 & 7 & 8 & 9 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Vertical height of each mesh layer within 1.5 km. Physical processes are crucial for determining the source and redistribution of momentum, heat, and water vapor in the atmospheric motion. These processes have a great impact on the overall dynamics of the atmosphere. Microphysics schemes mainly affect the precipitation simulation, among which the Purdue-Lin scheme and Thompson scheme are two relatively complex calculation schemes. These schemes involve variables such as rain, ice, snow, and graupel, making them suitable for high-resolution simulations. The planetary boundary layer scheme affects the wind simulation, among which the MRF scheme can provide ideal simulation results for the boundary layer conditions with good mixing, such as instability or convection, and the YSU scheme can simulate the turbulent motion and energy conversion in the atmosphere well and has wide applicability. A radiative parameterization scheme is used to calculate the surface heat budget.