## 3 WRF-CFD Wind Field Downscaling Technology
### Simulation of Typhoon Field by WRF Model
#### 3.1.3 Typhoon Wind Field Characteristics  
Figure 3 shows the simulation area of the WRF model and a map of Typhoon Rammasun's path. The blue line is the best path provided by the Tropical Cyclone Data Center of the China Meteorological Administration. The red line is the WRF-simulated path. From the chart, it can be seen that Typhoon Rammasun, as a whole, moved from the southeast coast to the northwest one. There is a certain deviation between the simulated and observed paths. In this paper, the classic turbulent closure scheme is used, which considers local turbulent transport. Although the typhoon wind speed can be simulated well, there is a certain deviation in the wind direction. This study focuses on the extraction of near-surface wind profiles under the action of Typhoon Rammasun. In general, the simulation of Typhoon Rammasun by the WRF model can effectively provide the required wind profile for the CFD inlet boundary. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**WRF Parameter** & **D01 Area** & **D02 Area** & **D03 Area** \\ \hline Horizontal resolution & 13.5 km & 4.5 km & 1.5 km \\ Integration time step & 240 s & 120 s & 40 s \\ Planetary boundary layer & Mellow–Yamada–Janjic\({}^{\prime}\) (MYJ) boundary layer & \\ Cumulus convection parameterization & & Kain–Fritsch scheme & \\ Land surface process & & Heat diffusion scheme & \\ Longwave radiation & Rapid Radiative Transfer Model (RRTM) scheme & \\ Short-wave radiation & & Dudnia scheme & \\ Near-surface & & Monin–Obukhov scheme & \\ Microphysical process & & [PERSON] et al.’s scheme & \\ Pattern dynamic framework & Advanced Research WRF (ARW) non-static & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter settings of the WRF model. Figure 4 shows the comparison curve between the maximum stable wind speed near the typhoon center simulated by the WRF model and the data [26] measured by the Japan Meteorological Agency (JMA). The comparison curve shows that the trend of the maximum wind speed near the typhoon center over time is consistent with the corresponding measured data, which both increase first and then decrease. The initial and mid-to-late simulation data are in good agreement with the measured data, while the mid-term simulation data exhibit a slight overestimation with respect to the measured data. The WRF simulation data concern the maximum wind speed in all areas near the typhoon center, while the JMA measured data concern the maximum wind speed at the location of the wind measurement mast. The effectiveness of the typhoon simulation is verified by analyzing the wind field characteristics of the typhoon. Figure 5 shows the WRF model simulation results for the wind field sea-level pressure, wind speed, equivalent potential temperature, and absolute vorticity when Typhoon Rammasun was logged on 18 July 2014. The analysis shows that the typhoon center has a low-speed and low-pressure distribution. The wind speed and pressure around the center gradually increase and the high-level airflow with higher potential temperature flows into the boundary layer through the coiling effect and then mixes and warms the low-level airflow. The strong wind field energy of the typhoon causes the absolute vorticity of the surrounding wind field to weaken outward from the typhoon center. The wind pressure in the central area of the typhoon simulation is 930-970 hPa and the wind speed is 30-34 m/s, which is consistent with the information recorded by the Central Meteorological Typhoon Network. Figure 4: Schematic diagram of WRF simulation. Figure 3: Schematic diagram of WRF simulation. (**a**) Simulation domain. (**b**) Typhoon track. Figure 6 shows some WRF simulation results for cloud maps of the velocity streamline of Typhoon Rammasun during different landing stages. As can be seen from the figure, before the typhoon landed, there was a complete vortex structure in the typhoon eye area, cloud wall area, and spiral rain belt area.