*2.1. Study Area*

This study covered the 18 prefecture-level cities in SCB, which were Chengdu (CD), Chongqing (CQ), Deyang (DY), Mianyang (MY), Meishan (MS), Leshan (LS), Ya'an (YA), Yibin (YB), Zigong (ZG), Luzhou (LZ), Neijiang (NJ), Ziyang (ZY), Suining (SN), Guangyuan (GY), Bazhong (BZ), Nanchong (NC), Dazhou (DZ) and Guang'an (GA). As shown in Figure 1, these cities were in the basin area confined by Tibetan Plateau to the west, Yunnan-Guizhou Plateau to the south, Wuling Mountain to the east and Daba Mountain to the North. The average altitude in the basin is about 400 m above sea level, while the altitudes of the plateaus and mountains are above 4000 m and 2000 m, respectively. The huge height drop creates lee-side calm area in the basin. Besides, the topography in the basin is also complicated, which could be divided into Chengdu Plain in the west, Hilly Area in the central basin and Ridge Valley area in the east by Longquan Mountain and Huaying Mountain.

**Figure 1.** The location (**left panel**) and topography (**right panel**) of the Sichuan Basin. In the left panel, the blue square presents the region where the synoptic classification was conducted and the red polylines presents the location of SCB.
