by Li Debiao, who would all shoot down five more of the Japanese bombers; the northern Chinese air-raid early-warning net headquarters in Xi'an observed 21 of the original 30 IJAAF bombers returning from Lanzhou. As the Imperial Japanese forces were preparing for Operation Z against the United States and other allies in Southeast Asia and the Pacific by mid-1941, the IJANF were busy training new aircrews in live combat action across China for the upcoming war in the Pacific; on 22 May 1941, Lt. Gao Youxin of the 21st PS, 4th PG engaged an IJNAF raid on Lanzhou in an I-16 fighter aircraft, while avoiding confrontation against the Zero fighters, he shot down a Mitsubishi G3M bomber north of Lanzhou. On the ground, the KMT Muslim Generals Ma Hongkui and Ma Bufang protected Lanzhou with their cavalry troops, putting up such resistance that the Japanese never captured Lanzhou. The city is the seat of a currently vacant Roman Catholic diocese and was previously the center of a vicariate apostolic (Vicariate Apostolic of Northern Kan-Su). Geography Area: Elevation: above sea level China's northwest geographical center More than of urbanisation along the southern banks of the Yellow River. Zonary basin Mountains are located on the south and north sides of the city: Qilian Ranges, Mt. Pingliang and Mt. Kongtong (the most noted in Taoism) River: The Yellow River flows through from west to east. Lanzhou is situated on the upper reaches of the Yellow River where it emerges from the mountains and has been a center since early times, being at the southern end of the route leading via the Hexi Corridor across Central Asia. It commands the approaches to the ancient capital area of Chang'an (modern Xi'an) in Shaanxi province from both the west and the northwest, as well as the area of Qinghai Lake via the upper waters of the Yellow River and its tributaries. Climate Lanzhou is situated in the temperate zone and enjoys a semi-arid climate (Köppen BSk) with hot summers and cold and very dry winters. In the urban core, based on 1971–2000 normals, the monthly 24-hour average temperature ranges from in January to in July. The mean annual temperature is , while annual rainfall is , almost all of which falls from May to October. The winters are so dry that snowfall is sometimes restricted to fall and spring. With monthly percent possible sunshine ranging from