Why does the northern slope of Lanzhou face the wind during the summer monsoon?

Why is the northern slope of Lanzhou the windward slope of summer monsoon? Due to the difference of thermal properties between land and sea and the seasonal movement of wind belt, the dominant wind direction in China is different in summer and winter:

① Summer: southwest monsoon+southeast monsoon, blowing from the Indian Ocean and the Pacific Ocean to the mainland, full of water vapor, bringing abundant precipitation to China;

② Winter: Northwest monsoon+Northeast monsoon, blowing from the mainland to the ocean, with dry air and little water vapor, which often brings strong winds and cool weather. Generally speaking, there is more precipitation on windward slopes (cold and humid) and less precipitation on leeward slopes (warm and dry). The specific reasons are as follows:

① windward slope. The wind (cold and wet) carrying water vapor is forced to rise when it is blocked by mountains, and the temperature gradually decreases with the increase of altitude, so the water vapor condenses to produce precipitation;

② Leeward slope. After the wind passed the top of the mountain (water vapor decreased, cold and dry), it began to sink, and the temperature gradually increased with the decrease of altitude. Therefore, the airflow on the leeward slope is warm and dry, which is easy to cause disasters such as melting ice and snow and forest fires.