Distribution of subtropical climate in China

China's subtropical climate zone is located between 25° and 35° north latitude. According to the climate zoning of the planetary wind system, it is an arid zone controlled by the subtropical high pressure belt. However, due to the difference between sea and land and the uplift of the Tibetan Plateau, The resulting changes in temperature and pressure fields and monsoon circulation have changed the circulation system of the planetary wind system in the near-surface layer, changing the arid continental climate into a humid subtropical monsoon climate, and changing the arid desert landscape into a humid evergreen broad-leaved forest landscape. After thousands of years of human intervention, this area has become a densely populated and economically developed area in my country. The average annual precipitation in this region is generally between 800 mm and 1600 mm, which is 1 to 2 times more than that in North China and richer than that in Southwest China. Precipitation distribution decreases from southeast to northwest. The annual precipitation in the hills of Zhejiang ranges from 1200 mm to 1800 mm, in the Nanling Mountains and Jiangnan hills around 1500 mm, and in the plains of the middle and lower reaches of the Yangtze River from 1000 mm to 1200 mm. Topography also has a significant impact on precipitation. Generally, there are more mountainous areas than flat areas, and more windward slopes than leeward slopes. For example, Tunxi and Huangshan in Anhui are very close to each other, but the precipitation in Huangshan is 700 mm more than that in Tunxi (1507.8 mm in Tunxi and 2263.9 mm in Huangshan). The annual precipitation in Luoxiao Mountain and Xuefeng Mountain can exceed 1800 mm.

Summer rains are the most common, followed by spring rains, autumn rains, and winter rains are the least. However, winter rains can also account for more than 10% of the annual precipitation. The middle and lower reaches of the Yangtze River are areas with the highest winter rain ratio and the most abundant spring rain in the country. This characteristic of seasonal distribution of precipitation is closely related to the geographical location of the middle and lower reaches of the Yangtze River and the atmospheric circulation process.

The middle and lower reaches of the Yangtze River are located to the east of the Qinghai-Tibet Plateau. In winter, the north and south jet streams of the high-altitude westerly belt converge over this area, forming a relatively stable shear line. Coupled with the strong influence of the Nanling Mountains, As a result, a relatively long-lasting quasi-stationary front in South China appeared on the ground level, cyclones passed frequently, and clouds and rain were common. The average precipitation in most areas in December and January was around 40 mm~50 mm, and in February it was around 60 mm~ About 80 mm. Starting from March, the warm and humid airflow from the south strengthens and the frequency of precipitation increases. In late March, southern Gansu first entered the spring rain period, and with this as the center, the spring rain gradually extended to the northwest, southwest and northeast. In most areas, the spring rain begins to end in early to mid-June. The hills in the south of the Yangtze River have heavy rainfall and a long rainy season in spring. In most areas, the spring rain starts in early April and ends in late May, with the rainy period lasting up to 60 days. There are more spring rains in this area. The rate of spring rains in Changsha, Nanchang and other places is about 40%, which is more than summer rains. In Nanjing, Shanghai, Wenzhou and other places, spring rain accounts for about 25% of the annual precipitation.