By using the real-time observation data and physical diagnosis, the causes and mechanisms of extreme hot weather in central eastern region of northwest China and the western region of Inner Mongolia from July 26 to 31, 2010 are analyzed and researched. The results show that the Tibetan high pressure at the upper layer and the continental high pressure at the middle layer of the troposphere are the main causes of this extreme hot weather, while the plateau warm ridge and ground heat low pressure at the lower layer contribute to this extreme high temperature. The Tibetan high pressure and continental high pressure are overlapped with each other, forming thick anticyclone warm high pressure system with the characteristics of convergence at the upper layer, divergence at the lower layer, and accompanied with sinking movement. The clear sky radiation heating and sinking adiabatic heating caused by the above two, as well as the temperature uplifting caused by plateau warm ridge and ground heat low pressure at the lower layer are the main causes and physical mechanisms of this extreme hot weather. The significances of various affecting systems are discussed, and a conceptual model for the forming of the extreme hot weather is provided.

Keywords: Extreme hot weather, Affecting system, Spatial structure, Conceptual model, Causes and mechanisms

Basically, extreme hot weather occurs in summer season, and mostly concentrates from late July to early August. It not only directly affects people’s daily life, but also seriously threatens people’s lives and property safety, as well as national energy, water resource, food, and other major economic security and social security. With the rapid development of the national economy, the economic losses directly or indirectly caused by the extreme hot weather are increasing. Therefore, the extreme hot weather attracts more and more attention.

In meteorology, the temperature of more than 35℃ is called as high temperature, and the high temperature which reaches to the extreme point is also called extreme high temperature. The relative humidity of northwest region is small, and it belongs to dry and hot high temperature. In the researches of hot weather, WEI Jie, etc. ^[1] studied the relationship between the abnormal east-west position of subtropical high pressure and the high temperature at the eastern area of China in summer. They thought that the continental high pressure causing strong downdraft is one of the systems affecting the hot weather in northern China. The researches of ZHANG Yinshang, etc. ^[2] show that the sinking motion heating caused by warm subtropical high pressure or continental high pressure (such as the high pressure at the bend of a river) is the main reason for the forming of hot weather. The earlier studies of XIE Zhuang, etc. ^[3] suggested that the sinking adiabatic heating in the air of high pressure system is the main physical mechanisms for the forming of hot weather. Through analyzing the high pressure structure at the bend of a river causing the continuous high temperatures in Beijing, QIAN Tingting ^[4] further thought that the radiation heating in clear sky, advection heating and sinking adiabatic heating associated with the high pressure at the bend of a river are the possible physical mechanisms for the forming of hot weather. HE Zhe, etc. ^[5] believed that the high pressure at the bend of a river is a thick negative vorticity system associated with the eastward shift of a wide range of warm air in the arid region in western China which favors to the forming of hot weather. ZHENG Zuofang, etc. ^[6] discussed that the forming mechanism of high pressure at the bend of a river is associated with the high temperatures in Beijing, and they also proposed a possible conceptual model for the hot weather of Beijing in the summer. The results from most of the above researches are obtained from analyzing the high-altitude situation and the structure of the dynamic field, but the discusses on warm low pressure which directly affects the changes of ground temperature are very rare. Due to this reason, the researches on the causes and mechanisms for the forming of hot weather in the past are not comprehensive, and the knowledge to it is not clear enough.