Airmass transformation over the semiarid region around North China and abrupt change in the structure of the baiu front in early summer

According to the observation data from 1975 to 1979, the “structure change” in the Baiu front in China occurs in late May in each year: the vanishment of the horizontal temperature gradient in the lower layer and the change in the stability for moist convection from stable state to nearly neutral state. The ground surface temperature in North China increases rapidly and its horizontal gradient along ~115°E vanishes in China around May in these years. In late May of 1979 the air temperature in the lower layer rises largely in the semiarid region (Takla Makan~Mongol Plateau~Gobi Desert~North China) and the polar frontal zone in the eastern part of Eurasia (90°E~140°E) shifts from ~45°N to ~60°N. The area where the deep mixed layer due to dry convection with a thickness of more than 200 mb (2000 m) begins to develop rapidly in late April over the semiarid region from Takla Makan to North China, a month before the “structure change” in the Baiu front in China. The heat budget in the atmosphere over Mongol Plateau suggests that the air temperature rise and the maintenance of high temperature in the lower layer in May are mainly due to the sensible heat supplied from the ground as well as in North China. The intense airmass transformation over that region warms the air parcel to some extent before it reaches North China. This seems to provide the more favorable condition for the change in the thermal structure in the Baiu front in South China. The “structure change” in the Baiu front in China and the northward shift of the polar front in the eastern part of Eurasia seems to be a response to the heating from the ground in the semiarid region from Takla Makan to North China.