A characteristic feature of diurnal air temperature variation in the upper Yellow River valley in a semi-arid region

摘要

In the upper Yellow River valley in a semi-arid region, the surface air temperature is high in the late afternoon and often peaking after 16:30 local time, and then decreasing rapidly during the growing season.To investigate this characteristic phenomenon, an intensive measurement of the heat budget of bare-soil surfaces was carried out at a field in the upper Yellow River valley in April 2008. Consequently, this phenomenon was found to occur at this field in the spring. The wind strengthened in the late afternoon, which was a valley breeze, and subsided after sunset, meanwhile the sensible heat flux was positive and remained considerable until sunset. Furthermore, the specific humidity decreased to minimal values when the air temperature approached its maximum for the day. These results suggest that the wind caused the vertical transport of the hot and dry air from the upper air advected from the surrounding desert, and maintained higher surface air temperatures, along with the large flux of sensible heat in the late afternoon. After sunset, however, the radiative cooling dominated the heat budget of the surface air layer and the air temperature decreased rapidly.