A Significant Evening Peak of Vapor Pressure at an Oasis in the Semi-Arid Region

Abstract

Significant evening peaks were observed in the diurnal variations of vapor pressure over an oasis surrounded by desert in the north-west part of China. They appeared not only in the surface layer measurement but also in the upper air soundings below 500 m. According to the surface flux measurements, it was observed that sensible heat flux was much stronger over the desert than over the oasis during the daytime. The upper-air sounding data shows that the oasis is a cool and moist island capped by dry and hot air which would be advected from the surrounding desert. In these situations, upper-layer convective mixing is considered to be much stronger than in the lower cool island, and this can lead to water vapor flux divergence in the afternoon. In the evening, convective mixing, which is suggested by the sensible heat flux from the ground, decreases over the desert and in the upper layer over the oasis. It was observed that evapotranspiration from the surface still continues upward. It is suggested that water vapor flux convergence can occur near the oasis surface.