SYNOPTIC DISTURBANCE AND RAINFALL OVER THE ARID AREA IN THE NORTHWESTERN CHINA

The synoptic weather situations around rainfall over the arid area in Northwestern China during HEIFE are further investigated as an extensi on of a previous study (Itano, 1997). The analysis shows a frequent pa ssage of the frontal troughes over this arid area and successive forma tion of cyclones on them at the northern edge of the Tibetan Plateau. The rainfall in this arid area is fundamentally caused by such westwar d moving troughes. With the passage of these frontal troughes, a signi ficant amount of rainfall is observed at the higher mountainous area b ut not necessarily at the lower area. Especially at the lowest desert area, rainfall is observed only when the relative humidity within the whole cloud base exceeds 60 percent. This fact seems to indicate the e vaporation of rain drops occurs within the dry boundary layer over the desert, which prevents the rain drops from reaching lower altitude un til whole boundary layer is moistened to some extent. This may be one of the principal reasons for scarce rainfall in the lowest desert area despite the frequent passage of synoptic disturbances and large numbe r of rain events.